Quantitative Evaluation Method and Application of Fault/Fracture Slip Risk Based on Geomechanics
Authors: Zhai Wenbao , Wang Qian , Chen Zhaowei , Feng Xiao , Huang Haoyong , Zhou Xiaojin , Xu Ersi , Yang Xinrui 
Published: September 2022

ABSTRACT:
Casing deformation is a key issue that limits the efficient development of shale gas in the Sichuan Basin, southwest China. In recent years, although the internal and external factors of casing deformation have been identified, the interaction mechanism between the internal and external factors has always been the bottleneck restricting the effective implementation of casing deformation prevention strategies. Therefore, firstly, high-resolution seismic data are used to identify potential faults/fractures and analyze the distribution characteristics of fault/fractures; Secondly, the rock mechanics and in-situ stress parameters are quantitatively obtained by using field multi-source data, and a three-dimensional geomechanical model is established to clarify the distribution characteristics of regional in-situ stress; Then, Based on the mechanical conditions of fault/fracture activation, the assessment method of fault/fracture slip risk based on geomechanics is established by using the Monte Carlo method considering the uncertainty of fault/fracture occurrence, in-situ stress, and other influencing parameters; Finally, taking the area of Ning X well as an example, the new method is used to quantitatively evaluate the fault/fracture slip risk. Meanwhile, the evaluation results and the prediction results based on fault/fracture and in-situ stress distribution are separately compared with the actual casing deformation position. The results show that the coincidence between the assessment results of fault/fracture slip risk based on geomechanics and the actual casing deformation position is more than 85%, while the prediction results are based on fault/fracture and in-situ stress distribution characteristics are less than 80%. The research results are helpful to clarify the interaction mechanism between internal and external causes of casing deformation, which can provide a reference for the field-targeted implementation of casing deformation prevention strategies.

1. INTRODUCTION
During the development of the Changning-Weiyuan National Shale Gas Demonstration Area in the Sichuan Basin, many shale gas wells experienced serious casing deformation(Chen, 2017a; Zhai et al., 2019). By the end of 2018, 113 wells have been fractured in the Changning block, 39 wells of which occurred casing deformation, accounting for 34.5%. As of October 2018, there are 36 casing deformation wells in the Weiyuan block, among 74 wells that have been fractured, accounting for 48.6%. By the end of 2020, the casing deformation ratio of deep shale gas wells in southern Sichuan is 30.77%(Huang et al., 2020). At present, casing deformation is a key engineering challenge that needs to be resolved urgently in the Changning-Weiyuan block. It not only increases the loss of the fracturing section reducing the production of a single well but also easily leads to well integrity problems shortening the life cycle of the wellbore, which seriously restricts the efficient development of shale gas.