Predicting Geomechanical Behavior: Empirical Relationships Between Mudstone Strength and Fluid Saturation

Abstract

Although mudstone is one of the most abundant rock types, its mechanical behavior has not been sufficiently quantified. This is due in part to inadequate preparation and moisture preservation of recovered samples as well as the inherent difficulties of working with these fragile samples. Developing a reasonably prescriptive set of procedures for sample recovery and preparation would benefit numerous disciplines (from geotechnical to geoscience), but is aimed herein at improving hydraulic fracture prediction in unconventional resource development. Geomechanical properties, including peak compressive and tensile strengths, Young’s modulus, and Poisson’s ratio of dry and saturated mudstone samples were obtained through systematic indirect tensile, unconfined compression and triaxial compression testing of plugs from a variety of mudstone sedimentary facies types. Based on the results from this testing program, we have developed a set of empirical relationships to predict how in-situ water saturation (Sw) conditions will control the geomechanical properties of a mudstone body. Overall, increased fluid saturation decreases maximum load bearing capacity. We observed a decrease between approximately 39 to 75% in peak strength between dry samples and water-saturated samples. Since different facies exhibit different strength to water saturation relationships, we developed a discrete linear function describing dry to saturated strength values per facies. This protocol can be applied to other lithofacies and different in situ conditions. The most drastic change in strength based on Sw (i.e., the function with the largest regressed variation) was observed within nominally structureless mudstone (F1-measured with horizontal plugs). Similar regressive functions were observed for heterolithic sandy mudstone (F2-measured with both horizontal and vertical plugs) and in bioturbated, sandy mudstone (F3-measured with horizontal plugs). Heterogeneous facies display a more gradual reduction in peak strength between dry and saturated samples. Strength to porosity plots show no distinct functional saturation dependences, indicating that porosity alone has little influence on the strength properties of mudstone within this dataset. This protocol provides predictive potential for geomechanical behavior in mudstones during induced fracturing and will ideally allow for more efficient planning of hydraulic fracture treatment design, lowering costs and reducing fresh water usage.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018