Innovative Leaching Tests of an Oilwell Cement Paste ...

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Energy Procedia 23 (2012) 472 – 479

TCCS-6

Innovative Leaching Tests of an Oilwell Cement Paste for CO2 Storage: Effect of the Pressure at 80°C Nadine NEUVILLEa,b,c*, Georges AOUADa,b, Eric LECOLIERc, Denis DAMIDOTa,b a

Univ. Lille Nord de France, 59000 Lille, France b EMDouai, MPE-GCE, 59500 Douai, France c IFP Energies nouvelles, 92852 Rueil-Malmaison, France * Corresponding author, presently at OXAND S.A., 77210 Avon, France

Abstract Old wells could be exposed to severe environment. This study reports the characterization of a class G oilwell cement paste exposed to a brine at high pressure and high temperature. The results showed that pressure had a strong impact on the phenomenology and the kinetics of oilwell cement paste degradation in selected downhole conditions. The alteration mechanism at high pressure appears to be more complex than the one observed at atmospheric pressure. At atmospheric pressure the diffusion rate governs the kinetics and thus is slower than the rate of the chemical reaction. On the other hand, it seems that an increase of pressure increases much more the diffusion rate than the rate of reaction. Future developments are proposed in order to improve the understanding of the long-term behavior of oilwell cement paste at downhole conditions.

©2012 2011The Published by Elsevier Ltd. Selection and/or peer-review under [name Energi organizer] © Authors. Published by Elsevier Ltd. Selection and/or peer-review underresponsibility responsibility ofofSINTEF AS Keywords : oilwell cement, brine, leaching, pressure, pre-alteration, durability, CO2 geological storage

1. Introduction Cement-based materials are used since a long time in the oil and gas industry for cementing oil wells [1]. The main role of the cement sheath is to permanently isolate all subsurface formations penetrated by the well [2]. These drilled geological formations contain fluids whose compositions are different from the cement pore fluids. This difference in composition induces chemical reactions which may lead to physico-chemical alteration of the cement paste. Consequently, some leakage of formation fluids from deep geological layers to shallow aquifers or to the surface can occur through the altered cementitious matrix or rock/cement/casing interfaces. Until very recently, the oil and gas industry was mainly interested in short- or medium-term integrity of the cement sheath. The investigations on the short-term

1876-6102 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SINTEF Energi AS doi:10.1016/j.egypro.2012.06.065

Nadine Neuville et al. / Energy Procedia 23 (2012) 472 – 479

cement sheath integrity were carried out to avoid costly problems during hydrocarbon production. With the growing interest for environmental concerns, the long-term durability of the cement-based materials used for well construction has become of paramount importance. Many studies have been devoted to cement paste, mortar or concrete durability under atmospheric temperature and pressure conditions (25°C, 105 Pa) [3,4]. The purpose of these studies was to understand the long-term behaviour of cement-based materials used for nuclear waste repository [5,6]. However, microstructures and mineral assemblages of cement pastes cured at elevated temperatures and pressures differ from those of pastes cured under atmospheric conditions [7,8]. Besides, the detailed mechanisms of cement degradation under downhole conditions are less documented [9,10]. During leaching, a concentration gradient is created between cement pore solution and the aggressive formation fluid. Thus ionic diffusion occurs and can modify the local equilibrium conditions that are likely to change the phase assemblages contained in the cement paste leading to its degradation. Among others, the degradation rate depends on temperature, fluid composition and renewal rate (periodical or continual renewal). However, the impact of pressure is not well known [11]. In the recent years, several studies dealing with the alteration of oilwell Portland cement by CO2-rich fluids have been carried out on sound cement-based materials [12-14]. However, in downhole conditions, the cement sheath might have been initially altered due to exposure to geological formation fluids for several decades when CO2 storage is foreseen in depleted oil and gas reservoirs. In this case, degraded cements are more representative of the cement annulus in the well at the beginning of CO2 injection. The one of the key issues to predict the long term behavior of cement paste with respect to CO2, is to improve the knowledge of its initial state, before its exposure to acid gas. Some studies showed that significant modifications of the well cement paste can be expected in old wells [14-17]. These modifications should be taken into account when modelling the chemical durability of cements exposed to geosequestered CO 2. Moreover, it is known that pressure has an impact on the kinetics of the chemical reactions involved during cement hydration and thus on the microstructure of the cement paste [8]. Therefore, geological fluid pressure is also expected to modify the kinetics of degradation and the associated changes of the microstructures leading to different degradation patterns. This study reports the characterization of an oilwell cement paste exposed to a formation fluid with low carbonates content (representative of some sedimentary basins), at high pressure and high temperature, in order to improve the knowledge of the long-term behavior of oilwell cement paste in downhole conditions (impact of pressure).

1.1. Materials and Methods 1.1.1. Sample preparation The Class G (Cemoil®) Portland cement used in this study was obtained from Italcementi company. The Bogue composition is given in Table 1 [18]. The cement powder was mixed with distilled water with a water/cement ratio (w/c) of 0.44. The samples were prepared according to ISO/API specifications. Table 1: Bogue composition for the Class G oilwell cement (wt.%) - in cement chemistry abbreviation, C = CaO, S = SiO2, A= Al2O3 and F= Fe2O3, from [18] C3S

C2S

C4AF

C3A

62.2

12

12.2

2.2

473

474


Cement paste samples were hardened for 1 month at 80°C either in pressurized small cells for curing at 70 bar or in curing chambers for curing at 200 bar. The pressurized cells and curing chambers were filled with water saturated with Ca(OH)2 (Ca2+ ≈ 15 mmol/L), NaOH (≈ 118 mmol/L) and KOH (≈ 245 mmol/L) to prevent alteration by leaching during the curing period (1 month).

1.1.2. Experimental setup and characterization performed on cement paste samples A specific innovative leaching experimental set-up was developed, making it possible to carry out degradation tests on cement pastes at high temperature and high pressure. The degradation tests were carried out at 80°C/1 bar, 80°C/70 bar and 80°C/200 bar on the cement paste samples. These samples were exposed to the brine (containing carbonates) with a constant renewal rate. All tests lasted one month. The chemical composition of the brine used for leaching tests is given in Table 2. Table 2: Chemical composition of the brine used for leaching tests Species Ca

2+

3.3

Cl

377.4 -

HCO3 +

K

2+

Mg

Na

+

SO42-

Concentration (mmolal)

5.7 33.6 1.8 342.9 2

After curing, the cement paste samples were cut into 20 mm cubes and positioned on a 6-tier stainless steel sample-holder (Figure 1). The sample-holder is directly placed in the pressurized cell, filled with the brine. Throughout the experiment, the samples are fully immersed in the brine (Figure 1). The cell is closed and temperature is gradually increased from room temperature to 80°C at a rate of 1°C/minute. In order to avoid mechanical stress within the samples, the fluid pressure (70 bar or 200 bar) is also increased gradually from 1 bar to 70 bar or 200 bar at a rate of 5 bar/minute.


N2

Pressure regulator

N2

P

Marcol

Safety valve P

Tank leachant

T

Thermostated bath

Pressure valve Purge valve

Pressurized cell Cement samples Lewa® Pump Brine used for tests

Figure 1: Experimental setup used for leaching tests at 80°C and various pressures (1bar, 70 bar or 200 bar)

A pressure regulator and a thermostated bath allowed constant pressure and temperature conditions in the cell. Throughout the tests, fluid pressure and temperature were recorded. Brine renewal rate was regulated at 3 litres/day with a Lewa® pump to maintain aggressiveness of the brine. At the end of the tests, cement paste samples were removed from the cell. To prevent any thermal/mechanical damage to the samples due to depressurization, pressure and temperature were slowly decreased before opening the cells (at the same rate than at the beginning of the tests). During the degradation test, the fluid is collected at the outlet of the leaching device for chemical analyses. Cement paste samples were characterized using advanced methods for chemical and mineralogical analyses: Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), (Thermogravimetric/ Thermodifferential Analyses (TGA/TDA) and 29Si/27Al Nuclear Magnetic Resonance spectroscopy (29Si/27Al NMR). Thermal analysis (TGA/TDA) consists in measuring the mass variations of a sample during a thermal treatment. This technique allows to quantify the amounts of Ca(OH) 2 and CaCO3. The thermobalance instrument used for this work was a NETZSCH STA 409 C. Samples were observed with a Scanning Electron Microscope (SEM). Which is a FEI QUANTA 200 coupled with a ROENTEC Energy Dispersive Spectrometer (EDS). The analysis conditions were 20 kV for the accelerating voltage, 10.1 mm for the working distance and the counting time was 40 s per point for semi-quantitative chemical analyses. Mercury intrusion porosity (MIP) enables us to measure total porosity on sound and degraded cement paste samples. Before MIP measurements, all samples were dried up at temperature of 80°C during one night. Porosity of hardened cement paste was measured by means of a high pressure porosimeter Micromeritics AutoPore IV 9500 with pressure range up to 41.3 MPa. The porosimeter measures a minimal access pore diameter of 3.6 nm.

475

476


2. Results 2.1. Leaching test at 80°C and atmospheric pressure First an experiment carried out at atmospheric pressure has been performed to be used as a reference in order to assess the effect of pressure. SEM observations by BSE (Back Scattered Electrons) images (Figure 2a) showed the presence of three different zones: x An inner zone of intact cement paste. XRD, TGA/DTA and NMR analyses showed that this zone was mainly composed of portlandite (20%), C-S-H, katoite and AFm; x A second zone (≈ 500 μm) where portlandite was no longer observed; the SEM image showed also that this zone had a higher porosity. Unfortunately the very small thickness of this zone did not allow us to perform MIP measurements. x A carbonated zone containing calcite (CaCO3) at the surface of the sample.

Zone2

Intact zone

Zone1

~500 μm

Intact zone

Altered zone (no Portlandite)

b 500μm Carbonated zone

Calcite layer

Zone2 Zone1

c

a 20μm

Figure 2: Cement paste samples exposed to brine during 1 month at 80°C and atmospheric pressure (SEM image (BSE), a), 70 bar (optical image, b) and 200 bar (optical image, c)

2.2. Leaching test at 80°C and 70 bar Macroscopic observations indicated the presence of three zones in the sample: x The first inner zone corresponded to the intact cement paste. x TGA/DTA analyses in the zone 1 (see Figure 2b) showed that portlandite was partially dissolved: the amount of portlandite was about 15% (see Figure 3a), whereas the amount of portlandite in the sound material was about 20%. This zone was also characterized by the presence of calcium carbonates (CaCO3 ≈ 6% in mass). MIP analyses showed that the

477


precipitation of calcite lead to a decrease of the porosity in this zone (≈ 23% instead of 28% in the sound material). x The outer zone (zone 2, see Figure 2b) was 2 mm thick and was characterized by the total dissolution of portlandite (Figure 3b), and by the presence of calcium carbonates (CaCO3 ≈ 20 % in mass). Porosity was not measured in this zone because of its small thickness. CaCO3

Zone 1

Portlandite = 0%

Zone 2

CaCO3 Zone 1 Zone 2

CaCO3 Portlandite = 15%

a

Portlandite = 14%

b

Figure 3: Thermal analyses results of cement sample after 1 month of leaching at 70 bar (a) and 200 bar (b)

2.3. Leaching test at 80°C and 200 bar Observations and analyses of the sample only showed the presence of two zones and thus the absence of an intact zone: x The inner zone corresponded to the zone 1 in Figure 2c. The microstructure was similar to the zone 1 of the sample leached at 70 bar: the portlandite was partially dissolved (14%, see Figure 3c) and calcite was detected. x The outer zone (zone 2, Figure 2c) did not contained portlandite (Figure 3c), but a larger amount of calcite was detected (CaCO3 ≈ 15 %). 3. Discussions The main results at atmospheric pressure indicated that leached cement paste exhibited a classical degradation pattern: the degraded thickness matched well with zone where portlandite was completely dissolved [19,20]. Ionic diffusion between the pore solution and the aggressive brine through the porous network of the cement paste lead to the departure from equilibrium between the hydrates and the pore solution. Then equilibrium was slowly recovered by dissolution of portlandite and precipitation of calcite. The tests carried out at 80°C/70 bar and 80°C/200 bar, highlighted a strong effect of the pressure leading to a mechanism of alteration inducing different patterns than those observed at atmospheric pressure. Cement paste samples leached respectively at 70 and 200 bar showed a different behavior: the thickness of the degraded zone (zone 1) was greater and the portlandite complete dissolution front (zone 2) was thicker at higher pressure. This was due to the presence of an inner zone where portlandite was partially dissolved.

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The alteration mechanism at high pressure appears to be more complex than the one observed at atmospheric pressure. At atmospheric pressure the diffusion rate governs the kinetics and thus is slower than the rate of the chemical reaction. On the other hand, it seems that an increase of pressure increases much more the diffusion rate than the rate of reaction. As a consequence, it exists some zone where portlandite is partially dissolved and calcite is present. Additional tests would be needed to better assess the concentration gradient of portlandite from the outer to the inner part of samples. The whole phenomena observed can be summarised in Table 3. Table 3: Summary of the impact of pressure on the degradation process of oilwell cement paste (from the core to the external zone of the samples)

Patm CH = 20 % (intact zone)

70 bar Diffusion Reactions ? CH = 0 % (~ 2 mm)

Diffusion >> Reactions ? CH = 0 % (~ 2,5 mm)

4. Conclusions This study showed that fluid pressure had a strong impact on the phenomenology and the kinetics of oilwell cement paste degradation by brine at downhole conditions. Degradation tests with brine showed that significant modifications of well cement mineralogy and microstructure can be expected in the external zone of the cement sheath of old wells, where dissolution processes govern. Nevertheless, calcium carbonate precipitation in the inner part of the samples (at 70 and 200 bar) demonstrates that transport properties of the material can be enhanced (thus diffusion process can decrease). Those can lead to also differences in the mechanical strength. The initial material used to carry out degradation tests under CO2 environments should account for these modifications, as it could have significant impact on well integrity and could defeat well confinement performance. Finally, additional tests should be performed with other type of brine in order to feed and validate predictive models that will take into account pre-alteration in order to assess longterm safety demonstration of CO2 geological storage projects. References [1] Ch. Noik, A. Rivereau, Ch. Vernet. Novel Cements Materials for High-Pressure / High-Temperature Wells SPE 50589-SPE European Petroleum Conference, The Hague, The Netherlands, October 1998. [2] Ch. Noik, A. Rivereau. Oilwell Cement Durability. SPE 56538 - SPE Annual Technical Conference and Exhibition, Houston, Texas, U.S.A., October 1999. [3] R. Barbarulo. Comportement des matériaux cimentaires : actions des sulfates et de la température. Thèse de Doctorat de l’Ecole Normale Supérieure de Cachan et de l’Université de Laval, Québec, 2002.


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