Water migration and methane desorption characteristics directly affect the performance of coalbed methane wells. In this paper, migration and desorption variability of methane and water in adsorption pores, seepage pores, and fractures were studied by laboratory simulation using an improved nuclear magnetic resonance displacement device. The results are as follows: Both adsorbed and bulk methane decreased logarithmically with the increase of the desorption time under the condition of one-stop desorption. The desorption process can be divided into the early rapid decline stage and the later slow desorption stage. In comparison to one-stop desorption, step-by-step depressurizing desorption can effectively increase the loss rate of the methane amount. For the two desorption modes, the variation rate of bulk methane is much higher than that of adsorbed methane at the same desorption time. The sensitivity of large pores to displacement nitrogen pressure is stronger than that of adsorption pores. In the process of methane displacement by water, the variation of bulk methane is larger than that of adsorbed methane, whereas the variation of adsorbed methane is more sensitive to injecting water pressure than that of bulk methane. The above results indicate that the quantity of water injected into the coal seam and the water drainage rate have an effect on methane desorption in the adsorption pore. Therefore, the parameters of depressurization value should be fully considered in the drainage system setting of coalbed methane wells.

Nuclear magnetic resonance (NMR) T2 cutoff value is an important parameter for pore structure evaluation. It is complicated and uneconomical to obtain T2 cutoff value by an experimental method; therefore, it is necessary to explore a prediction method of T2 cutoff value. In this paper, 10 samples of tight gas reservoirs in the eastern Ordos Basin were selected, and then saturation and centrifugal experiments of nuclear magnetic resonance were carried out. On this basis, multifractal theory was introduced to calculate the multifractal characteristics of the NMR T2 spectrum of each sample, and the relationship between multifractal parameters and T2 value was analyzed. The influencing factors of the T2 cutoff value were clarified, and the prediction model of the T2 cutoff value was constructed accordingly. The results show that the T2 spectra of sandstones in the study area can be divided into three types: single steeple peak, double steeple peak, and irregular double peak. The pore diameter of the three types is 1 nm ~ 3×104 nm, 1 nm ~ 104 nm and 1 nm ~ 4×103 nm, respectively. The T2 cutoff value ranges from 9.72 to 35.16 ms. The correlation analysis suggests that the symmetrical fractal dimension difference and symmetrical multifractal dimension ratio (Dmin−Dmax, Dmin/Dmax) shows a positive linear correlation with the T2 cutoff value. The value of T2 cutoff gradually decreases with the increase of the flow zone indicator (FZI). Therefore, three parameters, including symmetrical fractal dimension difference, symmetrical multifractal number ratio, and FZI are optimized, and the prediction model for the NMR T2 cutoff value of sandstone samples in the study area is proposed. The introduction of porosity‐related parameters compensates for the shortcomings of previous T2 cutoff value prediction models. At the same time, the prediction model is proven to be accurate and reliable by testing the measured data of the samples near the study area. The results of this paper can be used for further study of the NMR T2 cutoff value prediction of tight sandstone reservoirs in different areas.

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Methane adsorption of coal is essentially a process of energy and material state transformation. It is a complex process especially for tectonically deformed coals (TDCs). In order to clarify this process, 12 middle-rank TDCs were screened for the experimental and theoretical study. On the basis of the liquid nitrogen adsorption experiment and methane isothermal adsorption experiment, the nanopore structure and the methane adsorption capacity of the TDC samples were analyzed. Using the theories of Polanyi adsorption potential and surface free energy reduction (SFER), the variation of energy in the process of methane adsorption in nanopores was explored. The following results were obtained. (1) The Langmuir volume has a stronger positive relationship with the specific surface area of the ultra-micropore, mild-micropore, and micropore than with those of the transitional pore. The adsorption potential decreases as the adsorption space grows; however, it rises as tectonic deformation enhances. The increase in the ratio of specific surface area for the mild-micropore and ultra-micropore indicates that the tectonic deformation can enhance both the available adsorption area and adsorption potential for methane in TDCs, eventually resulting in the increase of the Langmuir volume and the SFER. (2) The methane molecules preferentially occupy the sites with strong adsorption potential on the coal surface during the strong adsorption stage, resulting in the sharp increase of the methane adsorption volume and SFER. Then, the number of sites with strong adsorption potential decreases and the methane molecules have to settle at the energetically suboptimal sites during the weak adsorption stage, leading to the slight increase of methane adsorption volume and SFER. The above results may help to improve the accuracy of coalbed methane resource estimations and gas outburst prediction in the area where TDCs are developed.

Low-field nuclear magnetic resonance has become one of the main methods to characterize static parameters and dynamic changes in unconventional reservoirs. The research focus of this paper is process simulation of coalbed methane (CBM) production. The dynamic variation of pore volume with different pore sizes during pressure drop, methane desorption–diffusion process, and methane–water interaction during migration is discussed. Moreover, the calculation principles of NMR single and multifractal models are systematically described, and the applicability of NMR fractal models within different research contexts is discussed. Four aspects need urgent attention in the application of this technology in CBM production: (1) overburden NMR technology has limitations in characterizing the stress sensitivity of shale and high-rank coal reservoirs with micropores developed, and we should aim to enable an accurate description of micropore pore stress sensitivity; (2) dynamic NMR physical simulation of reservoir gas and water production based on in-situ and actual geological development conditions should become one of the key aspects of follow-up research; (3) low-temperature freeze–thaw NMR technology, as a new pore–fracture characterization method, needs to be further applied in characterizing the distribution characteristics of pores and fractures; and (4) NMR fractal model should be used as the main theoretical method to expand the simulation results. The applicability of different fractal models in characterizing pore–fracture structure (static) and CBM production process (dynamic) needs to be clarified.

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The influence of the depressurization rate on coalbed methane desorption and percolation was studied using physical experiment and numerical simulation. First, low-field nuclear magnetic resonance technology provided a new approach to conduct desorption experiments with different depressurization schemes and obtain the compressibility ( C f ) of coal samples. Then, the productivity calculation of different depressurization schemes was carried out via numerical simulation. The results showed that the first-slow-then-fast (FSTF) depressurization scheme had the highest desorption efficiency (94%), followed by one-stop desorption (85%), first-fast-then-slow desorption (79%), and uniform depressurization desorption (61%). T 2 cutoff values and the corresponding compressibility were obtained by the saturation–centrifugation method and spectral morphology method, and a high-precision permeability expression for dynamic evaluation of numerical simulation was established by the historical production data fitting approach. Through numerical simulation, high production efficiency can be achieved using depressurization rates of medium (15 kPa/d) and FSTF schemes (8 & 50 kPa/d), and depressurization funnel expansion in the single-phase water flow stage plays a decisive role in stable and high-yield production in the later stage. Thus, the FSTF pressure reduction strategy could be advocated to promote gas production. Slow depressurization should be applied in ineffective desorption and the slow desorption stage for saturated coal seam or single-phase flow stage for undersaturated coal seam, given the higher single-phase water permeability. During the rapid and sensitive desorption stage, rapid depressurization is recommended because of large desorption capacity and low water phase permeability. This paper provides a possibility for the optimization of coalbed methane field production management.