Widened Safe Mud Window for Naturally Fractured Shale Formations: A Workflow and Case Study

Abstract

This work briefly reviews the literature reported in the References section on a widened mud weight window (MWW) by wellbore strengthening, including both hydraulic plugging and stress cage for shale formations with natural fractures. The workflow for designing the mud density for hydraulic plugging and the effectiveness of stress caging are presented. An example of widening the MWW is presented with a simplified model for a well section in a shale gas formation in a field in southwest China. With numerical simulation, the following conclusions are obtained: (1) the task of widening the MWW with the value of 1.66 lbm/gal can be realized by opening the fracture to 1.285 mm with hydraulic plugging; (2) the mud weight pressure required to create this value of 1.285 mm fracture opening is 39 MPa, which corresponds to a 15-lbm/gal mud weight gradient (i.e., 1.81 g/cm3 mud density); (3) because the value of the fracture opening is determined to be 1.285 mm, the size of the lost circulation materials (LCMs) is chosen accordingly. The case study described in this paper provides a workflow of widening the MWW for safely drilling through a naturally fractured shale formation. A best practice of this type of MWW design is also presented.