The role of rock composition, organic matter and rock fabric as controlling factors for acoustic and geomechanical characteristics of organic-rich shales

Abstract

Shale formations have unique properties that significantly differ from conventional formations because of low permeability, anisotropy and multiscale heterogeneity. These properties are the results of lithology variation, depositional process, diagenesis, fabric, and pore structure as well as distribution and maturity of organic matter in organic-rich shales. In this research, the influence of rock composition, organic matter and rock fabric on acoustic and geomechanical characteristics of organic-rich shales was evaluated by experiments. This thesis develops correlations describing the dependence of acoustic velocity and elastic moduli on stress, rock composition, organic matter, and rock fabric. The stress is defined by vertical stress minus horizontal stress. The correlations obtained from this work are of the following forms: Using laboratory and field data for Eagle Ford, Mancos and Green River shale formations, it was found that stress, rock composition, organic matter, and rock fabric (lamination orientation) were affecting sonic velocities and elastic moduli. The rock fabric was observed to be a dominant controlling parameter in deviated and horizontal laminated shales; therefore, rock fabric should be taken into account when rock properties of deviated and horizontal wells are estimated. Stress had minimal impact on acoustic and geomechanical properties of organic-rich shale especially in low porosity shale formations like the Eagle Ford. The stiff materials (e.g., calcite, quartz) have more influence on shale rock properties than that of soft materials such as organic matter. The correlations were developed at core scale to tie the stress, rock composition, organic matter and rock fabric to acoustic velocities and elastic moduli in organic-rich shales. Using well log data, these correlations were modified to include fluid saturation to lead to an accurate estimation of acoustic and geomechanical properties in both vertical and deviated wells. The correlations also can predict rock and organic matter composition and to probably estimate in-situ pore pressure in shale formations. In addition, three methods for organic matter quantification from well log data were evaluated. Knowing organic matter composition accurately helps in correctly estimating the acoustic velocity and elastic modulus.