Wellbore instability is frequently encountered in the process of shale oil drilling. Due to the anisotropic strength characteristics of shale, most conventional models are not suitable for wellbore stability analysis in layered formation. In this paper, taking the continental shale in the north of Songliao Basin as the research object, the anisotropic elasticity and strength parameters of shale are measured experimentally. Based on pore elastic mechanics, an anisotropic wellbore stability model of layered shale is established, and the variation of collapse pressure under different formation conditions is analyzed. The results show that considering the elastic and strength anisotropy, the collapse pressure is the largest, and the influence of strength anisotropy is significantly greater than that of elastic anisotropy. The stability of horizontal wells drilled along the horizontal maximum principal stress is the optimal, followed by vertical wells and small angle directional wells. The actual drilling conditions of G101 well are basically consistent with the prediction results, which confirms the accuracy of the model. According to the traditional wellbore stability model, the equivalent density of collapse pressure in horizontal well section is 1.58 ~ 1.76 g/cm3. It is much lower than the collapse density of 1.86 g/cm3 calculated by the anisotropic model in this study. The findings of this study can help for better understanding of the mechanism of wellbore instability in horizontal wells in shale formations. The model can be used to guide the drilling engineering design of shale oil horizontal wells and reduce the losses caused by the instability of the wellbore.

Background: Methods that are used to obtain oil and gas resources by penetrating deep strata are gradually being developed. The complex lithology of deep strata causes difficulties for traditional drill bits to cut into the rock effectively, which affects the drilling efficiency. Objective: The goal of this work is to optimize the bit structure and design a patented drill bit for more efficient rock breaking. Method: Through a series of tests, the rock mechanics parameters of deep strata in a block of Jilin Oilfield were measured, and the drill bit model XFXR5195 was selected to best match the lithology of the formation by using the Virtual Strength Index and the Technique for Order Preference by Similarity to an Ideal Solution. Simulation of the rock breaking process of the conical PDC cutter and cylindrical PDC cutter with the finite element method. Results: The simulation results show that the pre-crushing of the conical cutter can effectively reduce the difficulty of the subsequent cylindrical cutter in breaking and cutting rocks under 100 MPa compressive strength rocks; under 50 MPa compressive strength rocks, the combination of the conical cutter and cylindrical cutter are not effective with regards to drilling. Conclusion: The conical PDC cutter has a more uniform force range and more concentrated stress on the rock than the cylindrical PDC cutter. The rock-breaking ability of the conical PDC cutter is higher than that of the cylindrical PDC cutter under different compressive strength rocks and especially in high compressive strength rocks.

Fractures continuously extend and expand along bedding shale formations under the action of drilling fluid and eventually form a complex fracture network, which greatly reduces the strength of the rock. To determine the effect of the drilling fluid action time on the physical and mechanical properties of shale, nuclear magnetic resonance tests are carried out on shale soaked in oil-based drilling fluid for different soaking times. The fluid absorption rate of shale takes the form of a power function. The equations relating the shale mass, porosity, and permeability to soaking time are established. Then, in a sonic time-difference test, the change in the dynamic elastic parameters with the immersion time are observed. According to a triaxial strength test, the failure form and the deterioration degrees of the layered shale in different loading directions with immersion time are analyzed. Numerical simulation of the deterioration degree of shale under different water content conditions is carried out. When the water content increases, the collapse density of the surrounding rock of the well wall increases significantly. Finally, considering the anisotropy of bedding shale, the inclination angle, azimuth angle, and drilling fluid immersion time are substituted into the rock mechanics parameter deterioration model, the three pressure profiles of the formation are corrected, and the safe drilling fluid density window of the target interval is given.

The shale oil horizontal wells in the Songliao Basin are affected by a lack of mature theories, technologies and experiences in the direction of wellbore stability. Wellbore collapse may occur, and in severe cases, the wellbore may be scrapped, resulting in huge economic losses. Therefore, aiming at addressing the above problems, rock mechanics experiments were carried out. Based on the theories of elasticity and rock mechanics, this paper considers not only the influence of the bedding plane, but also the influence of hydration on the strength weakening of the shale body and the bedding plane. The analysis shows that no matter under which in situ stress mechanism, the wellbore in the vertical well section is the most stable, and when the inclination angle is approximately 45°, the wellbore is most likely to be unstable. Changes in water content do not affect the most stable or unstable regions. Under the same conditions, the equivalent density of collapse pressure increases with the increase in water content. In addition, field examples are also analyzed to verify the accuracy of this model, which can provide a theoretical and technical basis for the safe construction of continental shale oil horizontal wells.