A Reconstructed Core-Scale Model of the Lower Eagle Ford Shale Through FIB-SEM, SEM-EDS, and Microscopy for Gas Huff-n-Puff Simulation

Abstract

Abstract Simulation of gas huff-n-puff (HnP) in core models is often simplified by assuming homogenous, isotropic models, which are not representative of fine-grained multi-component shales with multiple types of pore space and anisotropic permeability. According to the previous literaturte, history matching of a 6-cycle HnP process and tuning of cross-phase effective diffusion coefficients using a homogeneous model presented a need for a detailed core-scale geomodel to capture variations in effective diffusion coefficients as a result of heterogeneity experienced during each diffusion cycle. A workflow for reconstructing a corescale volume of Lower Eagle Ford (LEF) shale is presented that combines rock characterization using high-resolution digital images at the micrometer/nanometer scale and transmitted light microscopy to provide models for detailed heterogeneity of the LEF shale for gas huff-n-puff simulation. Core-scale models form LEF are reconstructed using high-resolution SEM-EDS and FIB-SEM images. SEM-EDS and FIB-SEM images were coupled with transmitted light microsocpy and micro-computed tomography (CT) to constrain facies distribution, discrete fracture networks and to distribute petrophysical properties. The reconstruction method produced a more representative core model of the LEF and can be used for simulation of core-scale huff-n-puff experiments conducted in a diffusion cell. Common grain types observed in the SEM-EDS images at different resolutions include detrital quartz, calcite, microfossils and fragments, microfractures, clay minerals, traces of dolomite, and albite. Also common is diagenetic calcite, pyrite, kaolinite, and secondary organic matter (OM) infilling the chambers of foraminifer tests ("forams") and other mineral pores. FIB-SEM analysis provided organic bulk transport properties including tortuosity and permeability within the generated pore network models of the LEF shale. Transmitted light microscopy provided the lamina-scale variation of samples from the LEF shale to be incorporated in the core model. A detailed workflow is presented for reconstructtion of a LEF shale core-scale model using high- resolution images and photomicrographs. The model detailed the original core morphology, multiple pore space, and multicomponent mineral distributions for gas huff-n-puff simulation. Several realizations of the geomodel were prepared by varying mineral distributions within the layers as well as the range of petrophysical properties in order to present different gas huff-n-puff simulation scenarios. This provided the compositional model using a sophisticated heterogeneous component of tortuosity/permeability instead of a homogenous and isotropic model.