科研之友
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摘要
The occurrence characteristics of weak planes were analyzed according to outcrop, core, and SEM and FMI data of shale rocks. A strength analysis method was developed for shale rocks with multiple weak planes based on weak-plane strength theories. An analysis was also conducted for the strength characteristics of shale rocks with uniform distribution of multiple weak planes. A collapse pressure prediction model for horizontal wells with multiple weak planes was established in shale formation, considering the occurrence of each weak plane, wellbore stress condition, borehole azimuth, and in-situ stress azimuth. Finally, a case study was conducted of a horizontal shale gas well in southern Sichuan Basin. The results indicate that the intersection angle between a bedding plane and a structural fracture was large (nearly orthogonal). With the increase in the number of weak planes, the strength of rock mass declined sharply and was significantly influenced by weak planes. When there were more than four weak planes, the strength tended to be isotropic and the whole strength of rock mass was greatly weakened, significantly increasing the risks of wellbore collapse. With the increase in the number of weak planes, the drilling fluid density (collapse pressure) to keep borehole stability should be increased gradually. For instance, the collapse pressure was 1.04 g/cm3 when there were no weak planes, and 1.55 g/cm3 with one weak plane, and 1.84 g/cm3 with two weak planes. The calculation results of the horizontal well in this case study were in good agreement with actual situation and more accurate and practical than those by traditional models. This model effectively improved the accuracy of wellbore collapse pressure prediction of horizontal shale gas wells.
- 出版日期2014
