Study on mechanical properties and wellbore stability of deep brittle shale

Abstract

As the drilling depth of shale's oil and gas wells increases worldwide, the problem of borehole collapse and instability becomes increasingly serious. Due to the high content of brittle minerals in deep shale, the complex situation of an underground collapse of a site is often manifested as borehole wall caving and falling. On the basis of the deep shale of China's Longmaxi formation, we developed a simulation experiment involving the complex loading mechanics of underground working conditions. The influence of loading and unloading confining pressure speeds and the sealing properties of drilling fluid on the mechanical properties of rock were analyzed. We established the dynamic changes in borehole formation, combining seepage and pore pressure, the effective stress field around the well, and a method of evaluating wellbore stability in deep shale gas horizontal wells. The results show that the deep shale of the Longmaxi formation has a high brittle mineral content and is prone to brittle fracturing along parallel bedding surfaces under downhole pressure. The effective liquid column pressure at the bottom of the wells change noticeably under different working conditions. Changes in bottom hole fluid column pressure are the cause of the complicated situation of well leakage and overflow in the horizontal sections of the Longmaxi formation. The effective support confining pressure of borehole wall rock differs with different tripping out speeds. The faster the tripping out is, the less the support confining pressure of the wellbore rock and the easier it is to induce the rock to break down along the bedding. The effective sealing property of drilling fluid also affects wellbore stability. A good sealing property of drilling fluid can improve the supporting effect of the fluid on borehole wall rock and improve its resistance strength. On the whole, the pressure of the bottom hole liquid column is effectively controlled during drilling in the Longmaxi formation; this can avoid complicated situations such as well leakage. The results of this study provide a theoretical basis for the study of wellbore stability in deep shale formations during the drilling process.