Study on the Variation Law of Rock Mechanical Properties with Depth in Deep/Ultra-Deep Layers-Taking Shunbei Oilfield as the Rresearch Object

ABSTRACT At present, the development of petroleum industry is gradually developing from shallow oil layer to deep/ultra-deep layer, but with the increase of well depth, the rock strength is also getting higher and higher, which hinders the drilling work. In this paper, based on the well logging data, the variation of rock strength characteristics with well depth is analyzed, and the uniaxial compression model is established using the global cohesive element. The reason why the rock is difficult to be destroyed in the deep layer is analyzed, and the relationship between the numerical simulation results and the confining pressure and the well depth is analyzed. The results show that the confining pressure can greatly increase the mechanical strength characteristics of rock, and the rock strength increases faster and faster with the increase of well depth. INTRODUCTION Oil and natural gas are the main raw materials for obtaining energy in the world today. With the improvement of the development level of oil and natural gas, the depth of recoverable oil reservoirs is getting deeper and deeper, and the depth and number of deep/ultra-deep wells are also increasing. (Wang HG, Huang HC, Bi WX, et al. 2021). For these deep/ultra-deep wells, the traditional drilling methods cannot effectively drill, and it is difficult to meet the needs of rapid deployment of well patterns in the oilfield. (Tian S, Huang Z, Niu J, et al. 2009). Therefore, it is necessary to further study the influence of bottom hole confining pressure on deep/ultra-deep layers. Now, some studies have proved that confining pressure has an effect on rock mechanical properties. When the strain rate remains stable, the confining pressure can improve the compressive strength of rock. (Christensen et al, 1972;Hokka et al, 2016). Under the condition of dynamic loading, the strength characteristics of rock are still affected by the confining pressure. (Wang SM, et al., 2018). In order to study the influence of confining pressure on rock mechanical properties, many scholars have carried out sufficient theoretical research work from the microscopic field. From their research contents, it can be seen that confining pressure can reduce the initiation of cracks and hinder the expansion of cracks, and increase the energy required for crack expansion to cause block fragmentation. (Zhou et al., 2016; Zhou & Yang, 2007; Zhou et al., 2004).