Digital rock advances from a material point method approach for simulation of frame moduli and a sedimentary petrology-inspired method for creation of synthetic samples through simulation of deposition and diagenesis

Abstract

We compare hydromechanical simulation results that use two alternative sources of 3D digital rock input: micro-CT analysis and “synthetic rocks” created by using a newly developed process simulation methodology that more rigorously reflects knowledge from sedimentary petrology compared with previous efforts. We evaluate the performance of these alternative representations using St. Peter Sandstone samples where “dry” static bulk modulus ( K) and shear modulus ( G) are simulated using a new extension of the material point method that resolves contacts using high-resolution surface meshes and considers three alternative contact modeling approaches: “purely frictional,” “fully bonded,” and “cohesive zones.” We evaluate the model performance on two samples from the data set with multiple static moduli measurements (sample 1_2: porosity 24.6 vol%, K 10.2–14.7 GPa, and G 11.6–14.0 GPa; sample 11_2: porosity 12.4 vol%, K 13.5–24.6 GPa, and G 12.8–17.9 GPa). Purely frictional results underpredict measured modulus values, whereas fully bonded results overpredict them. Measured values are most closely approximated by results with cohesive zones that consider sets of discrete spring-like features at contacts. In contrast, shear modulus results from finite-element model simulations on structured grids tend to be significantly greater than measured values, particularly for samples with <18 vol% porosity. Permeability values from digital rock-physics simulations for the studied samples are within factors of 2–5 of conventional core analysis measurements (2860 and 58 mD for samples 1_2 and 11_2, respectively). We determine that the process modeling approach (1) accurately reproduces the measured rock microstructure parameters from thin-section analysis, (2) leads to simulation results for dry static moduli and permeability with accuracy comparable to simulations that use micro-CT samples, and (3) provides a rigorous basis for predicting diagenetically induced variations in hydromechanical properties over the range from unconsolidated sand to indurated rock.