Zhejun Pan

Laboratory study of gas permeability and cleat compressibility for CBM/ECBM in Chinese coals

Coal permeability is regarded as one of the most critical parameters for the success of coalbed methane recovery. It is also a key parameter for enhanced coalbed methane recovery via CO2 and/or N2 injection. Coal permeability is sensitive to stress and cleat compressibility is often used to describe how sensitive the permeability change to stress change for coal reservoirs. Coalbed methane exploration and production activities and interest of enhanced coalbed methane recovery increased dramatically in China in recent years, however, how permeability and cleat compressibility change with respect to gas species, effective stress and pore pressure have not been well understood for Chinese coals, despite that they are the key parameters for primary and enhanced coalbed methane production. In this work, two dry Chinese bituminous coal samples from Qinshui Basin and Junggar Basin are studied. Four gases, including He, N2, CH4 and CO2 are used to study permeability behaviour with respect to different effective stresses, pore pressures, and temperatures. The effective stress is up to 5 MPa and pore pressure is up to 7 MPa. Permeability measurements are also carried out at highest pore pressures for each adsorbing gas, at three temperatures, 35, 40 and 45°C. The experimental results show that gas species, effective stress and pore pressure all have significant impact on permeability change for both coal samples. Moreover, the results demonstrate that cleat compressibility is strongly dependent on effective stress. More importantly, the results show that cleat compressibility is also strongly dependent on pore pressure. Cleat compressibility initially decreases with pore pressure increase then it increases slightly at higher pore pressures. However, temperature only has marginal impact on permeability and cleat compressibility change.

Effects of pressure and temperature on gas diffusion and flow for primary and enhanced coalbed methane recovery

Due to the rapid increase of coalbed methane (CBM) exploration and development activities in China, gas adsorption and flow behavior for Chinese coals are of great interest for the industry and research community. How pressure and temperature affect the gas adsorption and flow on different rank coals are not only important for CBM recovery but also important for CO2 or N2 enhanced CBM recovery, since gases are often injected at a temperature different to the reservoir temperature. In this work, gas adsorption and permeability of three different rank Chinese coals are measured using CH4, N2 and CO2 at three temperatures, 20°C, 35°C and 50°C. Gas diffusivity and permeability with respect to gas species, pore pressure, effective stress and temperature are studied. The three coals are SQB-1 from Southern Qinshui Basin, JB-1 from Junggar Basin and OB-1 from Ordos Basin. Gas adsorption results show that both pressure and temperature have significant impact on adsorption behavior for SQB-1 and JB-1 using CH4. For higher rank coal SQB-1, adsorption isotherm tends to reach adsorption capacity quicker with respect to pressure. However, the maximum adsorption capacity is higher for the lower rank coal JB-1. Moreover, temperature has a stronger effect on reducing adsorption capacity for lower rank coal. Gas diffusivity results for OB-1 and JB-1 show that CO2 diffusivity is generally higher than that of CH4 and then N2. This could be related with their different kinetic diameters and their interaction with the coal. Both pressure and temperature have impact on gas diffusivity. In general, gas diffusivities increase with pressure and temperature. Permeability results show that it varies greatly with respect to coal rank with highest rank coal having the lowest permeability. Permeability is also strongly sensitive to effective stress and pore pressure. Temperature has a noticeable impact on permeability change. Permeability changes differently with temperature increase for the different rank coal samples studied. This may be attributed to the combined effect of coal strain change due to gas adsorption and thermal expansion. These results have significant implications for the design of enhanced CBM recovery and CO2 storage for different rank coals as injecting gas at different temperature and pressure would affect the CO2 injectivity and the CBM production rate.

Impact of various parameters on the production of coalbed methane

Coalbed-methane (CBM) reservoirs are naturally fractured formations, comprising both permeable fractures and matrix blocks. The interaction between fractures and matrix presents a great challenge for the forecast of CBM reservoir performance. In this work, a dual-permeability model was applied to study the parameter sensitivity on the CBM production, because the dual-permeability model incorporates not only the influence from matrix and fractures but also that between adjacent matrix blocks. The mass exchange between two systems is defined as a function of desorption time constant at the standard condition, coal matrix porosity, and the difference of gas pressure between two systems. Correspondingly, gas diffusivity in matrix is considered as a variable and represented by a function of shape factor, gas desorption time, and reservoir pressure. These relations are integrated into a fully coupled numerical model of coal geomechanical deformation and gas desorption/gas flow in both systems. This numerical approach demonstrates the important nonlinear effects of the complex interaction between matrix and fractures on CBM production behaviors that cannot be recovered without rigorously incorporating geomechanical influences. This model was then used to investigate the sensitivity of CBM extraction behavior to different controlling factors, including gas desorption time constant, initial fracture permeability, fracture spacing, swelling capacity, desorption capacity, production pressure, and fracture and matrix porosities. Modeling results show that the peak magnitudes of gasproduction rate increase with initial fracture permeability, sorption and swelling capacities, and matrix porosity, and decrease with gas desorption time constant and production pressure. These results also show dramatic increase in gas-production efficiency with decreasing magnitudes of fracture spacing. The comparison of the transient contributions of the desorbed gas and the free phase gas from the matrix system to gas production shows that the free phase gas plays the dominant role at the early stage, but diminishes when the adsorption phase gas takes over the dominant role, indicating the necessity of incorporating free phase gas impact in simulation models. The numerical model was also applied to match the history data from a gas-production well. A better matching result than that for the single-permeability model demonstrates the potential capability of the dual-permeability model for the forecast of CBM production.

Interactions and exchange of CO2 and H2O in coals: an investigation by low-field NMR relaxation

The mechanisms by which CO2 and water interact in coal remain unclear and these are key questions for understanding ECBM processes and defining the long-term behaviour of injected CO2. In our experiments, we injected helium/CO2 to displace water in eight water-saturated samples. We used low-field NMR relaxation to investigate CO2 and water interactions in these coals across a variety of time-scales. The injection of helium did not change the T2 spectra of the coals. In contrast, the T2 spectra peaks of micro-capillary water gradually decreased and those of macro-capillary and bulk water increased with time after the injection of CO2. We assume that the CO2 diffuses through and/or dissolves into the capillary water to access the coal matrix interior, which promotes desorption of water molecules from the surfaces of coal micropores and mesopores. The replaced water mass is mainly related to the Langmuir adsorption volume of CO2 and increases as the CO2 adsorption capacity increases. Other factors, such as mineral composition, temperature and pressure, also influence the effective exchange between water and CO2. Finally, we built a quantified model to evaluate the efficiency of water replacement by CO2 injection with respect to temperature and pressure.

Laboratory and modeling study on gas diffusion with pore structures in different-rank Chinese coals

The coalbed methane production rate is mainly controlled by two parameters: diffusivity in the coal matrix and permeability in the cleat system. As one key parameter, diffusion plays an important role in coalbed methane producing process. However, till now there is no systematic study on the relationship of diffusivity, pore size and coal ranks. In this work, three Chinese coal samples which belong to low, middle and high rank, respectively, were studied using experimental and modeling methods. Four gases, H2, N2, CH4 and CO2, are used to study the diffuse characters at four different pressures steps. The experimental results showed that the adsorption balance time varies with different testing gas type and it is closely related with coal rank. Balance time testing with CO2 is the longest, and then is CH4, while He is the slowest. Moreover, high rank coal takes the longest time to reach balance, while low rank coal takes the least. Generally, the sorption balance time for JCC-01 is about 150–350 S 0.5, it is about 250–400 S 0.5 for CZ-1, while it is about 900–1200 S 0.5 for JCC-01. Modeling results showed that for all these three ranked Chinese coals, the bidisperse model can be used to model the diffuse process. The β value, which is the ratio of macropore adsorption/desorption to the total adsorption/desorption, increases with the increasing of pore pressure, except for sample JCC-01 when measured using CO2. There is no regular law for both micropore and macropore diffusion coefficient. In order to study the relationship of diffusivity, pore structure and coal ranks, the experiments of mercury injection test and low-temperature liquid nitrogen experiment were done to analyse the relationship. The results show that, for the low rank coal sample TCG-1, the mesopore takes the majority while the macropore also takes part of the pore distribution, there are few even no micropore. For middle rank sample CZ-1, the mesopore also takes the majority and the macorpore accounts for a small percentage. While for high rank coal sample JCC-01, micropore takes the majority, and mesopore and macropore take small part of the pore structure distribution. The conclusion drawn from these testing results can be used to explain the adsorption and diffusion laws found before.

Impact of thermal processes on CO2 injectivity into a coal seam

The objective of this study is to investigate how thermal gradients, caused by CO2 injection, expansion and adsorption, affect the permeability and adsorption capacity of coal during CO2 sequestration. A new permeability model is developed in which the concept of elastic modulus reduction ratio is introduced to partition the effective strain between coal matrix and fracture. This model is implemented into a fully coupled mechanical deformation, gas flow and heat transport finite element simulator. To predict the amount of CO2 sequested, the extended Langmuir sorption model is used, with parameters values taken from the literature. The coupled heat and gas flow equations, are solved in COMSOL using the finite element method. The simulation results for a constant volume reservoir demostrate that thermal strain acts to significantly reduce both CO2 injectivity and adsorption capacity. These impacts need to be considered in the calculation of the optimum injection rate and the total sequestration capacity.

Influence and control of coal facies on physical properties of the coal reservoirs in Western Guizhou and Eastern Yunnan, China

Western Guizhou and Eastern Yunnan have abundant coal resources, and they are considered as the most prospective areas for coalbed methane (CBM) development in Southern China. To explore the potential of CBM production in these areas, this paper identifies the plane distribution of coal facies, and analyses its control on the physical properties of coal reservoirs. Four types of coal facies including a wet forest swamp, an intergradation forest swamp, a drained forest swamp, and a fresh-water peat swamp were distinguished. The results show that coal facies play a significant role in the control of the coal reservoir properties for coals with similar rank.

CO2 storage in coal to enhance coalbed methane recovery: a review of field experiments in China

ABSTRACT Coal reservoirs especially deep unminable coal reservoirs, are viable geological target formations for CO2 storage to mitigate greenhouse gas emissions. An advantage of this process is that a large amount of CO2 can be stored at relatively low pressure, thereby reducing the cost of pumping and injection. Other advantages include the use of existing well infrastructure for CO2 injection and to undertake enhanced recovery of coalbed methane (ECBM), both of which partially offset storage costs. However, ECBM faces difficulties such as low initial injectivity and further permeability loss during injection. Although expensive to perform, ECBM field experiments are essential to bridge laboratory study and large-scale implementation. China is one of the few countries that have performed ECBM field experiments, testing a variety of different geological conditions and injection technologies. These projects began more than a decade ago and have provided valuable experience and knowledge. In this article, we review past and current CO2 ECBM field trials in China and compare with others performed around the world to benefit ECBM research and inform future projects. Key aspects of the ECBM field projects reviewed include the main properties of target coal seams, well technologies, injection programmes, monitoring techniques and key findings.

Measurement of shale anisotropic permeability and its impact on shale gas production

Gas shales act as both the source rock and reservoir for the petroleum. One of the important characteristics of the reservoir is its low permeability, making gas production difficult. Although economic shale gas production has been achieved through horizontal drilling and multistage hydraulic fracturing, shale reservoir permeability is still one of the critical parameters in the evaluation of a shale gas play. In the present work, experimental measurement of shale anisotropic permeability is determined using a cubic shale sample in a triaxial cell. Anisotropic permeability was measured at a series of gas pressures and confining pressures. A permeability model incorporating stress and Klinkenberg effect was applied to describe the data. The model was applied in the reservoir simulator SIMED IITM to investigate the impact of anisotropic permeability and its change on shale gas production. Results are compared between using the vertical permeability, horizontal permeability, or anisotropic permeability as the reservoir permeability. The results show that using vertical permeability will significantly underestimate the gas production rate. This demonstrates that measuring directional permeability and using the most appropriate one is important for evaluating shale gas production and development of shale gas assets.

Simulation of coal permeability under non-isothermal CO2 injection

CO2 injection into coal seams is a non-isothermal process, which has significant impact on coal permeability but has not been well studied. In this paper, a non-isothermal model coupled with nonlinear gas flow and matrix deformation was developed. The effects of temperature change on each term of the effective strain during the CO2 injection scenarios, as well as the variations of fluid properties over a range of sub- and supercritical-thermodynamic conditions were investigated. This model involves the balance of thermal energy and the law of heat transfer. Two non-isothermal cases of CO2 injection were studied and compared with the isothermal case. The results show that CO2 injection into coal seams reduces coal permeability for all three cases. The coal matrix expands with temperature increase due to the thermal expansion and shrinks due to the decrease in adsorption amount. However, the final permeability with low-temperature CO2 injection remains lower than that with high-temperature gas injection since the effect of sorption-induced strain on permeability outweighs that of the thermal deformation. The increase in temperature leads to the reduction in coal swelling (with the decrease of CO2 adsorption capacity), resulting in larger cleat aperture and higher coal permeability for the cases studied in this work. [Received: November 29, 2015; Accepted: June 22, 2016]