Hydraulic fracturing of MECBM reservoirs for nutrient delivery

Microbially enhanced coalbed methane (MECBM) recovery is a novel method to increase gas production by injecting nutrient with/without microorganisms in depleted CBM wells. However, to be effective, methanogens require that nutrient must be delivered efficiently by aqueous solution to a maximally large reservoir volume for microbial colonization. This study seeks to improve understanding of solute transport, nutrient delivery and microbial gas generation in naturally fractured reservoirs that are both pristine and hydraulically fractured. An equivalent multi-continuum method is adopted to characterize fracture and matrix in coal that is regarded as a dual-porosity dual permeability model. An aperture evolution model includes increments related to shear dilation and hydraulic, mechanical, and propped opening to estimate permeability change. In this study, we mainly investigate permeability evolution and mineral concentration accumulation in different fracture types during injection. A field-scale numerical simulation is established to define the effectiveness of nutrient delivery in real reservoir. The complex pre-existing fracture pattern in the coalbed is represented by an overprinted discrete fracture network (DFN) to capture the natural heterogeneity and anisotropy of fracture permeability. A simplified PKN model is adopted to simulate hydraulic fracture propagation based on linear elastic fracture mechanics (LEFM). We find that the natural fracture network plays a significant role when stimulating the MECBM reservoir, at all scales, and impacts the evolution of the hydraulic fracture. Based on the simulated cases, hydraulically stimulated fracture pathways, especially when connecting with a natural fracture network, optimally deliver nutrient remote from the injection well, thereby increasing nutrient delivery. However, large proppant embedment occurring at low strength coal plays an important role in impeding nutrient delivery within propped fractures.