Quantitative Evaluation Method and Application of Wellbore Stability Based on Geomechanics

Abstract

ABSTRACT: Wellbore instability is one of the difficult problems restricting safe drilling. The traditional theoretical model of wellbore stability is easy to cause the actual drilling fluid density to be high and aggravate the risk of drilling lost circulation. Therefore, firstly, the rock mechanics parameter profile is quantitatively obtained according to the field multi-source data, and the single well geomechanics model is established; Then, the Monte Carlo method is used to comprehensively consider the uncertainty of geomechanical parameters, and a quantitative evaluation method of wellbore stability based on geomechanics is established to reasonably evaluate the wellbore stability probability and optimize the safe density window of drilling fluid; Finally, taking well X of an oil field in Western China as an example, the new method is used to quantitatively evaluate the wellbore stable section and unstable section, and the evaluation results are compared with the actual complex situation to dynamically optimize the density window of drilling fluid. The results show that the calculation results of the new method are consistent with the actual situation, and the probability of wellbore stability is more than 80%. This method can effectively evaluate the wellbore stability, and provide a reasonable density window of drilling fluid, reducing the drilling complexity and cost, which is of great significance to meet the requirements of safe and efficient drilling. 1. INTRODUCTION Drilling engineering is an indispensable and important link during oil and gas energy exploration and development, but wellbore instability causes frequent drilling complex problems, which seriously restricts the drilling cycle. Therefore, a reasonable evaluation of wellbore stability is conducive to broadening the safety density window of drilling fluid and effectively reducing drilling complexity, and shortening the drilling cycle(Darvishpour et al., 2019). In recent years, many scholars have made comprehensive analysis on the factors that may affect the wellbore stability(Germanovich and Dyskin, 2000; Germanovich et al., 1994; Li et al., 2022; Mokhel et al., 1996; Moos et al., 2003; Tran and Abousleiman, 2010; Zoback et al., 1985). Traditionally, it is considered that wellbore instability is caused by far-field stress conditions or by stress acting on a natural plane with too little or no cohesion(M. Zoback et al., 1985), which is related to shear and yield. For plastic rocks, the wellbore collapse can be described by using the plastic yield criterion(Germanovich and Dyskin, 2000), but for brittle rocks, the description of wellbore collapse can not only consider the shear failure hypothesis but also consider the shape of rock fragments, i.e. tensile failure and brittle failure (Germanovich et al., 1994).