Presented at 44th U.S. Rock Mechanics Symposium and 5th U.S.-Canada Rock Mechanics Symposium, Salt Lake City, Utah, June 2010.
A Geomechanical Approach to Enhancing Well Design and Increasing Drilling Performance in the Sichuan Xinchang Gas Field, China
Abstract
The Xinchang gas field is characterized by multiple over-pressured gas formations that are composed of hard, fractured rocks. Optimizing the mud weight and well design to improve drilling speed and reduce non-productive time is challenging in this reservoir. In 2007, we built a Geomechanical model based on a detailed quantitative analysis of the rock properties, pore pressure, and stress state in the study wells (including borehole collapse pressure and fracture gradient). The interpreted strike-slip stress regime (ShminSHmax) was validated in 2008 by the deadly 8.0 magnitude Sichuan earthquake. We used the geomechanical model to simplify the casing design by eliminating one casing string and to optimize mud weight for a new well which was successfully drilled without any major drilling problems. The average ROP was nearly doubled to 2.95m/h and drilling time was significantly reduced (133 days compared to previous 339 days). The well is now producing gas at 10.3×104 m3/d. The study demonstrates the robustness of stress state and rock strength determination based on modeling the co-occurrence of breakouts and drilling induced tensile fractures. The paper highlights the importance of correlating data from different sources to confirm the accuracy of the geomechanical model. The Sichuan basin is a large petroleum-bearing introcratonic sedimentary basin in southwestern China (Fig. 1). It is the first and principal gas-producing region of the country. The Xinchang gas field is close to the northwestern margin of the Sichuan Basin where the Longmen Shan thrust belt was seimically active in 2008 [1, 2]. The Xinchang field is dominated by clastic sedimentary sequences ranging in age from the Late Triassic to Quaternary [3]. Gas is produced from multiple pay zones within the Upper Triassic tight sandstones at depths of around 5,000 meters. The quality of the reservoir sandstones is generally poor. The average porosity of reservoir is about 5% and the permeability is less than 1 mD. The Xinchang gas field is characterized by multiple over-pressured gas formations that are composed of hard, fractured rocks. High mud weights often damage gas reservoirs, lead to lost circulation problems, and reduce the rate of penetration (ROP); while low mud weights may fail to prevent gas kicks or result in wellbore instabilities, causing tight hole and stuck pipe (Fig. 2). Average ROP is commonly less than 1.5 m/h in this field. We carried out a geomechanics analysis of the Xinchang gas field in 2007 with specific applications to enhancing well designs and increasing drilling performance. The analysis was designed to construct a geomechanical model and apply the model to optimizing the mud weight program and the casing design to reduce drilling time in vertical wells in the Xinchang gas feild without compromising the integrity of the well. The paper presented here details data analysis to determine the key components of the geomechanical model, model verification by correlating data from difference sources and comparing predicted hole enlargment with observed wellbore failure, and its successful application to simplify casing program and optimize mud weight.