High-Temperature Water-Based Mud Faces Challenges Offshore Sarawak, Malaysia
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The reservoir section of a gas field offshore Sarawak, Malaysia, consists of two massive pinnacle carbonate structures with heterogeneous porosity and permeability and many vugs and faults. Severe to total loss of circulation was expected along with a high bottomhole temperature (BHT). Considering the high risk of lost circulation, the drilling-fluid strategy involved designing and testing a high‑temperature (HT) -tolerant water‑based mud (WBM). While the main hole was drilled successfully with the HT WBM, the sidetrack experienced a significant reduction in rate of penetration (ROP).
Well Design and Execution Challenges
During the design stage, determination of the severity of losses in the carbonate formation was met with high uncertainty. Consequently, a WBM system was selected to drill the 8½-in. reservoir section. A shale formation was predicted to be above the carbonate section, and a 9⅝-in. casing was planned to be set as close as possible at the top of the carbonate to isolate the shale. With the 9⅝‑in. casing in place, the WBM was to be used to drill the shoe track and the remaining small shale section before entering the carbonate reservoir.
The BHT of the reservoir was predicted to be 370°F, which was extrapolated from data from offset wells. In addition, the pore pressure in the reservoir was overpressured approximately 14 lbm/gal because of a gas column in the carbonate. The high BHT coupled with the high mud-weight requirement of up to 14.5 lbm/gal led to the design of an unconventional HT WBM, which was a first for the operator. Special attention also was given to the corrosion risk on the drillstring from expected hydrogen sulfide levels up to 150 ppm and carbon dioxide levels up to 20% in the reservoir.
Predrill Laboratory Study
Laboratory tests were conducted to address major well challenges and overall performance of the system under extreme borehole temperatures. To ensure the mud system was able to withstand temperatures up to 400°F, a synthetic polymer was introduced to the mud design. Several mud formulations were tested and fine-tuned to achieve well-specific mud specifications for optimal hydraulic capability.
Laboratory tests in accordance with American Petroleum Institute Recommended Practice 13B were conducted to evaluate the mud properties with various concentrations of additives. Four samples—Samples A, B, C, and D—were prepared and submitted to the same test procedure to understand the mud rheological behavior upon exposure to the HT environment.
Sample A had a very high plastic viscosity (PV), which may cause problems with equivalent circulating density during drilling. The yield point (YP) and 6-rev/min test data were observed to be out of specification, indicating a highly viscous mud. A similar high-PV trend was observed with Sample C along with high high-pressure/high-temperature (HP/HT) fluid loss, which could increase the risk of differential sticking because of a thicker filter cake. Sample B had the optimal mud properties, with the lowest PV and low HP/HT fluid loss compared with the other mud samples. Other properties, such as low-end rheology, gel strength, and YP, were within the required range. Subsequently, Sample B was chosen as the mud formulation for the HT WBM system.
HT WBM Operational Performance
In the first well of the development campaign, the 9⅝-in. casing was set much shallower from the top of the carbonate because of geological uncertainties. Consequently, a total of 120 m of shale was drilled using the HT WBM system. No mud-related operational issues were observed in the first well. Nevertheless, the operator faced challenges in subsequent operations that led to sidetracking from the main hole of the first well. Before drilling the sidetrack, the formulation of the HT WBM was enhanced with additional shale inhibitors in anticipation of a long shale interval similar to that observed in the main hole.
Surprisingly, the operators faced a totally different experience while drilling the sidetrack. The operator encountered difficulty with slide drilling and frequently observed stalling of the drillstring. In addition, the operator faced a significant reduction in ROP—1–2 m/hr compared with 5 m/hr with the main hole. The operator made several attempts to address bit balling by pumping pills that consisted of concentrated potassium chloride (KCl) and a nut plug to improve ROP. However, the operator observed only a temporary increase in ROP, up to 3 m/hr, and finally decided to pull out the drillstring.
At surface, the drilling bit was partially balled up with clay (Fig. 1). This led the operator to replace the HT WBM with synthetic-based mud to continue drilling the sidetrack. No drilling-related issues were observed, and the ROP improved to 3–5 m/hr.
With a large amount of nonproductive time recorded, suspected to be the result of poor performance of the drilling fluid, the WBM formulation was re-examined and retested to determine the root cause of the problem. These challenges posed a learning curve for the operator to improve the mud system and serve as a mud-design benchmark for future HP/HT wells that require a WBM system.
The mud performance from the sidetrack was a surprise to the operator because no operational issues were observed in the main hole. The operational problems in the sidetrack were suspected to be caused by inadequate mud inhibition.
The HT WBM used in the main bore clearly was not an inhibitive mud system compared with the one used in the sidetrack, which included shale inhibitors such as glycol, sulfonate asphalt, and KCl. Further investigation was made to understand the shale reactivity through X-ray-diffraction analysis of the cuttings from the sidetrack. The analysis revealed the dominant clay minerals to be illite (10%) and kaolinite (6%). In addition, a cation-exchange-capacity study on the cuttings shows 8 meq/100 g, which classifies the shale as nonreactive.
Further analysis was conducted on the mud properties to explore the potential root cause. The mud properties, however, do not differ much compared with those of the original hole, except in the results of the methylene blue test (MBT). The results of the MBT of the HT WBM system in the original hole were higher compared with that in the sidetrack, indicating that the system is more dispersive in nature. A dispersive nature for the system leads to the theory that the less-reactive shale is easily dispersed in the mud, preventing any bit balling. For the sidetrack, the HT WBM system appears to have insufficient inhibition to handle the dispersive shale, indicated by the presence of sticky clays at the bit. Further testing, such as shale-accretion and linear-swellmeter tests, were conducted on the HT WBM system to improve the mud formulation further to address shale inhibition
Improvement of the HT WBM System
The operator used a nonstandard testing approach to understand the risk of bit balling. In this case, a shale-accretion test was conducted to compare the performance of HT WBM in the main bore and in the sidetrack.
Shale-Accretion Test. The results show higher shale adherence for the HT WBM in the sidetrack compared with that in the original hole, which supports the bit-balling claim. Further evaluations were conducted with different concentrations of KCl and the introduction of an antiaccretion material to minimize the sticking tendency of the clay on the drillstring.
The shale-stickiness tendency, or bit-balling risk, also has been reduced with a reduction in KCl concentration, in addition to the inclusion of an antiaccretion agent. The antiaccretion agent forms a lubricant film on the metal surface that eliminates cuttings adherence. Sample B has a shale adherence that is 41% lower compared with that of Sample A and 35% lower compared with that of the sidetrack HT WBM. However, the HT WBM system from the main bore has the lowest shale adherence because of its dispersive nature. All the mud samples also underwent a linear-swellmeter test to investigate inhibition performance and the effects of the antiaccretion additive.
Linear-Swellmeter Test. The linear-swellmeter test highlights that the mud sample with a lower concentration of KCl and the addition of an antiaccrection agent has the lowest swelling tendency for the particular shale reactivity. The addition of the antiaccrection agent also improved the mud inhibition performance by up to 18% and reduced the shale-adherence tendency by up to 35%. In conclusion, the revamped HT WBM formulation is more robust and able to handle well uncertainties in the future.
High-Temperature Water-Based Mud Faces Challenges Offshore Sarawak, Malaysia
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