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company man to make decisions for controlling the mud rehology. The clay and lead contamination can make the mud to be gelled up. On the other hand it will ...
Clay Contamination Effects On Heavy Weighted Drilling Mud 1 LASHKARI ,

2 KHALILNEJAD

3 NEZHAD

R. A. and A. TABATABAYEE 1 Sahand University of Technology, Tabriz - Iranian Central Oil Field Company (ICOFC) 2 Islamic Azad University, Marvdasht Branch, Marvdasht, Iran 3 Sahand University of Technology, Tabriz, Iran

RESULTS

INTRODUCTION For drilling in high pressure formations it is necessary to control the flow and the pressure of the reservoir by increasing the drilling mud weight. Saponja and Adeleye (2006) said increasing in drilling mud weight will enhance the weight of fluid column which avoids the formation fluid to invade the well during drilling. Some agents will be added to the drilling mud to control some of the properties which can affect the operation of the drilling mud. Green starch is the additive which added to the drilling mud to control the fluid loss and viscosity. Barite is known as the most famous additive for increasing the mud weight. The density of barite is about 267.8026 lb/ft3. While drilling in a high pressure formation, adding barite to the drilling mud in a special concentration prevents the inflow of the well. Ilmenite and siderite are also used as weighting agents. The weight of the mixtures repels the force of the well fluids, which allows the rig operators to prevent the kick or blowout occurrence (Caenn and Chillingar 1996). Henry and Gray proved the weight of the mud can be raised to 11.5 lb/gal by the suspension of the formation solids into base phase. To achieve the values above 11.5 lb/gal barite should be added to the base phase. Another additive which uses simultaneously with barite to raise the weight of drilling mud is hematite. Selection of the appropriate concentrations can improve the efficiency of the mud and each mistake in this field can cause great damages on the formation. Lost circulation will take place in a case when drilling mud invades to the other zones instead of returning to the surface. During drilling in low pressure fractured formations or high permeable formations lost circulation is a possible phenomenon (Qoutab 2004).

METHOD At a well that used heavy weighted drilling mud, the mud weight was equal to 145pcf. The mud gradually weighted up with barite and hematite. Green starch used for controlling the fluid loss during drilling. Since the mud viscosity was very high, red starch did not add to the mud. All the components added to the drilling mud with their concentrations have been represented in table 1.

Table 1- Description of Drilling Mud Components Components

Unit

Water Salt Soda Ash Lime G. Starch Barite Hematite

Bbl lb/bbl lb/bbl lb/bbl lb/bbl lb/bbl lb/bbl

Minimum Concentration

Maximum Concentration As Need 0-125

0.2 0.5 12

Since the pH plays an important role in working with heavy weighted drilling mud, controls of pH had been done to prevent the problems. After several pilot tests the results showed that the mud could be workable for drilling at pH values between 8 to 8.5. At higher pH values the mud was gelled up. The range of pH values for activation of green starch is 10 to 11. Therefore the green starch will not be activated at pH values between 8 to 8.5. So the fluid loss will increase significantly. When fluid loss takes place, stuck pipe and formation damage become probable. To determine the reason of problems, The first doubt was about ionic contamination. Ionic contaminants such as sulfate and calcium can increase the viscosity and gel strength. The entry of various ions into drilling mud can be done through formation drilling or water that used to make drilling mud. For this reason, magnesium,carbonate,calcium and sulfate ions detection tests was carried out on the drilling mud and water . The results of the test are presented in Table 2. The results shown that there was no carbonate and phosphate ions contamination, and the presence of calcium and magnesium ions was low, which indicates that the mud and water are clean. Table 2- Ionic test of drilling mud & water Ionic Test Calcium (Ca ++) Magnesium (Mg ++) Sulfate (SO4 =) Carbonate (CO3 =)

Mud 800 mg/l 80 mg/l 0 mg/l 0 mg/l

Water 200 mg/l 60 mg/l 0 mg/l 0 mg/l

Then, some of the additives checked. Green starch changed with an approved one. Changing the green starch did not solve the problems. The next guess was to evaluate the specific gravity of barite for contamination. The specific gravity was 262.197 lb/ft3. The measured value was approximately near to standard value of pure barite density (267.8026 lb/ft3).

Due to the behavior of mud rheology with respect to pH, clay contamination was the other probability for the reason of the problems. Two types of drilling mud (one with barite which had been used in this well and the other with another barite which had been used in another well at this formation without any problem) analyzed by pilot tests to determine and compare their rehology. The results of pilot tests have been shown in table 3. Table 3- Comparison the results of pilot test in two types of drilling mud Pilot Test NaCl Soda Ash Green Starch Barite Mud Weight (pcf) Temperature(F) R600/R300 Gel 10s/10m MBT (lb/bbl) pH

Mud type 1(barite used in this well) 0.2 lb/bbl 125 lb/bbl 14 lb/bbl 695 lb/bbl 132 120 87/55 18/26 12.5 7.5

For more investigation XRD (X-ray diffraction) tests had been done over the barite added to the drilling mud and the results showed an amount of Aluminum Silicate Hydroxide and Pb element (Lead) in barite. The results of XRD test are presented in figure 1.

Mud type 2 (Another barite) 0.2 lb/bbl 125 lb/bbl 14 lb/bbl 695 lb/bbl 132 120 96/54 4/6 2.5 7.5

As shown a high difference in the MBT. The MBT of the drilling mud which had been used in this well was 12.5 while it was equal to 2.5 in the other drilling mud. Van Dyke defines MBT as the methylene blue capacity index, which can illustrate the clay content in the fluid. MBT defined by equation 1: Methylene .Blue , (cm 3 ) MBT  Drilling .Fluids , (cm 3 )

All the additives used in both of the drilling fluids were similar except barite. So clay contamination assumed for the barite.

Figure 1- Results of XRD peak matching. The row labeled as peak list in figure-1 shows the reference peaks for the barite which had been added to the drilling mud. The reference codes for founded elements in barite had been represented in table 4.

Table 4- Identification of the reference codes Ref. Code 00-005-0448 00-049-0096 00-004-0686

Chemical Formula BaSO4 Al3Si2O7(OH)3 Pb

Compound Name Barite Aluminum Silicate Hydroxide Lead, syn

As figure 1 illustrates the existence of Pb element with refrence code of 0040686 and Aluminum Silicate Hydroxide with refrence code of 049-0096 have been proved. Aluminum Silicate Hydroxide is a member of clay minerals group, which can be considered as representative of clay content. Clay contamination had been observed in the drilling mud. The reason for existence of Pb element might be due to the barite ore contamination and poor segregation of barite ore .On the other hand clay content will reduce the density of the barite and this reduction will be neutralized by adding Pb element to the barite.

0.5 1 14 132-700 10-200

There were great problems reported during drilling, casing and cementing of this hole. The problems listed as below: • The drilling mud was gelled up in the hole and could not be pumped and circulated after trips while the mud stays in the static form for a long time. The measured 10 min gel was very high. • During casing running in the hole, the casing did not reach to the bottom of the hole because of the mud was gelled up. • The pump pressure had been raised significantly when bit was working in the gelled up mud zone. By applying higher pressure for circulation, complete fluid loss occurred. • During trip pipes out of the hole, the level of the mud in the annulus did not drop. The volume of the hole filled with formation fluid. • The drilling mud properties were not constant in surface and the measured properties changed after a while. Basically the drilling mud properties should be static at surface and in mud tanks. In this case the drilling mud intensive properties changed just a few hours after measurements. This phenomenon made much of uncertainties in the mud rheology. • Air enters to the mud, during process of pumping and while mixing with agitators in the mud tanks. Air should come out from the drilling mud gradually with respect to difference in gravities. Air did not exit from the base phase and trapped as bubbles in the base phase and lowered the mud weight (about 4 pcf), which can cause kick during drilling. • Significantly increasing in the mud volume in the mud tanks which caused by air trapping, had been made the concept of kick theory. With respect to kick occurrence and decreasing the mud weight, barite was added to the drilling mud with higher concentrations. Addition of more barite for weighting the mud decreased the ROP (Rate of Penetration) of the drilling. • The mud weighted very highly with barite. Cement should be weighted more than the drilling mud. The cement pumped to the well could not fellow up behind the casing, because the cement did not have the ability to displace the mud upward. Increasing the pressure made the complete loss for the cement. • The viscosity of the mud enhanced at high temperatures and lowered in low temperature. This behavior is in contrast with normal cases.

CONCLUSIONS Extra clay contamination in heavy weighted mud will affect the drilling mud properties which can misguide the mud engineer and company man to make decisions for controlling the mud rehology. The clay and lead contamination can make the mud to be gelled up. On the other hand it will affect the viscosity and weight and demolish the cementation process. During drilling with the barite which had been contaminated with clay, the volume of drilling mud dump enhanced significantly, which imposed much of cost to the drilling company. Its better to get XRD tests randomly from mud materials before sending them to the well sites to avoid usage of not qualified materials

REFERENCES [1] Caenn, R.Chillingar, V.G. [1996] Drilling fluids: State of the art, Journal of Petroleum Science and Engineering, 14, 221-230. [2] Darley, H.C.H. and George, R.G. [2011] Composition and Properties of Drilling and Completion Fluids (6th edition). Gulf Professional Publishing. [3] Qoutab, H. [2004] Underbalanced Drilling; Remedy for Formation Damage, Lost Circulation, & Other Related Conventional Drilling Problems. Abu Dhabi International Conference and Exhibition, Abu Dhabi, SPE-88698-MS. [4] Sponja, J. and Adeleye, A. [2006] Managed-Pressure Drilling (MPD) Field Trials Demonstrate Technology Value. IADC/SPE Drilling Conference, Florida, SPE-98787-MS. [5] Van Dyke, K. [2000] Drilling Fluids: Lesson 2. Petroleum Extension Service Continuing and Extended Education. Austin, Texas.

ACKNOWLEDGEMENTS The authors acknowledge with grateful appreciation the kind assistance and financial support provided by the Iranian Central Oil Field Company (ICOFC) for Research .

CONTACT INFORMATION • Reza.Lashkari [email protected], [email protected] Sahand University Of Technology, Tabriz, Iran Iranian Central Oil Field Company (ICOFC)Tel: +989173382196 • Ali.Khalilnejad [email protected] Islamic Azad University, Marvdasht Branch, Marvdasht, Iran Tel:+989370159590 • Prof.Alireza.Tabatabayee Nezhad [email protected], [email protected] Sahand University Of Technology, Tabriz, Tel: +984133449481