Study on desorption and diffusion dynamics of coal reservoir through step-by-step depressurization simulation——an experimental simulation study based on LF-NMR technology

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.

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Section: Cbm Well Description and Materials And Methodsmentioning

confidence: 99%

Physical Experiment and Numerical Simulation of the Depressurization Rate for Coalbed Methane Production

et al. 2020

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“…Sampling of methane-bearing coal seams by freezing was proposed, and the coalbed gas desorption characteristics under freezing temperature were studied to verify the feasibility of this method [14]. Calibration, isothermal desorption, pressure step-down desorption, and pore dynamic experiments were conducted with high-rank coal samples using low-field nuclear magnetic resonance technology [15]. Wang et al [16] obtained the effect of water on diffusion dynamics during methane desorption.…”

Section: Introductionmentioning

confidence: 99%

Experimental Study on Desorption Characteristics of Coalbed Methane under Variable Loading and Temperature in Deep and High Geothermal Mine

Wang

Jia

et al. 2020

Advances in Civil Engineering

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Due to the influence of deep high stress, geothermal heat, and other factors, the law of desorption of methane in coal seams is more complicated in the process of mining deep coal seams, which is prone to methane over-limit, coal and gas outburst, and other accidents. In order to study the desorption characteristics of coalbed methane under different loading and temperature conditions, the desorption tests at different deformation stages of coal containing methane were carried out in the process of loading-adsorption-desorption-reloading until the coal sample was destroyed by using the seepage-adsorption-desorption test system on coal and rock mass, and the test programs were different combinations of gas pressure 1.2 MPa, two kinds of confining pressure, and three kinds of temperature. The results show that the cumulative methane desorption amount corresponding to each deformation stage presents a convex parabolic increase trend with the increase in desorption time, while the desorption rate presents a power function decay trend. Under the condition of the same desorption time, the cumulative methane desorption amount from large to small is residual deformation stage, compaction stage, near the peak stress, plastic deformation stage, and elastic deformation stage. Under the same confining pressure, temperature, and methane pressure, the maximum desorption rate from large to small is residual deformation stage, near the peak stress, plastic deformation stage, compaction stage, and elastic deformation stage. The desorption and diffusion of methane are promoted under the higher temperature and lower confining pressure, which presents a certain mechanism of promoting desorption. The thermal movement of methane molecules is intensified with the increase in temperature, and the adsorption effect between methane molecules and the molecules at the surface of the coal is weakened. The cumulative methane desorption amount and the maximum desorption rate increase with the increase in temperature. The cumulative methane desorption in the residual deformation stage is obviously greater than that in other deformation stages. The increase in confining pressure inhibits the development and expansion of pore fractures in raw coal specimen and hinders the increase in the effective desorption surface area. The cumulative methane desorption amount and the maximum desorption rate decrease with the increase in confining pressure.

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“…28−30 Zhao and Wang 31 and Liu et al, 32 by studying the effect of CO 2 injection on the methane adsorption process in shale with the NMR-based methodology, observed that the methane adsorbed onto the shale pore surface can be strongly replaced by CO 2 and then only the desorbed methane changes to the free-state methane in the pore center and is hardly gets extracted from organic pores. Furthermore, Quan et al 33 utilized the low-field NMR technology to study the methane desorption−diffusion dynamics of coals and discovered that the dynamic variation of the methane diffusion process caused by the pore deformation behavior is related to different desorption characteristics under the conditions of various depressurization schemes. However, the above-stated research mainly concentrates on the application of the NMR-based technology in the identification of multiphase methane gases, quantitative characterization of gas adsorption capacity, and its influencing factors in coal or shale gas reservoirs.…”

Section: Introductionmentioning

confidence: 99%

“…Some researchers studied the influence of adsorbed/nonadsorbed water on methane adsorption/desorption process of coals using NMR spectral measurements and found that the water film in pore walls and the water droplet near the pore throat can seriously reduce or block the methane adsorption/desorption process on the micropore. − Zhao and Wang and Liu et al, by studying the effect of CO 2 injection on the methane adsorption process in shale with the NMR-based methodology, observed that the methane adsorbed onto the shale pore surface can be strongly replaced by CO 2 and then only the desorbed methane changes to the free-state methane in the pore center and is hardly gets extracted from organic pores. Furthermore, Quan et al utilized the low-field NMR technology to study the methane desorption–diffusion dynamics of coals and discovered that the dynamic variation of the methane diffusion process caused by the pore deformation behavior is related to different desorption characteristics under the conditions of various depressurization schemes.…”

Section: Introductionmentioning

confidence: 99%

Evaluation of Methane Dynamic Adsorption–Diffusion Process in Coals by a Low-Field NMR Method

Liu

Xie³

et al. 2020

Energy Fuels

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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.

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Study on desorption and diffusion dynamics of coal reservoir through step-by-step depressurization simulation——an experimental simulation study based on LF-NMR technology

Cited by 17 publications

References 52 publications

Physical Experiment and Numerical Simulation of the Depressurization Rate for Coalbed Methane Production

Physical Experiment and Numerical Simulation of the Depressurization Rate for Coalbed Methane Production

Experimental Study on Desorption Characteristics of Coalbed Methane under Variable Loading and Temperature in Deep and High Geothermal Mine

Evaluation of Methane Dynamic Adsorption–Diffusion Process in Coals by a Low-Field NMR Method

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