HOMOGENIZATION MODEL TO DETERMINE CHANGE OF SHALE ELASTIC PROPERTIES DUE TO CO2 INJECTION

The analyses of mineralogical composition of the Middle-Bakken member of the Williston Basin from Field Emission Scanning Electron Microscope, the elastic properties of the shale reservoir were characterized using nanoindentation technique and rock physics homogenization model. Three distinct microstructural zones are identified in the shale reservoir: carbonate matrix, quartz matrix, and clay minerals. The elastic properties of the of different zones were determined using nanoindentation testing. From the statistical analysis of the nanoindentation result, the spatial distribution of the elastic properties was obtained. Rock physics models including the dilute scheme and Mori-Tanaka scheme were used for homogenization. The rock physics model predicted values are compared with dynamic and static elastic properties calculated from density, p-wave and s-wave log values. The sample was exposed to CO2 for up to 60 days and the elastic properties of the mineral grains in the sample were measured using nanoindentation testing. Gridding and spatial distribution approach were used to observe the changes in the mechanical properties of the mineral grains and the changes in the volume fraction of the minerals before and after CO2 saturation. In general, the elastic properties of the sample decreased by 40%. The gridding and spatial distribution showed a volume concentration decrease of the minerals, based on their elastic properties after 60 days of saturation. Discrete distribution map was used to visualize the spatial distribution of the individual mineral grains present in the rock. The discrete distribution map showed the elastic modulus and hardness of the shale reservoir pre-CO2 and post-CO2 saturation. A heterogeneous distribution of both mechanical properties in the shale reservoir was observed. The images roughly display the microstructure morphology of tested area and then indicate the approximate surface fraction of different constituent phases.