This study evaluates gas transport through matrix pores in Barnett and Haynesville shale gas plays. Gas is mainly stored in a compressed phase in small pores or as a desorbed phase on the surface of the pores. At the early stage of production, gas decompression/expansion within networks of large fractures connected directly to the wellbore is the dominant production mechanism; however, after some time (a couple of weeks up to few months) diffusive flux from the body of kerogen is triggered in response to the pressure drop realized at the fracture face. In this study we present a comprehensive study of the importance of various diffusion patterns to predict the productivity of shale reservoirs at the later stage of production.
We present transient diffusion models to address gas diffusion from kerogen bodies through connected pores with different shapes (slit or cylindrical) toward fractures. In this model we use a new set of boundary conditions as opposed to traditional Dirichlet and Neumann types of boundary conditions. The analytical solution is validated using a set of experimental data.
In the next step, we choose wells from gas producing parts of Barnett and Haynesville shale plays. Based on detailed studies of scanning electron microscope (SEM) images, we identified two different governing patterns for diffusive flux in the shale matrix: inter diffusion in Barnett and intra diffusion in Haynesville. We used production data to differentiate the matrix contribution using conventional methods of rate transient analysis. Using the analytical model, we compared the matrix contribution parts of production data between Barnett and Haynesville shale plays. We found an excellent match between observed small-scale pore shapes and field production data in our transient diffusion model. Based on results, the inter diffusion pattern provides greater flux; while everything else remains equal, Barnett wells benefit more from matrix contribution than those of Haynesville. Results indicate that the distribution of connected pores in the shale matrix has significant effect on the matrix contribution.
The main contribution of this work is providing a transient flow model for gas transport through shale matrix accounting for the impacts of dominant shape of the connected pores and their abundance in the matrix texture (namely: organic or inorganic part). Our results suggest that the inter diffusion in Barnett shale play outperforms the intra diffusion mechanism in the Haynesville field. In addition, the more cylindrical pores, the better the performance. In general, the topology of the connected pores as well as their abundance within kerogen or inorganic matters can greatly influence long-term productivity of gas wells.