A Horizontal Drilling Breakthrough in Developing 1.5 m Thick Tight Gas Reservoir - Case Study in Risk Uncertainty Management, Ordos Basin, North China

Abstract

Dual-lateral horizontal wells have been the strategy applied in Changbei field to maximize economic development of a tight gas sandstone reservoir. A motor and measurement-while-drilling (MWD) bottomhole assembly (BHA) is normally used to drill 2-km dual-lateral wells within the quartz arenite (QA) sand reservoir that has an average thickness of 15 m. After completing most of the drilling in the thicker axial region of the channel belt, the field development has now shifted toward the channel margin boundary that has more straigraphic complexity and much thinner reservoir. A pilot hole, drilled to provide better understanding of the reservoir thickness near the channel boundary, has revealed that the reservoir was about only 1.5 m in thickness. The application of motor and MWD BHA would have not been adequate to drill and place the well within the very thin reservoir. Therefore, the combination of RSS and the high-resolution resistivity image LWD BHA were applied to replace the motor and MWD BHA. This paper features the successful approach taken using an integrated application of RSS, optimized bit, high-resolution resistivity image LWD, and dip determination well placement technique to overcome the subsurface challenges and to improve the drilling efficiency. RSS and LWD technologies were deployed to provide real-time, full-bore, high-resolution resistivity and gamma ray images to evaluate the structural dips, image stratigraphic events, and provide better trajectory control with near-bit survey capability. The field real-time data will be discussed. This integrated application has helped to place the well optimally along the thin target reservoir, thus optimizing the reserve recovery despite the large discrepancy of actual reservoir profiles relative to the original predrill model. The authors also share the successful approach, best practices, and valuable lessons learned that have provided the breakthrough in developing the tight gas reservoir under such complex geological uncertainties and in a very hard, abrasive formation. Introduction Changbei gas field is located in ShanXi province, northcentral China. It resides on the edge of the Mauwusu desert in the Ordos basin, one of the largest sedimentary basins in China. It is also the first tight gas field in the Ordos basin to successfully use horizontal dual-lateral wells as the basic development concept. Dual-lateral horizontal technology was introduced to achieve high production rate without applying fracturing technology. The horizontal well can penetrate several potential baffles in the reservoir, enlarge the reservoir exposure length, and extend the drainage area. The main Upper Paleozoic clastics reservoirs in Changbei field occur 2800 to 3000 m below the surface. The target gas reservoir is located in the Lower Permian Shanxi formation (P1S2 QA). The P1S2 QA interval is defined in core and calibrated to logs as a low-gamma quartz arenite main reservoir sand. The main P1S2 QA reservoir has an average gross thickness of approximately 15 m (1.5 to 39 m range) and displays a regional SW-directed dip of ~1°. Drive mechanism is depletion drive with dry gas.