Controls on the Geomechanical Properties of Unconventional Resource Formations
Historic interest in the geomechanical properties of shales arose from understanding frac barriers and borehole stability. Little effort was focused on understanding the controls on the geomechanical properties of shales. Now that shales represent a plentiful source of liquid and gas hydrocarbons requiring stimulation, i.e. hydraulic fracturing, there is a renewed interest. Shales are differentiated from mudstones by their fissility which imparts an intrinsic mechanical anisotropy. The degree of anisotropy is strong and often attributed to the clay and organic content; however, the anisotropy is typically ignored, and shales are treated as isotropic elastic materials where required characteristic geomechanical properties are reduced to two elastic moduli, typically Young’s modulus and Poisson’s ratio and failure strength (UCS). Shales present formidable sampling challenges and are often not measured in their preserved state. Moduli measurements can be static or dynamic; logs produce dynamic measurements averaged over the wavelength of the logging tool. The smoothing masks the importance of highly laminated shale interfaces. The static measurements place stricter sample requirements in requiring length/diameter ratios greater than two. To overcome some of these restrictions, researchers have turned to new technologies like nanoindentation and atomic force microscopy to extract geomechanical properties from friable and limited sample quantities, including cuttings. However, these technologies are limited to measurements at ambient conditions and at modest temperatures. The problem with geomehcanical properties is their intrinsic dependence on many independent variables such as saturation, mineralogy, organics, pore pressure, stress levels, etc. and in the case of shale, orientation. The wealth of data reported to date— some 260 measurements of Young’s modulus and Poisson’s ratios and some 417 measures of failure strength—are devoid of the required conditional information to allow trends and systematics to be developed. The collective data sets lack sample orientations, mineralogies and specified testing stress conditions. A very small subset possesses sufficient details to begin to analyze cause and effect, but the numbers are too small to be statistically significant. However, for failure strengths reported as a function of confining pressure, there is a clear increase in strength with applied stress, roughly 2 MPa for each MPa increase in confining pressure. The geomechanical properties of shale are strongly influenced by age; the younger shales and those rich in smectite, tend to be more ductile and cause borehole problems and are more resistant to fracture stimulation. Many of the unconventional shale resource plays are naturally fractured, and these fractures are commonly mineralized. The mineralized fractures are inherently weaker than the host shale and represent the weakest interfaces during stimulation. To understand the geomechanical properties of shales, we need to understand the elasticity of the matrix, the role of anisotropy and natural fractures both filled and open. What is clearly needed going forward is a better and more comprehensive and consistent reporting of sample and test conditions.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019