Effect of Parent Well Production on Wellbore Breakout of Child Well in Coalbed Methane Reservoirs

ABSTRACT: The accurate estimation of parent well production induced in-situ stress change is of great significance for evaluating wellbore breakout of child wells. For conventional reservoirs, poromechanical effect is considered to be the main factor that controls in-situ stress evolution. However, for coalbed methane (CBM) reservoirs, gas desorption-induced coal shrinkage also contributes to the stress change, which is rarely included in the theoretical models. In this work, the in-situ stress evolution during coalbed depletion is first investigated through combining the effect of poromechanics and gas desorption. Then, the calculated stresses are incorporated into the geomechanical model to analyze the effect of gas desorption-induced coal shrinkage on wellbore breakout of child wells. The obtained results indicate that with pore pressure decreasing, both the maximum and minimum horizontal stresses decrease at a larger rate when gas desorption effect is considered. For vertical wellbore section of child wells, the larger Langmuir strain leads to the smaller breakout length and width, while for horizontal wellbore section of child wells, the larger Langmuir strain leads to the larger breakout length and smaller breakout width. The modeling results offer new insights for understanding the wellbore breakout of child wells in CBM reservoirs. 1. INTRODUCTION With the worldwide energy consumption rate steadily increasing, more drilling practices are taken place in the mature oil and gas fields (Yang and Liu, 2021). For instance, in CBM reservoirs, infill-wells are drilled and stimulated around the parent-wells to improve the production performance of the depleted formation (Salmachi et al., 2013). When an infill-well is drilled, stress concentration will occur around the borehole which may damage the rocks if the concentrated stress exceeds the rock strength (Zoback et al., 1985). Breakout is a typical type of wellbore instability problem during the drilling process (Mohammadali et al., 2021) and, in vertical wells, borehole enlargement orientation is consistent with the minimum horizontal stress direction, as shown in Fig. 1. However, in depleted reservoirs, pore pressure decline alters the original in-situ stresses and thus the stresses around the wellbore. Therefore, drilling mud weight window changes for the new wells and also the wellbore breakout shape will be different from the old ones.