No abstract

Mercury intrusion porosimetry (MIP) has been utilized for decades to obtain the pore size, pore volume and pore structure of variable porous media including inorganic rocks and organic rock (e.g., shales and coals). Diffusivity and permeability are the two crucial parameters that control gas transport in coals. The main purpose of this work is to derive the CH 4 effective gas diffusivity and permeability in different rank coals with vitrinite reflectance of 0.46-2.79% R o,m by MIP. Furthermore, regular CH 4 diffusivity and permeability measurements are conducted to compare with the results of the derived CH 4 diffusivity and permeability with MIP data. In this work, CH 4 diffusivity and permeability of different rank coals are acquired with established equations, which are basically in accordance with the experimental values. However, the coal rank (maximum vitrinitere flectance, R o,m) exhibits no significant relation to the effective diffusion coefficient (De) and gas diffusivity (D). The cementation factor (m values) varies from 2.03 to 2.46, which tends to exhibit a semi-consolidated structure for coals compared with other rocks (e.g., dolomite, limestone, sandstone and red brick). The results show that the cementation factor could be an important factor for gas flow in coals. The correlation of CH 4 diffusivity to porosity and permeability of 12 coal samples were explored, and it appears that CH 4 diffusivity exhibits an increasing trend with an increase of permeability, and two different exponential relationships respectively exist in diffusivity versus porosity and permeability versus porosity. Therefore, this study could be conducive to gas sequestration or gas production during enhanced coalbed methane (CBM) recovery.

Quantitative characterization of multiphase methane and investigation of the methane dynamic adsorption process of coals were performed by a low-field nuclear magnetic resonance (NMR) method. Meanwhile, methane diffusion behaviors during the step-by-step pressurization adsorption process were evaluated by three diffusion models. The results indicate that the transverse relaxation time (T 2 ) spectra of methane demonstrate two distinct peaks of adsorbed methane (P1, T 2 < 2.5 ms) and porous mediumconfined methane (P2) at a low-pressure step (∼1.0 MPa), and the third peak of bulk methane (P3) obviously appears when the pressure >2.0 MPa. The integrated T 2 amplitude of adsorbed methane increases quickly during the first 2 h (>75% of total) and then gradually reaches a maximum value in the last 4 h during the initial pressure step of ∼1.0 MPa, whereas it reaches >90% of total amplitude in 1 h as the pressure is increased step-by-step. According to the strong linear relationship between the adsorbed methane volume and the integrated T 2 amplitude, the real-time methane adsorption volume can be determined, and adsorption isotherms from the NMR method are found to be mostly overlapped with those of the volumetric method. Moreover, the effective diffusion coefficient of the unipore model (10 −6 to 10 −5 s −1 ) coincides with the micropore diffusion coefficient of the bidisperse model and the slow diffusion coefficient of the multiporous model, which is generally 1−3 orders of magnitude less than the macropore diffusion coefficient (10 −3 to 10 −2 s −1 ) and the fast diffusion coefficient (10 −2 s −1 ) or transitional diffusion coefficient (10 −4 to 10 −3 s −1 ). The dynamic changes in diffusion parameters with pressure may be related to the comprehensive effects of methane diffusion mechanisms and coal matrix swelling under different adsorption pressures.

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.