Deformation and Failure Characteristics and Borehole Instability Simulation of Ultra-Deep and High-Stress Dolomite
Authors: Li Daqi , Li Ji , Cheng Zengwei , Zhang Yayun , Chen Junhai , Lu Yunhu 
Published: September 2022

ABSTRACT:
Sudden collapses frequently occur in the surrounding rocks of the borehole in dolomite formations deeper than 6000 meters in western China. Research on the discontinuous deformation and failure behavior of dolomite under high stress is of vital significance to clarify the stability evolution and control mechanism of the surrounding rock and to analyze and predict the stability of the borehole. With the help of high-temperature and high-pressure rock mechanics tests, the characteristics of stress-strain curves of dolomite under high-stress conditions are studied, and the damage evolution of dolomite under the condition of ultra-deep, high temperature, high stress, and fluid soaking are summarized. A dolomite borehole stability evaluation model is established to predict the collapse degree of different dolomite boreholes. The study found that: the dolomite deformation experiences the process of pre-peak dilatation and post-peak rupture; the deterioration law of the strength parameters of dolomite is obtained; the prediction results of the borehole stability evaluation model established are consistent with the actual engineering situation, which verifies the accuracy of the model and provides a reference for the borehole stability control of 6000 m deep dolomite in western China.

1. INTRODUCTION
Borehole instability in ultra-deep dolomite formations has become one of the key technical problems in the drilling process. During the drilling process in deep dolomite formations above 6,000 meters in western China, the sudden collapse of the surrounding rock of the wellbore frequently occurs, which seriously restricts safe drilling. Therefore, it is of great significance to study the stability evolution and control mechanism of the surrounding rock in the ultra-deep formation under the conditions of high temperature, high stress, and drilling fluid soaking.
The predecessors have made a series of achievements in solving the problem of borehole collapse. The collapse volume can be determined by using the imaging logging while drilling method, and then the problem of wellbore collapse can be solved by adjusting the drilling fluid density in real-time (Tingay M et al., 2008, Alkamil E H K et al., 2018). By using managed pressure drilling technology, the bottom hole pressure can be changed by adjusting the surface back pressure, so as to solve the problem of wellbore collapse without changing the density of the drilling fluid (Benny B et al., 2016). A probabilistic wellbore stability model can be established based on Monte Carlo simulation to determine the optimal wellbore trajectory to avoid wellbore instability (Al-Ajmi A M et al., 2010). Some scholars regard wellbore collapse as the limit point instability and believe that wellbore collapse occurs at the critical point at the end of the plastic softening stage and the beginning of the residual stage and established a model to determine the collapse pressure (Youquan Y et al., 2009, Zhang L et al., 2020). Previous scholars have done sufficient research to solve the problem of wellbore collapse by accurately collapsing pressure, adjusting bottom hole pressure, and optimizing wellbore trajectory. However, there are relatively few studies on the internal mechanism of the instability evolution of dolomite boreholes under the condition of ultra-deep, high temperature, high stress, and fluid soaking.