Evolution of a Versatile HTHP Water-Based Drilling Fluid Improved Drilling Efficiency Offshore Vietnam

Abstract

Abstract The fast pace of exploration and production level increase over recent years has led operators to expand the horizon for exploration of hydrocarbons in more complex and exigent reservoirs which includes the high temperature and pressure (HTHP) reservoirs. Typically wells in the North of Vietnam have been drilled using a Non-Aqueous Drilling Fluid (NAF) for their lower and reservoir intervals. The challenges of multi-string design, potential of acid gas contamination, well bore stability, down hole losses and a high temperature environment require a well-engineered NAF to meet the overall well objectives. Alternatively, Water-Based Fluid (WBM) has significant environmental benefits compared to NAF. This paper presents the technical development, evolution and lessons learned from using a high performance HTHP WBM over a series of extremely challenging HPHT exploration wells Offshore Vietnam, with temperatures expected to reach 170°C (338°F) and pressures over 12,000 psi. These conditions place severe limitations on the drilling fluid design and can often lead to failures in open-hole wireline logging operations, which is probably the most important objective in an exploration well. To ensure the successful drilling, wireline logging and completion operations under the thermal conditions, the development of the HPHT WBM for these exploration wells needs to address the HTHP filtration control properties, filter cake quality, inhibition and the thermal stability of the drilling fluid. Extensive laboratory testing was performed to optimize the rheological properties for mud weights up to 18.7 ppg to ensure minimal sag tendency after static condition, while also providing excellent filter cake quality, system inhibition and hole condition to allow the well to be fully logged and evaluated. The enhancements made in the HPHT WBM design led to an improved drilling performance in drilling these challenging exploration wells. Inhibition of the reactive formations was improved with each well drilled, as was the rate of penetration and wellbore stability. Extensive logging was performed with the wellbore remaining in excellent condition throughout the long open-hole time.