Matrix Compressibility and Its Controlling Factors of the Marine Shale Gas Reservoir: A Case Study of the Ning228 Well in the Southwest Sichuan Basin, China

Abstract

Exploring the compressibility of the deeply buried marine shale matrix and its controlling factors can help achieve efficient petroleum production. Taking ten sets of deeply buried marine shale core samples from Ning228 wells in the Yanjin area as an example, the matrix compressibility and pore characteristics of deeply buried marine shale reservoirs were investigated by applying mercury intrusion porosimetry (MIP) and nitrogen adsorption/desorption isotherms at a low temperature of 77 K. Mathematical models (based on MIP and nitrogen adsorption/desorption isotherms) were established to analyze the effects of TOC, mineral components, and pore structure on matrix compressibility. The relationship between the compressibility coefficient and the brittleness index was also established. The results show that the compressibility of the shale matrix is significant when the mercury injection pressure ranges from 8.66 to 37 MPa. For deeply buried marine shale, the matrix compressibility is in the range of 0.23 × 10−4–22.03 × 10−4 MPa−1. The influence of TOC and minerals on matrix compressibility is mainly reflected in the control effect of pore structure. High TOC content decreases the overall shale elastic modulus, and high clay mineral content enhances shale stress sensitivity, resulting in a significant matrix compressibility effect. For the effect of pore structure on compressibility, the pore content in shale has a positive effect on matrix compressibility. In addition, the pore-specific surface area is critical to the effective variation of shale matrix compressibility, indicating that the complexity of the shale pore structure is a key factor affecting matrix compressibility.

Matrix Compressibility and Multifractal Nature of Nanoporous Shale

“…Generally, in shale or coal reservoirs, matrix deformation can be attributed to two common effects: (i) deformation caused by adsorption/desorption with matrix swelling/shrinkage 12,13 and (ii) pressure-induced deformation characterized by geomechanical matrix compression. 15,18 Extensive research has been conducted on adsorption swelling and desorption shrinkage. 12,13,19 However, there is limited research on geomechanical matrix compressibility for shale reservoirs.…”

“…12,13,19 However, there is limited research on geomechanical matrix compressibility for shale reservoirs. 15,20 Compared to nonporous materials, the compression behavior of nanoporous shale is more intricate. Mercury injection capillary pressure (MICP) measurements with the combination of low-pressure N 2 adsorption (LPN 2 A) were employed to evaluate compression deformations in porous materials.…”

“…Pore heterogeneity significantly impacts the storage conditions and flow behaviors of oil and gas . Shale oil and gas recovery would alter the reservoir pressure, inducing response in matrix volumes and pore structures, which in turn determines the matrix permeability and seepage capacity of shale reservoirs. ,− The volumetric response in the shale matrix is not directly accessible but can be estimated through matrix compressibility. − Matrix compressibility serves not only to predict dynamic permeability during extraction but is also closely associated with shale oil and gas production and CO 2 -EO/GR effectiveness . Thus, matrix compressibility and pore heterogeneity are the critical properties of shale reservoirs, yet limited attention has been paid to their quantitative characterization and geological governing factors.…”

“…Generally, in shale or coal reservoirs, matrix deformation can be attributed to two common effects: (i) deformation caused by adsorption/desorption with matrix swelling/shrinkage , and (ii) pressure-induced deformation characterized by geomechanical matrix compression. , Extensive research has been conducted on adsorption swelling and desorption shrinkage. ,, However, there is limited research on geomechanical matrix compressibility for shale reservoirs. , Compared to nonporous materials, the compression behavior of nanoporous shale is more intricate. Mercury injection capillary pressure (MICP) measurements with the combination of low-pressure N 2 adsorption (LPN 2 A) were employed to evaluate compression deformations in porous materials. − Liquid mercury can penetrate the nanopores in dense rock samples due to its extremely low compressibility, making it more resistant to compression compared to the solid matrix. , With the increasing intrusion pressure, the matrix skeleton would be compressed by high-pressure mercury, , and the volumetric deformation from matrix compression would provide noticeable intrusion volumes .…”

“…Figure15. Plots of the multifractal parameters (Δα and H) of pore structure vs the TOC content (a, e), clay content (b, f), quartz content (c, g), and easy-dissolve mineral content (d, h) for the shale samples.…”