Abstract
For horizontal wells in Longmaxi Formation, oil-based drilling fluid that soaks for a long time is more likely to cause a wellbore collapse. Therefore, in this paper, the downhole core method of shale formation, in Longmaxi Formation, was adopted. First, rock samples were selected from different sampling angles and soaked with field drilling fluid. Second, a triaxial mechanics experiment was carried out. Based on the anisotropic wellbore stress distribution model, the stability of shale wellbore was calculated and analyzed. The results show that the compressive strength and cohesion of the shale are reduced after soaking in the drilling fluid. Hence, the reduction range of various sampling angles obviously differs as well. Shear failure occurs in vertical stratification; shear slip failure occurs along the weak plane, showing strong anisotropy. Combined with the experimental results, the collapse pressure is calculated, and it is found that the weakening effect of drilling fluid makes the overall collapse pressure rise by about 0.2 g/cm3. Finally, the shale bedding dip and dip direction have a great influence on the collapse pressure. The lower critical mud weight always takes the minimum value when the borehole axis is perpendicular to the bedding.
Highlights
Shale accounts for more than 75% of the oil and gas deposits drilled globally [1]
An accurate understanding of the collapse rule, for the Longmaxi shale formation in Jiaoshiba, and the selection of the most optimal drilling design both have a significant influence on the efficient development of shale gas
In order to better understand the influence of oil-base drilling fluid on the strength of shale with different drilling angles, the underground core of the Longmaxi Formation was used to carry out a triaxial mechanics experiment
Summary
Shale accounts for more than 75% of the oil and gas deposits drilled globally [1]. The Longmaxi Formation, in Sichuan Basin, is rich in unconventional oil and gas resources. In order to better understand the influence of oil-base drilling fluid on the strength of shale with different drilling angles, the underground core of the Longmaxi Formation was used to carry out a triaxial mechanics experiment. The strength of shale at different angles, before and after drilling fluid immersion, were tested in order to provide parameters for the study of borehole wall stability in shale formation. All these parameters are input parameters for the lower critical mud weight analysis of the Longmaxi shale formation in Fuling. The mechanism of shale mechanical failure, bedding occurrence and weakening effect of drilling fluid, are all very important in the wellbore stability analysis of shale formation
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