A novel collapse pressure model with mechanical-chemical coupling in shale gas formations with multi-weakness planes

摘要

The shale gas reservoirs are usually characterized by tight matrix, rich in clay minerals, well-developed bedding planes and micro-fissures, which makes it highly water-sensitive and strong anisotropic, therefore the most important factors of wellbore instability for horizontal drilling in shale gas reservoirs are mainly the mechanical-chemical coupling. But apart from known problems until now drilling engineers still face the wellbore collapse problems for horizontal drilling in shale gas reservoirs. To overcome the above problem, the main objective of the present work is to propose a novel collapse pressure model with mechanical-chemical coupling. Firstly, the effective stress tensor around a wellbore is proposed based on the pore pressure propagation model and the stress distribution model. Secondly, the failure criterion of shale rock with multi-weakness planes (MWPs) is proposed, and the evolution of rock strength is involved. Thirdly, the effective stress tensor has been transformed into the local coordinate system of each group of weakness planes (WPs) to determine the shear failure of wellbore, and the solution method and computer program of collapse pressure have been developed. Fourthly, the distribution characteristics of pore pressure, water content, strength parameters and their evolution laws are analyzed, and the influence of WPs and soaked period on the failure zones (FZs) and critical mud weight (CMW) is also analyzed. Finally, the present model is applied in a field case study. The results show that the pore pressure increased under the water based mud (WBM) condition, but decreased under the oil based mud (OBM) condition. The pore pressure propagation and the strength variation have significant impacts on wellbore stability, the increase of pore pressure is detrimental to the wellbore stability, and the decrease of rock strength is also detrimental. The horizontal wells, drilling in shale gas reservoirs with MWPs, are more unstable than other intact one. The FZs occur on the wellbore wall during the initial stage, and gradually expand into the inner of formation around the wellbore with time increased. The FZs' shape is very complex when considered the influence of MWPs, instead, the traditional model occurs only on the wellbore wall and presents as a symmetrical crescent shape. The CMW required to prevent collapse gradually increases with time under the WBM condition, but decreases under the OBM condition. The field case study shows that the present model seems to be consistent with the drilling practices. The present model can be used to analyze the pore pressure, water content and shale strength profiles, simulate the FZs around the wellbore, calculate the CMW required to prevent collapse, optimize the mud weight and the well path. Moreover, the present model has been used to guide horizontal drilling of the Lower Silurian Shale Formation in Southern Sichuan Basin, China. (C) 2016 Elsevier B.V. All rights reserved.