The Behavior of Fractures and Weak Planes in Response to Thermal and Mechanical Stress Changes During Drilling, Fracturing and Production

Abstract

Fractures are present at some scale in most formations currently being developed onshore North America. In some cases it has been seen that the behavior of fractures in response to stress changes induced by drilling, injection, and production operations can dominate the observed reservoir response. We have investigated several formations that are currently being exploited in North America to better understand the relationship between fracture behavior and measureable fracture properties. In this work we relied primarily on image log interpretation and core analysis to characterize fractures at the well scale. We quantified surface properties such as surface roughness, shear properties, and permeability as measured in core samples and correlated to image log features. We also quantified the thermal dependency of fracture frictional properties to adequately model the response to fracturing and injection scenarios. With some additional constraints, we then constructed a discrete fracture network around the wells to enable geomechanical modeling of 3D stress-strain behavior of the fractured reservoir. In this way we simulated thermal and mechanical stresses induced during drilling, fracturing and injection/production. Various investigators have previously identified several fracture types in the Montney and Duvernay Formations of Western Canada. These include cemented and partially cemented natural fractures, polished tectonic slip faces, and weak cohesive surfaces that are initially intact in the subsurface. Of these, the latter two types can be difficult to identify on image logs and are often overlooked in borehole stability analyses and hydraulic fracture designs. However, these features can be highly sensitive to the shear stresses induced by drilling, fracturing and reservoir depletion. Our results indicate that dilational shear behavior, with associated enhanced permeability, may be a relevant feature in these reservoirs despite their great depth. In addition, we observed that thermal dependency of shear behavior may dominate predictions of shear stability of some fractures and weak planes in the subsurface.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016