Ensuring shale stability while drilling the reservoir and completing a well is critical to guarantee well integrity and to enable the reservoir to product at its maximum potential. Drill-in fluid type, completion brine, and shale inhibitors play an important role in maintaining well integrity and preventing formation damage as they have different effects on shale. Reactivity of shale samples from offshore Gulf of Mexico was studied using several reservoir and completion fluids. The shale was first characterized using X-Ray diffraction (XRD) and X-Ray fluorescence (XRF). Then, the effect of fluids on shale was determined by performing several tests including linear swell meter (LSM), capillary suction time, and cuttings dispersion. Water-based (WB) and oil-based (OB) drill-in fluids, synthetic formation water, and completion brines with and without shale inhibitor were used to study the shale reactivity. XRD and XRF test showed presence of 5wt% illite and 6wt% kaolinite in the shale sample. Performance tests conducted on the shale samples showed a similar trend of high reactivity with improvements when inhibitor is added. Some brines showed poor synergistic effects when inhibitors were present in the fluid formulation. The type of drill-in fluid has a large impact on the LSM test results. Type of brine in the WB drill-in fluid also showed a major influence on the shale behavior. Oil-based drill-in fluids are commonly used to drill sections with very reactive shales. Nevertheless, water-based fluids are sometimes required due to performance preferences, environmental concerns, economic and logistic reasons, and/or synergistic effects with logging tools. Filtrate invasion and drill-in fluid/completion brine losses could cause detrimental effects in the reservoir if the fluids have not been designed to inhibit the shale hydration and swelling or fines migration. Therefore, improving the clay inhibition and shale stability while using water-based fluids is fundamental when drilling through reactive shale sections. Performing a comprehensive test matrix to determine the feasibility of using water-based fluids is imperative. Achieving excellent inhibition for a very reactive GOM formation shale while maintaining performance is possible when a precise and well-engineered combination of brine/shale inhibitor in the drill-in and completion fluids are found.
The design of an invert emulsion fluid that can hold up more than 24,000 psi and 435°F is a complicated and exhausting process. The challenges presented in the design of an UHPHT drilling fluid formulation are the base oil selection and the complete drilling fluid formulation. Flash point measurement of base oil before and after heat ageing to ensure that the base fluid were stable and the overall stability of the mud system need to be assured (rheology, gelation, fluid loss and weight material suspension) after both dynamic and static ageing for at least 96 hours at anticipated maximum BHST. Laboratory pilot testing allows us to find a chemical balance where the rheology profile and mud properties fulfill the UHPHT project requirements. The laboratory testing protocol validation is the most important factor to prevent different issues during the drilling phase. This testing will be a contributor factor to conclude a well successfully. Another phase of the design that we need to consider is an exact dynamic temperature modelling because a more precise Bottom Hole Temperature profile helps optimize the drilling process to increase the possibility of borehole stability.
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