Gas injection into and migration through layered sedimentary sequences

Abstract:

Gas migration within sedimentary basins is central to both carbon capture and storage (CCS) and hydrocarbon exploration, both of which involve fluid flow over very large length and time scales (10² to >10⁵ m; 1 to >10⁶ y). Seismic imaging is the primary tool for in-situ gas detection in both cases, but it is limited in resolution and can only provide qualitative information about the presence and distribution of gas. Theoretical models can be a strong complement to seismic observations, providing quantitative tools to inform physical understanding, answer basic feasibility questions, and, in CCS, to assess environmental risk and estimate storage capacity. Here, we develop a new model for gas injection into and migration through a multi-layered sedimentary sequence, where the source may be a wellbore (as in CCS) or a natural influx from a deeper source rock or reservoir. We idealize the sequence as consisting of alternating reservoir-like and seal-like layers, where seals are thinner and more fine-grained. For simplicity, we use a sharp-interface model for gas and water migration within each layer, adopting the assumption of vertical-flow equilibrium. We allow for compressibility in both fluids and, crucially, we also allow for leakage of both fluids across the seals, where the gas is subject to a capillary threshold. The model allows us to better understand how leakage enables inter-layer pressure communication, and how this impacts the evolution of the gas distribution in the sequence both during and after injection. The model also supports lateral and vertical heterogeneity in the permeability field, and in the capillary threshold, allowing us to evaluate the effect of leakage within highly-heterogeneous sequences.