Geological Control of Irreducible Water Within the Coal Matrix and Its Quantified Evaluation Model

Abstract

This paper adopts the measurement of mercury intrusion porosimetry and nuclear magnetic resonance (NMR) to analyze the pore system and the pore structure of coal samples, and the measurement of maceral group composition, scanning electron microscopy, and energy dispersive X-ray spectroscopy to obtain the organic/inorganic composition of coal samples. Gravimetric and NMR methods are both used to calculate irreducible water saturation of the samples, and qualitative and quantitative research studies are therefore conducted. The following knowledge is obtained. Coal samples can be classified as micro-trans-pore-dominated samples, meso-macro-pore-dominated samples, cleat-dominated samples, and even development samples. The main composition of the samples is organic, and a little kaolinite and pyrite can be observed. Irreducible water saturation obtained by the gravimetric method is almost close to that gained by the NMR method. The influencing parameters can be divided into two categories. The first category contains the maximum vitrinite reflectance, volumetric factor, fixed carbon yield, volatile yield, vitrinite percentage, and inertinite percentage, which have a strong correlation with irreducible water saturation. The second category includes the buried depth and median radius, and they have a weak correlation with irreducible water saturation. Multivariate regression shows that there is a linear quaternion equation between irreducible water saturation and independent variables such as maximum vitrinite reflectance, volumetric factor, volatile yield, and vitrinite percentage.

Review on the Application of Low-Field Nuclear Magnetic Resonance Technology in Coalbed Methane Production Simulation

“…Before the pressure drops to the critical desorption pressure of methane in coal reservoirs during CBM drainage, the increase of effective stresses is the dominant factor leading to pore–fracture structure variation of coal reservoirs. − To separately describe the stress sensitivity of pores with different diameters under the effective stress, Li et al initially used NMR to obtain the T 2 spectrum distribution of low-, medium-, and high-rank saturated coal samples (25 × 50 mm cylindrical samples) by increasing the confining pressure; the volumetric compressibility coefficients of adsorption pore, seepage pore, and fracture were calculated; a volumetric compressibility coefficient model using LF-NMR tests was constructed. The results show that the compressible space of adsorption pores in the same coal sample was significantly lower than that of seepage pores and fractures, and the corresponding pore volume varied exponentially with increasing stresses.…”

“…Some scholars think that the D A value less than 2 does not satisfy the fractal range, and therefore the adsorption pores are not considered to have fractal characteristics . Other scholars think that this pore size range has fractal characteristics and can be used to calculate the fractal dimension value to analyze the non-homogeneous variation of pore distribution of adsorption pores . The coexistence of two seemingly contradictory conclusions has also affected the application and expansion of fractal models. The applicability of the fractal model to characterize the dynamic changes of pore–fracture inhomogeneity needs to be further validated.…”

Geological evaluation and gas bearing analysis for shale reservoir in Ziyun-Luodian typical area of southern Guizhou province, China

“…The figure indicates that the gas content has a positive and linear relationship with clay mineral, with a fitting degree of about 0.4. There are many pores developed within clay minerals, which supports effective space for gas storage (Hill and Milburn, 1950;Aringhieri, 2004;Wang and Reed, 2009;Zou et al, 2020). Therefore, clay mineral has a positive relationship with gas content.…”

Classification of Pore–fracture Combination Types in Tectonic Coal Based on Mercury Intrusion Porosimetry and Nuclear Magnetic Resonance

“…It is generally considered that three spectral peaks with relaxation times at 0.5–2.5, 20–50 ms, and greater than 100 ms separately represent micropores, mesopores, and macropores and microfractures. 10 − 13 The SEM observation method can be used to directly observe the morphologies of nano-/micropores on the surfaces of coal samples 14 , 15 but cannot characterize the three-dimensional (3-D) structure of coal.…”

“…For high-pressure MIP, by analyzing types of mercury injection and ejection curves and combining the data with fractal theory, critical values for distinguishing diffusion from seepage pores are separately determined and connectivity between pores and fractures measuring more than 7.2 nm is evaluated. − The liquid nitrogen adsorption method describes and characterizes the distribution of pores with a size of 2–100 nm and adsorption characteristics of pores to nitrogen by analyzing morphologies of hysteresis loops on adsorption and desorption curves and calculating fractal dimensions. − As for the NMR method, by measuring 1 H signals of fluid in rock and coal and plotting T 2 spectra, the types of pore structures in coal were classified. It is generally considered that three spectral peaks with relaxation times at 0.5–2.5, 20–50 ms, and greater than 100 ms separately represent micropores, mesopores, and macropores and microfractures. − The SEM observation method can be used to directly observe the morphologies of nano-/micropores on the surfaces of coal samples , but cannot characterize the three-dimensional (3-D) structure of coal.…”