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SPE 95089 What Level of Sulphate Reduction is Required to Eliminate the Need for Scale Inhibitor Squeezing? L.S. Boak, SPE, H. Al-Mahrouqi, E.J. Mackay, SPE, C.E. Inches, K.S. Sorbie, SPE; Heriot-Watt University, UK, and M.C.M. Bezerra and R.O. Mota, Petrobras, Brazil © 2005 Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE 7th International Symposium on Oilfield Scale held in Aberdeen, United Kingdom, 11-12 May 2005. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the SPE, their officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract Scale deposition is often controlled using chemical scale inhibitors. However, for high scale severity, sulphate reduction in the injected seawater may provide a more attractive economic solution to the problem. This study uses data based on Marlim Leste (Campos Basin, offshore Brazil) reservoir brine compositions and conditions for both reservoir modelling and experiments. Using reservoir modelling, supersaturation and precipitation potentials in the system are first calculated assuming no precipitation reactions in the reservoir. Sensitivity calculations are then performed to investigate the effect of brine mixing both deep in the reservoir and at the production well. Supersaturation and precipitation levels are then recalculated assuming in situ stripping has reduced [Ba2+] and/or [SO42-] levels at the wellbore. Static (uninhibited) BaSO4 precipitation experiments have been conducted to determine the limiting sulphate level that removes the requirement for applying squeeze treatments. A simple rate law is found and this is used to define “safe operating envelopes” where the barite precipitation is sufficiently slow that we expect no squeeze treatment is required. Some risk is associated with these envelopes due to uncertainties in the determination of the rate constant, k, for barium deposition. Based on further kinetic analysis using data on the safe envelopes, we establish what rate the barite deposition may occur thus identifying where the system would deposit barite slowly and where it would reach its full equilibrium deposition limit quickly. This work presents both a methodology and also some analytical modelling tools for establishing, on a sound technical basis, the answer to the question: What level of sulphate reduction is required to eliminate the need for scale inhibitor squeezing?

Introduction Scale inhibitors (SI) have been used extensively to prevent mineral scale deposition in oilfields1. Nowadays, new field developments present further challenges; e.g. when there is limited access to wet subsea wellheads especially with the development of long subsea tiebacks, or when difficulties arise from SI placement in remote ultra-deep horizontal wells2. Complicated well structures also pose problems with squeeze treatments as does the chemical analysis of return fluids where producing streams co-mingle.3 Thus, the problems arising from scale formation require an even more rigorous assessment than previously and this should be incorporated into the Front End Engineering Design (FEED) in the field development. An alternative scale control strategy is to reduce the sulphate level in the injected seawater before its deployment, using a sulphate reduction plant (SRP). This would reduce or avoid scale formation when the low sulphate seawater (SW) mixes with the reservoir formation water (FW) during production. Other options may be to combine desulphation with squeeze treatments, or to utilize alternative sources of injection brine such as from aquifers or produced water. Several operators have assessed their fields to determine which of the various scale control methods are most technically viable and economically attractive.2,4,5 Technical and Economic Analysis Economics: Traditional scale management using chemical SIs, is becoming increasingly more difficult, especially in deepwater production systems. Indeed, this approach is in some cases thought to be economically unfeasible. When the scaling problem is assessed in the FEED stage, it is often found to be a significant cost to the development through reduced PI due to uncontrolled scale build up, coil tubing squeeze treatment interventions or scale removal operations. Thus, the cost of reactive scale management may have a significant effect on the project economics. It has been suggested that scale control should be considered as CAPEX (capital expenditure) rather than OPEX (operating expenditure). Thus, sulphate removal is often thought of as CAPEX which solves the scale problem “up front”, whereas performing squeeze treatments is seen as OPEX where the problem is solved continuously as the reservoir is developed. The advantages, disadvantages and risks for both squeeze treatments and for sulphate removal has been discussed4.

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L.S. Boak, H. Al-Mahrouqi, E.J.Mackay, C.E. Inches, K.S. Sorbie, M.C. Bezerra, R.O. Mota

Desulphation: One alternative scale control strategy involving the desulphation of injected seawater was first introduced to the industry in 1988 by Marathon in the Brae A reservoir in the North Sea6. This method employs a sulphate reduction plant (SRP) using, for example, nanofiltration membranes that remove the sulphate ions while allowing other ions in the seawater to pass through the system. This can reduce sulphate concentrations from about 3000ppm, depending on the location and water temperature of the injected SW, to less than 40ppm. However, even at 40ppm sulphate there may still be a scaling potential and squeeze treatments may still be required in some cases. Another type of sulphate removal uses reverse osmosis but it is very expensive2. Previous work has discussed desulphation in greater detail.2,3 Modelling: There are a number of thermodynamic prediction programs,2,7,8 reservoir fluid flow simulators1,9 and analytical models10 that can be utilised to aid the risk assessment process. A combination of such modelling programs can highlight various aspects that need to be considered when optimising scale management in a group of reservoir wells. Several examples have been published of how a combination of reservoir modelling and scale prediction software can be used to assess the severity and location of scaling problems, with or without reservoir stripping 7-11. Reservoir flow (and thermodynamic) modelling may play a major role in determining how different reservoirs and brine compositions react to specific conditions such as field temperature and pressure. However, such calculations should be supported by performing laboratory tests that are representative of field conditions with respect to reaction kinetics. Summary of the Current Study Reducing the sulphate concentration to low levels may still give some level of scaling potential and squeeze treatments may still be required2,5,11. The FEED study should address the question: What level of sulphate reduction is required to eliminate the need for scale inhibitor squeezing? This question has been pursued in the study described here using the Marlim Leste (Campos Basin, Brazil) example. An economic evaluation study was previously published for this field4, and it has been determined that a sulphate removal plant, similar to that deployed in the Girrasol Field3, would be beneficial. The Marlim Leste field4 is a deepwater offshore oilfield situated in the Campos Basin off Brazil (Fig. 1). The main reservoir age is Oligo-Miocene and the depth is 2700m, in water depths of 800-2000m. For Marlim Leste, we wish to establish the required [SO42-] to eliminate the need to squeeze altogether. To do this, we use several modelling packages along with supporting laboratory barite precipitation experiments. Marlim Leste reservoir brine compositions and conditions12,13, are used as in Table 1 with sensitivity studies conducted at various barium and sulphate concentrations. However, the methodology is applicable for assessing required desulphation levels for any reservoir. The supersaturation and precipitation potentials of the system are calculated assuming no reactions in the reservoir (mixing in the wellbore only). Various sensitivity calculations are then performed to assess the effect of brine mixing in two locations, (a) deep in the reservoir and (b) at the production

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well (63oC, 275bar). In addition, the impact of in-situ deposition/stripping on the degree of desulphation is investigated. Further modelling is then carried out to recalculate supersaturation and precipitation levels assuming in situ stripping has reduced [Ba2+] and/or [SO42-] at the wellbore. Alongside this study, a series of static experiments are conducted to determine the optimum sulphate level that negates the use of squeeze treatments. The experimental data is then analysed using a simple novel analytical approach to develop “safe envelopes” of [Ba2+] and [SO42-] within which we should work. Furthermore, some estimate of likely barite deposition rates are calculated along these envelopes. Experimental Procedures Brine compositions: The brine compositions in Table 1 were used in the (uninhibited) barite precipitation experiments. Various [SO42-] levels were studied for the SW and various [Ba2+] levels were investigated for the FW. A matrix of tests was performed for several combinations of [SO42-] and [Ba2+]. The base case SW and FW prepared for use in the laboratory static tests were the reservoir brine compositions but with the respective [SO42-]= 2780ppm and [Ba2+]= 80ppm missing to allow different concentrations to be examined. Static barite precipitation kinetic tests for (uninhibited) solutions: The experiments performed were uninhibited (no SI) static barite precipitation kinetic experiments. These tests were performed at 63oC, at pH 6.6 for a 50:50 mix of SW:FW. The detailed experimental procedure is given in Appendix 1. Scale Prediction and Reservoir Calculations Modelling approaches using MultiScale and STARS: In the present work, the impact of sulphate removal was modelled using the thermodynamic scale prediction code MultiScale7 and the reservoir simulation code STARS14. A two-stage methodology was followed, (a) using MultiScale to model scaling reactions in the wellbore, and (b) setting up a vertical injector-horizontal producer model in STARS to predict barite precipitation in the formation. The base case model had a low [Ba2+] = 80ppm, and [SO42-] = 2780ppm (normal SW). We note that the sulphate removal process not only decreases the concentration of SO42- in the seawater but can alter the concentration of other ions such as calcium and magnesium.11 Neither the modelling nor laboratory tests in these studies takes into account the potential reduction in these ions, as MultiScale predictions indicate that the loss of these additional ions does not significantly influence the scaling tendency of the resultant brine. However, if scale inhibitor was present, then the reduction in the cations, Ca2+ and/or Mg2+, could detrimentally alter the efficiency performance of the scale inhibitor if it was a phosphonate species.15 STARS model setup: A three-dimensional (3D) reservoir model of 30 x 21 x 10 cells, each 100ft x100ft x 50ft, was set up for STARS calculations to predict barite precipitation within the reservoir under different sensitivity scenarios. The model is homogeneous and of high porosity. It is expected that using a grid block size of 100ft will introduce numerical dispersion problems, resulting in an overestimation of the degree of mixing and errors in other possible outputs (e.g.

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What Level of Sulphate Reduction is Required to Eliminate the Need for Scale Inhibitor Squeezing?

SO42- breakthrough time). Numerical dispersion may be overcome by using streamline calculations or by increasing the grid resolution (finer)1,8,16 but these entail longer run times. MultiScale and STARS Modelling Results and Discussion MultiScale sensitivity results – scale prediction modelling: Base Case. Scaling tendency is measured by the Saturation Ratio (S.R):15 (1) S .R. = ( a A .aC ) / K spCA where a A and aC are the activities of ions A and C and K spCA is the solubility product of CA (= Cation-Anion) at the solution temperature (and pressure). Scale may precipitate if SR > 1 2,8. The base case for this study assumes that in the FW [Ba2+] = 80ppm and in the SW [SO42-] = 2780ppm. The MultiScale SR values for BaSO4 and SrSO4 in Fig. 2 show that the greatest driving force for the reaction (SR maximum) is at 55% SW for both BaSO4 and SrSO4. However, the precipitation levels (mg/L) in Fig. 3 show that the greatest masses of BaSO4 and SrSO4 precipitation occur at 10% and 45% SW fractions, respectively. Assuming no reaction in the reservoir, the following observations were made from the MultiScale results for various desulphation sensitivities (63oC, 275bar); - SR of both SrSO4 and BaSO4 decreases as [SO4] decreases; - No SrSO4 precipitation occurs at [SO4] ≤ 1000ppm; - BaSO4 precipitation was predicted for the full range of [SO42-] (2780 Æ 5ppm) for an initial [Ba2+] of 800ppm; - No BaSO4 precipitation was predicted for the following combinations of Ba2+ and SO42-: 229ppm Ba2+ ≤ 5ppm SO4280ppm Ba2+ ≤ 20ppm SO4245ppm Ba2+ ≤ 20ppm SO42- With more desulphation, maximum precipitation occurs at higher SW fractions; - The mass of BaSO4 decreases with increased desulphation. STARS– Reservoir Mixing Simulations STARS is a reservoir simulation model in which chemical reactions between components may be included, e.g. to predict barite scale deposition within the formation due to in situ mixing of Ba2+ and SO42- ions. Flow assurance risks are more severe when these ions are co-produced causing barite deposition in the production tubing. The problem severity may be reduced or eliminated if sufficient scale deposition occurs within the reservoir. STARS simulations are used to study the effect of sulphate reduction on in-situ scale deposition. For un-treated injection water and a high [Ba2+] = 800ppm, the results in Figure 4 show that some deposition occurs in the formation and proportionately a larger fraction of the total barium than sulphate is consumed. There is also a delay of ~300 days in injection water (sulphate) breakthrough if precipitation is allowed in the reservoir and the overall scaling potential is reduced. Fig. 5 shows that this delay in sulphate breakthrough is increased to ~2000 days when the injected SW has [SO42-] levels reduced to 50ppm. This desulphation significantly reduces the barium deposition (Fig. 5) and it is the produced sulphate concentration that decreases dramatically. Figs. 6 and 7 show sensitivities for a low [Ba2+]

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case and a case where both ion concentrations are low. For similar levels of barium and sulphate, equal amounts of deposition occur and desulphation further delays SW breakthrough. In conclusion, sulphate is delayed to higher SW fractions due to the effect of in-situ deposition. The scaling problem which then occurs is less problematic since the scaling potential of the remaining brines is substantially reduced. Thus, desulphation offers the potential for greatly reducing the number of squeeze treatments. Further investigation on the effect of in-situ deposition. The STARS results give barium and sulphate concentrations and seawater fraction over 7000 days of production. The produced brine compositions were then used in MultiScale to calculate profiles of SR and mass of BaSO4 (mg/L of produced brine). This allows us to assess the effect of in-situ deposition when desulphation is used. Figs. 8 and 9 show comparisons between the original MultiScale results (no precipitation) and the new results with precipitation. Results shown are for high and low barium reservoirs considering desulphation from high sulphate seawater (HSSW = 2780ppm) to low sulphate seawater (LSSW = 50ppm). Clearly, the in-situ deposition has a significant impact in stripping out the barium and sulphate ions and has delayed the scale deposition in the well to high seawater fractions; e.g. when desulphation to 50ppm is applied in the high-barium reservoir. It has eliminated scaling when low-barium is considered (Fig. 9). Normally, the scaling risk is present over the entire lifetime of a well with SW in the range 0 > SW > 100%. However, the occurrence of in-situ deposition not only reduces the scaling potential but also narrows the window of SW fraction where a potential scaling problem exists. Experimental Results – Static Barite Precipitation Tests Experimental Measurements: In a typical experiment, the barium and sulphate (and sometimes strontium) concentrations vs. time were monitored. The sulphate is obtained from the ICP measurement of S- and Ba2+ is measured directly by ICP. The tests with lower [Ba2+] or [SO42-] levels were not thought to have achieved equilibrium at 22 hours due to slow kinetics and were extended to 48 and 72 hours. From the ICP analysis, no loss of Sr2+ was found at initial [SO42-] ≤ 1000ppm as predicted by the MultiScale software; i.e. no precipitation of SrSO4 occurred. Plots of % [Ba2+] and % [SO42-] remaining vs. time were established. The results indicated the combinations of initial [Ba2+]0 and [SO42-]0 where no barite precipitation was seen over 22 or 72 hours, and also where the “onset” of precipitation is located (Fig. 10, Table 2). A trend observed was that the deposition rate for SW of high [SO42-] mixed with a FW of medium [Ba2+] is similar to that for SW of medium [SO42-] mixed with a FW of high [Ba2+]. More precisely, the order of rates of deposition of barium follows the order of the initial product [Ba2+]0 x [SO42-]0 as shown in Table 3. Kinetic Analysis of Barite Deposition The basic rate law: At lower levels of [Ba2+] and [SO42-], we expect the precipitation rate to be quite low and hence very little scale formation is expected. In order to investigate this effect, we assume a simple rate law for the removal of barium

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(and sulphate) from solution as follows:  d [ Ba 2+ ]  2+ 2− (2)   = k .[ Ba ].[ SO4 ]  dt  where k is the precipitation rate constant. [Ba2+] and [SO42-] are molar (M), time is in hours and k is in units of M-1.hour-1. This rate law is the simplest one possible that is consistent with the observed deposition rate results in Table 3. The initial barium and sulphate concentrations, [Ba2+]0 and [SO42-]0, and defining, α = [SO42-]0 - [Ba2+]0 and X(t) = [Ba2+](t), we can show by integrating the rate equation (Eq. 2) that, X(t), is given by the expression: (3) α [ Ba 2+ ](t ) = X (t ) =   α + 1 − 1 eα .k .t   X0  The corresponding value of the, [SO42-](t), is then given by: [ SO42− ](t ) = [ SO42− ]0 − [ Ba 2 + ]0 + [ Ba 2+ ](t ) = α + X (t ) (4) For example, using the following model parameters; rate constant k = 10 M-1.hour-1 and k = 75 M-1.hour-1 and initial conditions, [Ba2+]0 = 400ppm (0.00291M) and [SO42-]0 = 1000ppm (0.0104M) (since M Ba =137.34 and M = 96.06), SO 2− 4

then the decline in the barium and sulphate concentrations are predicted by Eqs. 3 and 4 to be as shown in Fig. 11. In this numerical example, the rate constant k =10 appears to be too low since a 400ppm [Ba2+] in a 1000ppm [SO42-] brine would precipitate much faster than appears in Fig. 11(a) and the higher figure of k = 75 (Fig. 11(b)) seems more likely. Estimating k from several experiments: We may use Eq. 2 to make an estimate of the rate constant, k, by plotting the initial rate (at t = 0) of barium removal from solution, (d[Ba2+]/dt)0, vs. the product of initial (M) concentrations [Ba2+]0.[SO42-]0. Results from a number of experiments must be used to construct such a graph as shown in Fig. 12, and k is the slope of the resulting line. The initial rate is measured over the first hour in the higher concentration deposition experiments and the latter initial quantities have been. Note that there is a reasonable correlation between these quantities, but there is considerable scatter in the results since we are measuring the initial derivative which is an inherently inaccurate process. However, the slope obtained from this scattered data gives a value of, k = 74.3 M-1.hour-1, but this is subject to some error. Direct matches to individual experiments. Another way of using Eqs. 2 and 3 is to match the [Ba2+](t) and [SO42-](t) results directly by adjusting the rate constant k, for various initial values of [Ba2+]0 and [SO42-]0. This is done for two lower concentration barite precipitation experiments where we have data over 72 hours. A comparison is shown between analytical and experimental results for the conditions: - Fig. 13: [Ba2+]0 = 40ppm and [SO42-]0 = 250ppm, (a) linear and (b) log scales; best fit is k = 25 M-1.hour-1. - Fig. 14: [Ba2+]0 = 22.5 ppm and [SO42-]0 = 500ppm (a) linear and (b) log scales; best fit IS, k = 12 M-1.hour-1.

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Results in Figs. 13 and 14 are presented using both a linear and a log scale for [Ba2+] and [SO42-] values since this shows the fit/misfit in various concentration regions. Note that the linear plots look very good but the logarithmic scales emphasise the fact that the lower concentration tail is not accurately predicted. Also, the range of value, k = 25 and 12 M-1.hour-1 as best fits to this data are rather less than the value of k = 75 M-1.hour-1 found in the aggregated data for many experiments (Fig. 12). However, it was noted above that there was a large scatter on this value and these lower values support this contention and tend to indicate that the rate constant is in the rather broad range, k = 15 – 75 M-1.hour-1. Defining Safe Working “Envelopes” for Desulphation We now use our simple rate law for the barium precipitation rate to define some “safe envelopes” where the barite precipitation is sufficiently slow that we expect no squeeze treatment is required. Such envelopes essentially define the level of desulphation for a given [Ba2+] that is required in order to avoid the need for squeeze treatments. However, some risk is associated with these envelopes due to the significant uncertainties in the determination of the rate constant, k (k ≈ 15 to 75 M-1.hour-1). Here, we make some pragmatic choices based on our observations and then take “relaxed”, “moderate” and “conservative” limits as explained below. We proceed as follows: Suppose we take some limiting “acceptable” barium deposition rate, R, based on (a) our observations of [Ba2+] decline rate in our experiments or (b) on the rate law of Eq. 2 with a best guess value of k (to be conservative k = 75 M-1.hour-1). That is, we decide on an acceptable rate of barium (or barite) deposition R, where: (5) R = k .[ Ba 2+ ]0 .[ SO 2− ]0 4

Hence, for the acceptable rate, R, for a given level of barium concentration in a produced brine mixture, [Ba2+], the allowable level of sulphate would be given by: R (6) [ SO42− ] = k .[ Ba 2+ ] For example, combining both approaches (a) and (b) above, then we might observe experimentally that a limiting case of [Ba2+] = 80ppm (0.00058 M) and [SO42-] = 80ppm (0.0008 M), showed an appropriately low acceptable deposition rate, R. Furthermore, we can calculate this rate for [Ba2+] = 0.00058 M and [SO42-] = 0.0008 M assuming k = 75 M-1.hour1 and we would obtain, R = (dBa/dt)0 ≈ 5 g of barium deposition (~8.5 g of BaSO4 deposition) per 100L of produced brine per hour. Likewise, we perform the same calculation for other cases where k is taken as 35.07 M-1.hour-1 and 20.31 M1 .hour-1. Note that for these latter 2 cases, for the deposition rate to be the same as R, then the acceptable product [Ba2+].[SO42-]would be higher and they would represent more lax conditions. Each of these cases is shown in Fig. 15 where the various acceptable [SO42-] levels for a given level of [Ba2+] are plotted for the 3 cases described above, that is: ♦ “conservative” assuming the fastest rate, k = 75 M-1.hour-1 which will give the most strict requirements for desulphation; ♦ “moderate” assuming the slower rate, k = 35.07 M-1.hour-1; ♦ “relaxed” assuming the slower rate, k = 20.31 M-1.hour-1.

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Three points are also shown on Fig. 15 which correspond to 3 of our experiments, (i.e. 3 combinations of initial [Ba2+]0 and [SO42-]0) where we did not observe a very significant drop in [Ba] over 22hours for 229ppm Ba2+ and over 72hours for 45 and 80ppm Ba2+. These experimental results are very consistent in form to the predicted safe [SO42-] envelopes and they indicate that k = 35 M-1.hour-1 gives a reasonable value for practical calculations for this system. In fact, the curves in Fig. 15 will not be universal and will need to be established for specific brine mixing systems and conditions (temperature in particular). However, it is the procedure presented here which can be applied in a general case where – if the rate expression is known – then a simple model can be developed which allows us to construct envelopes of the type shown here. The actual acceptable deposition rate in terms of kg of barite per produced volume (e.g. per 100L or 1000 bbl) per day can be decided upon. The acceptable deposition rate will depend upon whether the concern is loss of flow or access to the well (scale deposition on pipe walls) or damage to sensitive equipment (eg valves, gauges, pumps, etc.) This procedure allows us to assign risk levels to this deposition rate as shown in Fig. 15 which we can then use in the field. However, there is an even more novel use for the kinetic analysis presented in this study and this is explained in the following sub-section. Kinetic Considerations in Desulphated Systems Kinetic procedure on “safe envelopes”: The “safe envelope” idea presented above and illustrated by Fig. 15 can be extended to incorporate the kinetics of barite deposition in a simple but useful manner. This is explained by considering just one of the safe envelopes in Fig. 15 and we will take as our base case the “moderate” case where k = 35 M-1.hour-1. The new procedure we suggest is shown schematically in Fig. 16 and is explained as follows: Step 1: At each point X on the chosen safe envelope (Fig. 16), calculate the maximum amount of barite scale dropout that is possible as a function of [Ba2+], supposing that the limiting ion is completely precipitated. This quantity, (in g/L) is given by the following expression: Mass BaSO4

 [ Ba 2+ ] [ SO42− ]  MassBaSO = Min.  : . 137.34 + 96.06 ) (7)  137.34 96.06  ( 4   This is a good approximation for barite and the resulting curve of maximum mass of barite (in g/100L) vs. [Ba2+] for the base case is shown in Fig. 17. Note that, at lower [Ba2+] (by definition high sulphate, since [Ba2+].[SO42-] = constant), then the barium is the limiting ion and at [Ba2+] >140ppm, sulphate is limiting and there is a clear maximum mass of barite dropout at this crossover point. Step 2: At Point X (Fig. 16), we may then invoke the simple kinetic model (Eq. 2) proposed above which gives an analytical solution (Eq. 3) for X(t) = [Ba2+](t). This is then used at each point along the envelope – for that value of initial [Ba2+]0 and [SO42-]0 on the actual envelope - to calculate the drop in [Ba2+] (and hence [SO42-] ) over given times. Step 3: The mass of barite dropout for each point on the envelope can then be calculated from the analytical model at each time (3, 6, 12 and 24 hours in this case) and can be

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plotted against the initial value of [Ba2+]0 as shown in Fig. 18. We note that for the rate assumption of this case (k = 35.07 M-1.hour-1), the system does not reach equilibrium even after 24 hours for [Ba2+] > 75ppm. At the point where potentially most scale could form, only 50% of it has been deposited after 24 hours. The mass of barite data on Fig. 18 is shown as % values of the maximum possible amount of scale in Fig. 19. Plots such as those in Figs. 18 and 19, can then be made up for all of the safety envelope scenarios that are envisaged by the operator. This is shown for the 3 envelopes in Fig. 15 in Figs. 20(a) to 20(f). The combined approach of (a) defining the safety envelopes for desulphation conditions (Fig. 15), and (b) then applying the kinetic models for each envelope (Fig. 16) to produce plots such as those in Fig. 20, gives us a simple, novel and useful way of analysing such systems. This can be applied in the field using simple spreadsheet calculations using the model proposed in this chapter. Applying this procedure with both reservoir simulation studies and complementary laboratory work of the type described here gives a very powerful and novel way of assessing the target levels for any desulphation process. Also, it is easy to see how risks may be evaluated by taking multiple safety envelopes and assigning probabilities to these based on laboratory kinetic data, field brine compositions and likely performance levels of the desulphation plant itself. Summary and Conclusions Modelling: A two-well simulation model including barium sulphate precipitation on mixing of injected and formation brines, has been used to investigate the impact of seawater desulphation on the scaling problem at the producing well. Various concentration sensitivities were selected and used to determine the optimal sulphate level that could eliminate the need for any squeeze treatments. The conclusions of this study are qualitative since a relatively simple reservoir model was used although we expect some of them to carry over into field scale practice and these are summarised as follows: - Numerical effects due to the grid size tend to overestimate the degree of brine mixing and time of SW breakthrough. - For all sulphate concentrations, the period during which both barium and sulphate continue to cause scale problems is predicted to increase as the barium concentration increases. - In all cases, desulphation of the injected seawater results in delaying the time of sulphate breakthrough to higher seawater fractions due to scale deposition within the reservoir. Clearly, the amount of barium precipitated within the reservoir is also reduced as the level of desulphation increases. We note that neither the STARS simulation calculations nor the MultiScale calculations takes into account the kinetic considerations developed from our experimental results above. This work should be incorporated along with simulation and scaling calculations in future analysis in order to give a fuller assessment of the desulphation problem. Experimental barite precipitation tests: A large number of barite precipitation experiments were performed starting from different combinations of initial scaling ions, [Ba2+]0 and At higher concentrations of both ions, the [SO42-]0. precipitation rate is very rapid but this slows considerably as

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we reduce the concentration of one of the ions to a sufficiently low level. The experimental ordering of the barite deposition rates (Table 3) was observed to be broadly consistent with a simple rate law given in Eq. 2, which has a simple analytical solution (Eq. 3). This rate law led to some useful ways in which we could analyse the experimental data, as follows: (i) k from data from several experiments. Data from a number of deposition rate experiments was used to make an estimate of the rate constant, k, in this expression leading to a value of k = 74.3 M-1.hour-1 but a wide scatter was seen in the results and it is likely that k is subject to considerable uncertainty. (ii) Direct matching of individual barite deposition experiments. An alternative approach was to take the profiles of [Ba2+](t) and [SO42-](t) vs. t directly for some of the experiments and fit these directly using the analytical expression in Eq. (3). This gave a satisfactory match to the experimentally observed profiles but (a) led to a range of values of k from k = 12 to 25 M-1.hour-1, and (b) gave a good fit in the early time decay but in the longer time/lower concentration region it over predicted barite drop out (it would therefore be conservative if used to predict scale kinetically). (iii) Safe operational envelopes in desulphation. Approaches (i) and (ii) above indicate a wide range of values of k. This suggests that k should be used as an uncertain parameter in various scaling rate scenarios. We use the rate law to define safe working envelopes for operating in terms of the combined allowed concentrations of barium and sulphate that lead to lower levels of actual mass deposited and in a regime where this occurs kinetically slowly. Such envelopes are defined for 3 rate scenarios which we identify as “conservative” (deposition is fast), “moderate” and “relaxed”. Deposition in the latter case is slow and hence it would be safer in this case to work with barium and sulphate concentration whose product ([Ba2+].[SO42-]) is higher. (iv) Kinetic considerations in the safe envelopes. Finally, based on further kinetic analysis using data on the safe envelopes, we can establish the rate at which barite deposition occurs. This goes further than the simple equilibrium thermodynamic scale prediction models and helps to identify where the system would deposit barite slowly and where it would reach its full equilibrium deposition limit quickly. This work uses the type of analysis described in (i) to (iv) above to present a novel approach for managing desulphation applications in the field. This gives us some analytical tools for establishing, on a more sound technical basis, the answer to the question: What level of sulphate reduction is required to eliminate the need for scale inhibitor squeezing? Acknowledgements The Flow Assurance and Scale Team thank the following sponsors for their support of this research work: Baker Petrolite, BioLab, BP, Champion Technologies, ChevronTexaco, Clariant, ConocoPhillips, Halliburton, M I Swaco, Nalco, Norsk Hydro, Petrobras, REP, Rhodia, Shell, Solutia, Statoil and Total.

SPE 95089

Appendix 1: Experimental Details of Static Barite Precipitation Tests The procedure for these (uninhibited) tests is detailed below: 1. Prepare 20 litres of Base Case SW and FW (no SO42- or Ba2+ included). 2. Using Base Case SW, dilute the amount of sodium sulphate to be dissolved to make a SW solution of 2780 (8.22g/2l). Repeat for all other Sodium Sulphate concentrations. 3. Repeat the process for Barium Chloride to make various FW’s. 4. Prepare the buffer, ensuring that the 50:50 mixed solution of Base Case SW: Base Case FW gives a mixed pH of 6.6. Buffer; 34g Sodium acetate 3-hydrate and 0.05g of acetic acid in 250ml DW. 5. Ensure sufficient KCl/S40 for quenching the tests (Preparation; 5g S40 and 28.55g KCl in 4.8 litres and then adjust to a pH value between 8 - 8.5 before making up to 5 litres). The normal KCl/PVS quenching cannot be used as it contains sulphur ions. This will interfere with the ICP analysis that will be quantifying the concentration of Ba2+ and S- left in the test solutions after each of the residence times. 6. Depending on Ba2+ and SO42- concentrations, the quenching dilutions for the tests were either x2 or x10 to allow accurate analysis for both Ba2+ and S-. Some tests required both x2 and x10 dilutions. 7. Filter each of the FW and SW. 8. Measure out 2x100ml of each FW and SW. Put the FW into 150ml bottles and then the SW is put into the 250ml test bottle. Add 2ml of buffer to each of the SW solutions and put these bottles into the waterbath at 63oC to warm up to temperature for an hour. Place the FW bottles in the oven at 63oC for an hour. 9. After an hour, add the FW to the SW, cap and shake the bottle and then return it to the water bath. 10. One hour after mixing, for a x2 dilution - take a 5ml sample from the test bottle and pipette it into a test-tube containing 5 ml of the quenching solution. For a x10 dilution – take a 1ml sample from the test bottle and pipette it into a test-tube containing 9ml of the quenching solution. 11. Repeat the quenching process outlined in step 10 after residence times of 2 and 22 hours after mixing. For the lowest [Ba2+]=45 and 80ppm, some tests were also sampled after residence times of 48 and 72 hours. 12. All the quenched test solutions are now ready for ICP analysis of the remaining barium and sulphur concentrations (and strontium). 13. A control sample is run alongside the test samples of the ICP. The control sample simulates the mixing (50:50%) and quenching regime that the test samples have under gone. A control will have to be made for each of the different SW and FW mixes. For a x2 dilution control; 5ml of SW X, 5ml of FW X are quenched into 10ml KCl/S40.For a x10 dilution control; 1ml of SW X, 1ml of FW X are quenched into 18ml KCl/S40.

References 1. 2.

3.

Mackay, E.J.: “Predicting In-Situ Sulphate Scale Deposition and the Impact on Produced Ion Concentrations”, Trans IChemE (March 2003) 81 (A) 326-332. Collins, I.R., Stalker, R. and Graham, G.M.: "Sulphate Removal for Barium Sulphate Scale Mitigation a Deepwater Subsea Production System", SPE 87465, 6th International Symposium on Oilfield Scale, Aberdeen, UK, 26-27 May 2004. Vu, V.K., Hurtevent, C. and Davis, R.A.: “Eliminating the Need for Scale Inhibition Treatments for Elf Exploration Angola’s Girassol Field”, SPE 60220, 2nd SPE International Symposium on Oilfield Scale, Aberdeen, U.K., 26-27 January, 2000.

SPE 95089


4.

Mota, R.O., Bezerra, M.C.M, Rosario, F.F. and Paris, F.: “Forecast and Alternative Analysis for Sulphate Removal or Chemical Treatments for Barium and Strontium Scale Deposition – Offshore Brazil”, SPE 87464, 6th International Symposium on Oilfield Scale, Aberdeen, UK, 26-27 May 2004. 5. Graham, G.M. and Collin, I.R.: "Assessing Scale Risks and Uncertainties for Subsea Marginal Field Developments", SPE 87460, 6th International Symposium on Oilfield Scale, Aberdeen, UK, 26-27 May 2004. 6. http://www.chemguide.co.uk/inorganic/group2/solubility.html 7. Petrotech a.s Knowledge: "Multiscale", P. O. Box 575, N-5501 Haugesund, Norway. 8. Mackay, E.J., Jordan, M.M. and Torabi, F.: “Predicting Brine Mixing Deep Within the Reservoir, and the Impact on Scale Control in Marginal and Deepwater Developments”, SPE 73779 & SPE 85104, SPE Prod. & Facilities 18 (3) 210-220, August 2003. 9. Mackay, E.J. and Graham, G.M.: “The Use of Flow Models in Assessing the Risk of Scale Damage”, SPE 80252, SPE International Symposium on Oilfield Chemistry, Houston, Texas, 5-8 February 2003. 10. Mackay, E.J. and Sorbie, K.S.: “Brine Mixing in Waterflooded Reservoirs and the Implications for Scale Prevention”, SPE 60193, 2nd SPE International Symposium on Oilfield Scale, Aberdeen, U.K., 26-27 January, 2000. 11. Chekani, M. and Mackay, E.J.: “Impact on Scale Management of the Engineered Depressurisation of Waterflooded Reservoirs: Risk Assessment Principles and Case Study”, SPE 86472, SPE

12.

13.

14. 15.

16.

17.

International Symposium and Exhibition on Formation Damage, Lafayette, Louisiana, 18-20 February 2004. Maria C. M. Bezzera, Rosario, F.F., Rocha, A.A., Sombra, C.L.: “Assessment of Scaling Tendency of Campos Basin Fields Based on the Characterization of Formation Waters”,SPE 87452, 6th International Symposium on Oilfield Scale, Aberdeen, UK, 26-27 May 2004. Mackay, E.J., Matharu, A.P., Sorbie, K.S., Jordan, M.M. and Tomlins, R.: “ Modelling of Scale Inhibitors Treatments in Horizontal Wells: Application to the Alba Field”, SPE 39452 SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 18-19 February 1998. Steam, Thermal, and Advanced Processes Reservoir Simulator (STARS), a product of the Computer Modeling Group (CMG), Calgary, User manual, 2003 Graham, G.M., Boak, L.S., Sorbie, K.S.: “The Influence of Formation Calcium and Magnesium on the Effectiveness of Generically Different Barium Sulphate Oilfield Scale Inhibitors, SPE 81825, SPE Production & Facilities, 28-43, (2003). Mackay, E.J.: “Modelling of In-Situ Scale Deposition: The Impact of Reservoir and Well Geometries and Kinetic Reaction Rates” paper SPE 74683 & SPE 81830, SPE Prod. & Facilities (Feb 2003) 18 (1) 45-56. McElhiney, J.E.: “Deepwater Project Economics Demand Sulphate Removal to Ensure Scale-free Operation”, Offshore (May 2003).

Table 1: Composition of Seawater and Formation Water Constituents

Formation water (mg/l)

Seawater, (mg/l)

Sodium

26,535

10,900

Potassium

1,906

380

Calcium

2,033

405

Magnesium

547

1,300

Barium

80

0

Strontium

417

0

Chloride

48,700

19,800

Sulphate

0

2,780

Table 2: Limits for the onset of barium sulphate scaling in the static barite precipitation tests 2+

7

2-

[Ba ], ppm

Onset [SO4 ] concentration, ppm

45 80 229 800

300 < [SO4 ] < 500 2175 < [SO4 ] < 250 250 < [SO4 ] < 150 220 < [SO4 ] < 50

2-

8


Table 3: Observed order of barium deposition rate that is shown to correlate with the initial product [SO42-]0 x [Ba2+]0 Fastest Kinetics ψ

Slowest Kinetics

Æ ÆÆ

SWA:FWD > SWB:FWD ~ SWA:FWC > SWB:FWC ~ SWA:FWA > SWB:FWA ~ SWA:FWB > SWB:FWB

2780:800* > 1000:800* ~ 2780:229* > 1000:229* ~ 2780:80* > 1000:80* ~ 2780:45* 2224000+ ψ

800000+

636620+

229000+

222400+

80000+

> 1000:45*

125100+

45000+

This row of the table shows the observed order of rates of precipitation from fastest to slowest

* Shows the initial (t = 0) composition of the two brines as [SO4] 0 and [Ba]0, respectively e.g. 2780:800 is [SO4] 0 = 2780 ppm and [Ba] 0=800 ppm 2

+ This is the product [SO4] 0 x [Ba]0 in ppm e.g. 2780 x 800 = 2224000 ppm

2

+

NOTE: the rate orderψ is the same as the order of the product [SO4]0 x [Ba]0 which is consistent with the proposed simple rate law in Eq. (2)

17

Fig. 1: Location map of the Campos Basin Oilfields

SPE 95089

SPE 95089


9

Fig. 2: The saturation index of BaSO4 (a) and SrSO4 (b) for the base case 120

(b)

1.8

SrSO4 Saturation Ratio

100

BaSO4 Saturation Ratio

2.0

(a)

80 60 40 20

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2

0 0

20

40

60

80

0.0

100

0

20

40

% seawater fraction

60

80

100

% seawater fraction

Fig. 3: The amount of precipitation (mg/l) of BaSO4 (a) and SrSO4 (b) for the base case 250

(a)

120

(b) SrSO4 Precipitation (mg/l)

BaSO4 Precipitation (mg/l)

140

100 80 60 40 20 0 0

20

40

60

80

200

150

100

50

0

100

0

20

40

% seawater fraction

60

80

100

% seawater fraction

2-

2+

Fig. 4: Barium and sulphate concentrations as a function of time (a) and % Seawater cut (b) for initial 800ppm Ba and 2780ppm SO4

3000

Ba Ba (no precip) SO4 SO4 (no precip)

600

2000

500 1500

400

(a)

300

1000

Ba (no precipitation) Ba S04 (no precipitation) SO4

700

2500 sulphate concentration (ppm) barium concentration (ppm)

barium concentration (ppm)

700

800

600

2500 sulphate concentration (ppm)

3000 800

2000

500

1500

400

(b)

300

1000

200

200 500 100 0

0 0

1000

2000

3000

4000

time (days)

5000

6000

7000

500 100 0 0

10

20

30

40

50

60

sea water fraction (%)

70

80

90

0 100

10


SPE 95089 2-

2+


50

500 40 400

(a)

300

30 20

200

0

600 500

30 400

1000

2000

3000

4000

5000

6000

(b)

300

20

200 100

0

0

40

10

10

100

50

0

7000

0

20

time (days)

40

60

sulphate concentration (ppm)

600

700

60


700

60


800

70



800 barium concentration (ppm)

900

80

900

0 100

80

sea water fraction (%) 2+

2-


40

50

3000


45

2500 sulphate concentration (ppm)

35

2000

30 25

1500

20

(a)

15

1000

10

500

40

2500

35


45 barium concentration (ppm)

3000



50

2000

30 25

1500

20

(b)

15

1000

10

5

500

5

0

0

0

1000

2000

3000

4000

5000

6000

0

7000

0

20

time (days)

40

60

0 100

80


2+

2-

Fig. 7: Barium and sulphate concentrations as a function of time (a) and % Seawater cut (b) for initial 45ppm Ba and 50ppm SO4 80

70

60 50 40

20

30

(a)

20

10

0 0

1000

2000

3000

4000

time (days)

5000

6000

7000


30

60



70 Ba Ba (no precip) SO4 SO4 (no precip)

40

60 Ba (no precipitation) Ba S04 (no precipitation) SO4

50

50

40

40 30 30

(b)

20

10

10

0

0

20

10

0

20

40

60


80

0 100


50

SPE 95089


11

Fig. 8: Impact of in-situ deposition on saturation index in high and low barium reservoirs: (a) no desulphation and (b) desulphation to 50ppm Ba800_SO4_2780

Ba45_SO4_2780

mixed profile single stream

mixed profile

single stream 70

1200 1100

60

900

Saturation Ratio of BaSO4 (SR)


1000

800 700 600 500 400

50

40

30

20

300 200

10

100 0

0 0

10

20

30

40

50

60

70

80

90

0

100

10

20

30

40

50

(a)

60

70

80

90

100

% SW

% SW

Ba45_SO4_50

Ba800_SO4_50



1.4

25

1.3 1.2 1.1



20

15

10

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3

5

0.2 0.1 0 0

10

20

30

40

50

60

70

80

90

0

100

0

% SW

10

20

30

40

50

60

70

80

90

100

% SW

(b)

Fig. 9: Impact of in-situ deposition on pptn. of BaSO4 in high and low barium reservoirs: (a) no desulphation; (b) desulphation to 5 ppm Ba800_SO4_2780

Ba45_SO4_2780



80

1200 1100

70

1000 60 Precipitation of BaSO4 (SR)

Precipitation of BaSO4 (SR)

900 800 700 600 500 400

50

40

30

20

300 200

10

100 0 0

10

20

30

40

50

60

70

80

90

0

100

0

10

20

30

40

% SW

50

60

70

80

90

100

% SW

Ba800_SO4_50

(a) Ba45_SO4_50

mixed profile


single stream 100

6

90

5

70



80

60 50 40 30

4

3

2

20

1 10 0 0

10

20

30

40

50 % SW

60

70

80

90

100

0 0

10

20

30

40

50 % SW

60

70

80

90

100

(b)

12


2+

2-

Fig. 10: A typical plot of % [Ba ] and [SO4 ] remaining versus time for initial 80ppm Ba precipitation of BaSO4 has ceased

120 100 80 60 40 20 0

% [Ba] left

% [SO4] left

0

20

40

60

where precipitation still occurs (a) and where

Initial 80ppm Ba2+ and 175ppm SO42monitored over 1-72hours

80

120 100 80 60 40 20 0

% [ion] left

% [ion] left

Initial 80ppm Ba2+ and 250ppm SO42monitored over 1-72hours

2+

% [Ba] left

% [SO4] left

0

(a)

20

40

60

Fig. 11: Calculated decline in barium and sulphate concentrations with time for initial conditions, -1

-1

and rate constants (a) k = 10 M .hour and (b) k = 75 M .hour

[ Ba 2+ ]0 = 800ppm and [ SO 2 − ]0

= 1000ppm

4

(b) k = 75 M^-1.hour^-1 [Ba]o = 400 ppm; [SO4]o= 1000

1200

1200

1000

1000

[Ba](t) and [SO4](t) (ppm)

[Ba](t) and [SO4](t) (ppm)

(b)

-1

(a) k = 10 M^-1.hour^-1 [Ba]o = 400 ppm; [SO4]o= 1000

800 600 400 200

800 600 400 200

0

0 0

5

10

15

20

25

0

5

Tim e (hours)

10

15

20

25

Tim e (hours)

(

Fig. 12: The initial rate of barium removal from bulk solution, d [ Ba 2 + ] dt 2

80

Residence time (hr)

Residence time (hr)

-1

SPE 95089

-1

-1

)

0

(in M/hour), as a function of the initial product [ Ba 2 + ]0 .[ SO 2 − ]0 4 -1

-1

(in M ). The slope of this line is k in units of M .hour and this is approximately, k = 74.3 M .hour but is subject to considerable error.

Initial (d[Ba]/dt) vs. Initial [Ba][SO4]

Initial (d[Ba]/dt) (M/hour)

0.0035 0.003 0.0025 0.002 y = 74.258x + 0.0002 R2 = 0.7197

0.0015 0.001 0.0005 0 0

0.00001

0.00002

0.00003

Initial [Ba][SO4]

0.00004

0.00005

SPE 95089


13 -2

-1

Fig. 13: Calculated decline in barium and sulphate concentrations with time for the parameters, rate constant k = 25M hour and initial 2+

2+

2-

2-

conditions (t=0) of [Ba ]0= 40ppm and [SO4 ]0 = 250ppm for (a) linear and (b) log scale. Experimental points are ∆ = [Ba ] and X = [SO4 ]. 1000

250

[Ba2+] or [SO4] (ppm)


300

200 150 100 50

100

10

1

0 0

20

(a)

40

0

60

20

(b)

Tim e (hour s )

40

60

Tim e (hour s )

-2

-1

Fig. 14: Calculated decline in barium and sulphate concentrations with time for the parameters, rate constant k = 12M hour and initial 2+

2+

2-

2-

conditions (t=0) of [Ba ]0 = 22.5ppm and [SO4 ]0 = 500ppm for (a) linear and (b) log scale. Experimental points are ∆ = [Ba ] and X = [SO4 ]. 1000

500 400



450 350 300 250 200 150 100

100

10

50 0

(a

1

0

20

40

60

(b

Tim e (hours)

Fig. 15: Envelope of tolerable levels of

[ SO42 − ]

0

20

40

60

Time (hours)

by desulphation for given levels of barium concentration based on an acceptable rate of

barium loss from solution, R (see text); Tolerable [ SO 2− ] = R 4

( k.[ Ba ]) . 2+

“Conservative” conditions are the most strict in desulphation

requirements. The experimental points (•) are for experimental cases where very little barium deposition was observed over 22 hours.

Envelopes of safe operation in terms of desulphation for a given level of barium

Tolerable [SO4] (ppm)

600

“Conservative” envelope of operation taking k = 75 (see units) => product [Ba]*[SO4] = 6500

500

“Moderate” envelope of operation taking k = 35.07 => product [Ba]*[SO4] = 13900

400 300

“Relaxed” envelope of operation k = 20.31 => [Ba]*[SO4] = 24000

200 100 0 0

100

200 [Ba] (ppm)

300

14


SPE 95089

Fig. 16: Outline of how to calculate the kinetics of barite precipitation along a given “safe envelope” using the method explained in the text. 2.aApply analytical model [Ba2+](t) Envelopes of safe operation in terms of desulphation for given level of barium ("moderate" case, k = 35) [SO42-](t) at t =3, 6, 12, 24 hours 600 [SO42-]0

and

t

Tolerable [SO4] (ppm)

500

X

400

[Ba2+]0

1. At point X: [Ba2+]0 = 35 ppm

Time (hours) Æ

[SO42-]0 = 400 ppm

300

3. Calculate Mass barite deposited at each time t (g/100L), M for X = 100*([Ba2+]0 - [Ba2+])*(137 +96)

200 100 0

[Ba2+]0

0

100

200

4. Do for new point X – go to 1 300

[Ba] (ppm)

Fig. 17: The maximum mass of barite per 100 L of produced brine mixture if all the possible barite is deposited vs. [Ba] showing the barium -1

-1

limited and sulphate limited regions; i.e. limiting ion conc. drops to zero. For the “moderate” case (k = 35.07 M .hour ) in Fig. 15.

Max. possible Mass barite per 100L

Ba limited

Max. Mass barite in 100 L (g)

25

SO4 limited

20

15

10

5

0 0

50

100

150

200

250

300

[Ba] (ppm )

Fig. 18: The mass of barite deposition at various times per 100 L of produced brine mixture vs. [Ba] for the envelope of the “moderate” case -1 -1 for k = 35.07 M .hour . The max. barite dropout (from Fig. 17) is also shown.

Mass of Barite precipitation (gram s/100L) for "m oderate" case (k = 35.07) at various tim es

Mass Barite (g/100L produced brine)

30 Max. barite

25

3 hours

20

6 hours

15

12 hours

10

24 hours

5 0 0

50

100 150 200 250 300 [Ba] ppm

SPE 95089


15

Fig. 19: The % barite deposition at various times per 100 L of produced brine mixture vs. [Ba] for the envelope of the “moderate” case for k = -1 -1 35.07 M .hour . This is taken as a % of the max. barite dropout (shown in Figs. 17 and 18).

% Barite dropout of m ax. possible for the "m oderate case (k= 35.07) at various tim es

% barite dropout

100 80 3 hours 60

6 hours 12 hours

40

24 hours 20 0 0

50

100 150 200 [Ba] ppm

250

300

Fig. 20: Figures showing (a, c and e) the mass of barite deposition at various times per 100 L of produced brine mixture vs. [Ba], along with (b, d and f) the % barite dropout as a % of the total possible barite that could be formed (which varies for each case) vs. [Ba]. Cases are (a, b) -1 -1 -1 -1 the “relaxed case”, k = 20.32 M .hour ; (c, d) the “moderate case”, k = 35.07 M .hour (max. barite dropout shown in Fig. 17) ; (e, f) the -1 -1 “conservative case”, k = 75 M .hour . (b) % Barite dropout of m ax. possible for "relaxed" case (k =20.31) at various tim es

(a) Mass of Barite precipitation (gram s/100L) for "relaxed" case (k =20.31) at various tim es 100

% barite dropout

25

3 hours

20

6 hours

15

12 hours

10

24 hours

5

% barium deposition

30

0

80 3 hours 60

6 hours 12 hours

40

24 hours

20 0

0

50

100 150 200 [Ba] ppm

250

300

0

(c) Mass of Barite precipitation (gram s/100L) for "m oderate" case (k = 35.07) at various tim es

250

300

100 Max. barite

25

3 hours

20

6 hours

15

12 hours

10

24 hours

5

% barite dropout


100 150 200 [Ba] ppm

(d) % Barite dropout of m ax. possible for the "m oderate case (k= 35.07) at various tim es

30

80 3 hours 60

6 hours 12 hours

40

24 hours 20 0

0 0

50

0

100 150 200 250 300 [Ba] ppm

(e) Mass of Barite precipitation (gram s/100L) for "conservative" case (k = 75) at various tim es

50

100 150 200 [Ba] ppm

250

300

(f) % Barite dropout of m ax. possible for the "conservative" case (k = 75) at various tim es

30

100

25

Max. dropout

20

3 hours 6 hours

15

12 hours

10

24 hours

5

Series6

% barite dropout


50

80

3 hours 6 hours

60

12 hours

40

24 hours

20 0

0 0

50

100 150 200 250 300 [Ba] ppm

0

50

100 150 200 [Ba] ppm

250

300