An assessment of methane gas production from natural gas hydrates: Challenges, technology and market outlook

The negative environmental impact of carbon emissions from fossil fuels has promoted hydrogen utilization and storage in underground structures. Hydrogen leakage from storage structures through wells is a major concern due to the small hydrogen molecules that diffuse fast in the porous well cement sheath. The second-order parabolic partial differential equation describing the hydrogen diffusion in well cement was solved numerically using the finite difference method (FDM). The numerical model was verified with an analytical solution for an ideal case where the matrix and fluid have invariant properties. Sensitivity analyses with the model revealed several possibilities. Based on simulation studies and underlying assumptions such as non-dissolvable hydrogen gas in water present in the cement pore spaces, constant hydrogen diffusion coefficient, cement properties such as porosity and saturation, etc., hydrogen should take about 7.5 days to fully penetrate a 35 cm cement sheath under expected well conditions. The relatively short duration for hydrogen breakthrough in the cement sheath is mainly due to the small molecule size and high hydrogen diffusivity. If the hydrogen reaches a vertical channel behind the casing, a hydrogen leak from the well is soon expected. Also, the simulation result reveals that hydrogen migration along the axial direction of the cement column from a storage reservoir to the top of a 50 m caprock is likely to occur in 500 years. Hydrogen diffusion into cement sheaths increases with increased cement porosity and diffusion coefficient and decreases with water saturation (and increases with hydrogen saturation). Hence, cement with a low water-to-cement ratio to reduce water content and low cement porosity is desirable for completing hydrogen storage wells.

There are limited comparative studies on modeling fluid transport in fractured porous media. Hence, this paper systematically compares the steady-state creeping flow Stokes–Brinkman and Darcy–Darcy models for computational efficiency and accuracy. Sensitivity analyses were also conducted on the effect of fracture orientations, fracture sizes, mesh resolution, and fractures with Local Grid Refinement (LGR) under the FEniCS computational framework. Both models were validated numerically, and the accuracy of their solution is compared using the R-squared metric and L2 norm estimates. Key results showed that both models have similar pressure and velocity field solutions for a given fracture orientation. The computational time required for solving the Stokes–Brinkman models for a single fracture case was unusually lower than that of the Darcy–Darcy model when the pressure and velocity terms in the Darcy–Darcy model were solved simultaneously using two equations, contrary to where only one equation solves for the pressure and the velocity is obtained by projecting the gradient of pressure onto a vector space. The Stokes–Brinkman model is more sensitive to mesh resolution, and as a result, the Darcy–Darcy model tends to be more accurate than the Stokes–Brinkman model at low resolutions. Local Grid Refinement (LGR) can improve the Stokes–Brinkman model's accuracy at low mesh resolution. Furthermore, both models showed similar results when compared for complex fracture systems such as multiple fracture cases: interconnecting and isolated fractured porous media systems under low-velocity and steady-state creeping flow conditions. The FEniCS code in this paper is shared for future researchers to reproduce results or extend the research work.

Most numerical simulations for modeling acid reactive fluid transport and wormhole propagation during matrix acidizing, waterflooding, and CO₂ sequestration in carbonate formations are computationally expensive, limiting real-time reservoir management and deep learning training datasets generation for inverse modeling research. Therefore, there is a need for less computationally expensive acid-reactive fluid flow models with adequate accuracy. This study developed and validated a simplified acid reactive-transport model by integrating a simplified Stokes–Brinkman model (as opposed to Darcy’s law), an averaged continuum model, and a pseudo-fracture model. Using FEniCS, the model effectively simulates acid-reactive fluid transport and wormhole propagation in carbonate rocks, achieving a high R-square value of about 0.97 based on a quantitative comparison of the breakthrough volume with other models. The simplified model can also simulate wormhole propagation for the reciprocal of the Damköhler number (1/Da) ranging from 0.001 to 1 with adequate accuracy. Sensitivity studies on the natural fracture parameters such as orientation, length, width, and density showed that higher fracture density, wider fracture aperture, longer fracture length, and orientation aligned with the direction of acid injection contribute to lower pore volume to breakthrough ratio but may not increase long-term acid stimulation efficiency. Also, the presence or absence of fractures in the matrix does not alter the dissolving patterns and optimum injection rate. This simple acid reactive-transport model can generate large training datasets for developing surrogate models in deep learning research. Finally, the FEniCS code in this paper is shared so future researchers can reproduce the results or extend the research work.

Hydraulic fracturing creates long-lasting contact between aqueous fracturing fluids and gas-bearing ultra-low permeability matrix rock, making fluid retention and dissolved-species penetration important for interpreting matrix–fracture mass exchange and post-fracturing flowback behavior. These processes are controlled by the coupled effects of capillary imbibition, fracture–matrix pressure assistance, and species diffusion, but their relative importance is difficult to distinguish through a concentration profile. This study develops a regime-adaptive pressure-assisted imbibition and species-transport framework for interpreting fracturing-fluid species movement from a hydraulic fracture into a gas-bearing porous matrix. The framework couples a pressure-assisted imbibition (PAI) model with a one-dimensional advection–diffusion equation (ADE), where the PAI-derived velocity provides the transient advective input for species transport. The same formulation continuously represents capillary-dominated, mixed pressure–capillary, and pressure-dominated behavior without switching governing equations. Three diagnostic limiting cases, Capillary-driven Advection, Pressure-driven Advection, and Diffusion-only, are introduced to compare reduced mechanism-specific responses with the Full PAI-ADE response without interpreting them as additive components of the full solution. At 100 days, the Full PAI-ADE concentration front in the Ultra-Low Matrix reached 31.9 cm at $C=0.10$, compared with 18.0 cm for Capillary-driven Advection, 9.2 cm for Pressure-driven Advection, and 21.7 cm for Diffusion-only, indicating a mixed diffusion–capillary response. For the High-Perm Matrix at 0.20 day, the Full PAI-ADE and Pressure-driven Advection fronts reached 211.6 and 210.9 cm, respectively, with a normalized profile error of 0.003, identifying a pressure-advection-dominated limiting behavior. A dimensionless mechanism-weight analysis based on the pressure–capillary ratio (${\mathsf{\Pi }}_{\mathrm{p}\mathrm{c}}$) and front-scale Péclet number (Pe) further summarizes these behaviors. The results demonstrate that the proposed framework provides a compact diagnostic tool for linking pressure–capillary driving, advective–diffusive tendency, and observable species-transport profiles in ultra-low permeability gas-reservoir applications.