Drawing the Frame of the Shale Barrier Window

Abstract

ABSTRACT Well completion usually implies that long sections of the hole are left with an open annulus between rock and casing. In soft shale formations, the in-situ stresses may be sufficiently large to squeeze the rock up against the casing and seal off the annulus, thus forming a so-called shale barrier. While the shale pressure against the casing enables the shale barrier, it may also result in casing failure if the pressure is too high or uneven. A functional shale barrier may therefore exist within a window, framed by non-barrier on one side and casing failure on the other. This paper describes the principles of a semi-analytical model designed to draw the frame of the shale barrier window. Through a set of examples, the model is used to illustrate qualitatively how the shale barrier window is affected by various parameters such as well path, rock strength and stiffness, in-situ pore pressure and stress anisotropy, and casing quality. It is also shown how the shale barrier window may evolve with time. INTRODUCTION In numerous wells penetrating shale formations, it has been observed that the shale may creep in and close the initially open annulus between rock and casing, thus forming a sealing shale barrier (Williams et al., 2009; Kristiansen et al., 2018). This reduces the risk of leakage along the well and may represent huge cost savings for the operator during P&A operations. A shale barrier may form if the in-situ stresses, which are pushing the shale towards the casing, are able to overcome the fluid pressure in the annulus, which is pushing the shale away from the casing, and the stress arch formed by the hoop stress within the shale around the hole. The strength of this stress arch depends on the shear stiffness and shear strength of the shale. In stronger shales, the stress arch is able to resist the in-situ stresses, thus preventing the forming of a shale barrier. In very soft, weak shales on the other hand, the stress arch provides very little resistance against the in-situ stresses, thus leaving the heavy burden on the casing. This may result in casing failure unless the casing has the proper strength and dimensions.