Investigation of mass transfer in fractured shales

CH₄ and the other hydrocarbons are stored in the adsorbed state in the micro and mesopores of the shale rocks, and as free gas in the fracture networks. Although convective and diffusive transport accounts for the short-term behavior during shale gas production, desorption is thought to dominate the long-term dynamics of shale-gas generation. Out of all the secondary components of natural gas, C₂H₆ is typically the second largest component of natural gas and is thought to compete for the same adsorption sites as CH₄.

Therefore, it is important to study the role of competitive sorption of CH₄ and C₂H₆ during shale gas production. ❧ The key objective of this study is to investigate the interplay between adsorption/desorption and diffusive and convective transport phenomena of methane/ethane binary gas mixtures in gas shales, and its impact on the short-term and long-term shale gas recovery. ❧ Evaluating the pore structure of shales presents technical challenges due to the presence of a range of pores from the nanometer to the micrometer size.

Characterization of the entire range of pore sizes requires an all-inclusive study employing a variety of techniques. Such an integrated approach is presented in this work to understand the pore structure of Monterey shale samples, as obtained from the following characterization techniques: (i) Mercury porosimetry, (ii) Nitrogen adsorption experiments, (iii) High-Resolution X-ray Computed Tomography (HRXCT), and (iv) Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM).

These techniques are coupled with gas permeability measurements of the Monterey shale samples, as well as spontaneous water-air imbibition experiments. Calculated permeability values (via Lattice Boltzmann flow simulations) based on the pore characterization data are in good agreement with the gas permeability measurements. ❧ Next, in conjunction with the work done by our group using powdered shale, we study the adsorption/desorption behavior of pure CH₄ and C₂H₆, and their mixtures in a shale cube sample of ~1cm³ from the Marcellus formation, using Thermogravimetric analysis (TGA).

To do this, we first measured the steady state isotherms of the pure component gases and their binary mixtures, which are essential to predicting the gas storage capacity of the shale. We then analyzed the dynamics of reaching the steady state during the adsorption process to better understand the role of desorption during the later times of shale gas production. We used the well-established Langmuir model to predict the gas isotherm data and calculate the shale’s gas sorption capacity and sorption equilibrium constant.

This, in turn, facilitates our modeling and interpretation of the dynamic experimental observations, for which, we propose and execute a two-dimensional transport and Langmuir-type of sorption dynamic model. ❧ We succeed this by aiming to provide an improved approach to characterize gas shales’ petrophysical and transport parameters such as porosity, permeability, diffusivity, and storage capacity, using helium (He) and argon (Ar) in larger samples.

To this end, we performed gas expansion experiments using a full-diameter core and thermogravimetric analysis (TGA) using the same shale cube as used earlier, both from the same depth/location in the Marcellus shale formation. The experimental results show that argon (Ar), which has a similar sorption potential as methane, but is generally assumed to be inert, adsorbs onto the walls of the mesoporous and microporous regions of the shale samples as a monolayer of argon molecules, thus displaying Langmuir type sorption behavior.

We applied the Langmuir approach to model the argon isotherm data to calculate the shale’s gas sorption capacity and sorption equilibrium constant. A bi-disperse pore model (BPM) was then implemented to interpret sorption and transport in the shale cube. Assuming the shale cube represents a single matrix block of the core, the cube model was then upscaled to model the full-diameter core, while accounting for the fracture-matrix interactions via a dual-porosity formulation. ❧ Lastly, we simulate a model shale gas in our laboratory to understand the mass transfer of methane (CH₄) and methane-ethane (CH₄-C₂H₆) binary mixture (90-10mol%) during early and late times of production.

We do so, by performing “step-wise” pressure depletion experiments using the same full-diameter shale core. We performed depletion experiments using pure CH₄ while continuously monitoring the backpressure and the produced gas flow rate. This was subsequently followed by similar experiments using CH₄-C₂H₆ binary gas mixture, with continuous monitoring of the produced gas composition, in addition to the backpressure and the produced gas flow rate.

The binary mixture and pure component depletion experiments together help us to investigate the effect of C₂H₆ on the more desired CH₄ production during shale gas recovery, due to preferential sorption of C₂H₆. In accordance with characterizing the shale cube and the full-diameter core using helium and argon, the same bi-disperse numerical model was then used to describe the gas production from the shale core. ❧ The experimental observations and their analyses in this work pave a path to improving the interpretation of production data from shale-gas wells by leveraging an improved understanding of desorption dynamics and mass transfer of natural gas mixtures in shale.