Coal Shrinkage Research Articles

Subsidence associated with dewatering and gas extraction from coal seams: Contribution of desorption-induced coal shrinkage

Several coal seams are contained within the formations of Great Artesian Basin (GAB), the largest natural underground water reservoir in Australia and the world. The extraction of coal seam gas (CSG) and its associated water from thousands of wells within the coal measures of the GAB has led to tens of millimetres of subsidence in CSG development areas. Since highly developed farming systems located in these areas rely on very low slope landforms, even this scale of subsidence has caused significant community concern about the potential for CSG extraction to impair farming operations and productivity through changes in land slope and drainage. Coal seam compaction associated with dewatering and gas extraction has two key components: poromechanical compaction and desorption-induced bulk shrinkage. The former results from pore pressure depletion (due to dewatering and gas extraction) and an increase of vertical effective stress, while the latter is induced by gas desorption from the coal matrix, leading to further deformation. While poromechanical compaction of fluid-bearing formations has been extensively addressed in the literature, relatively little research has been conducted on the role of coal shrinkage in CSG-induced subsidence. This paper introduces an innovative practical modelling approach for assessing CSG-induced subsidence at a subregional/regional scale. The approach utilises an analytical model for CSG-induced subsidence derived from constitutive stress–strain relations for poroelastic-sorptive media. This is a novel approach in the context of CSG-induced subsidence which considers both poromechanical compaction and desorption-induced shrinkage. A further distinction to previous work is the integration of the geomechanical-sorptive subsidence model with a numerical groundwater model. Based on this approach, this paper examines the effect of coal shrinkage on subsidence, and its proportions with respect to total compaction for one of the major coal measures in the Surat Basin. Input data are derived from three-dimensional geological, geomechanical, and groundwater models, as well as methane adsorption and desorption reports. Results show that (a) the impact of coal shrinkage on CSG-induced subsidence is likely to be significant in the study area and (b) the contribution of coal shrinkage to CSG-induced subsidence depends on gas content, Langmuir isotherm, shrinkage strain parameters, and the saturation state of coal. This study provides important insights into CSG-induced subsidence and lays the foundation for the development of robust, efficient, and localised predictive models to support environmental impact assessment and management.

Experimental study on coal blockage removal based on pulverized coal blockage

The generation and transport of coal powder has caused great harm to coal reservoir channeling conductivity, affecting the capacity of coalbed methane wells, causing the failure of engineering equipment and great distress to production, while current unblocking methods have many shortcomings. In this study, a series of laboratory experiments on coal samples' plugging and reverse-water/gas injection replacement of pulverized coal to unblock coal samples under three-dimensional stress conditions were carried out using a gas–liquid–solid three-phasedisplacement simulation system. In addition, the characteristics of the change of coal permeability and coal powder output under different plugging pressures and unplugging media and pressures were explored. Furthermore, the influence of medium difference on coal powder migration and its mechanism were analyzed. The results were as follows: (1) under the same pressure, the gas carrying capacity for pulverized coal in the pores and fissures of raw coal is much greater than that of water; (2) only when the reverse-gas injection pressure is greater than or equal to the plugging pressure can the coal be unblocked. The reverse-gas injection unblocking the coal body has three stages such as gas-drive-water, gas-drive-water and pulverized coal, and gas production stages (complete unblocking stage of the coal body). The migration difficulty determines the output order of products; (3) during reverse injection hydrolysis of coal plugging, the liquid-phase permeability (LPP) of coal showed “fluctuation–stability” characteristics. The final liquid-phase permeability is lower than the initial value, which is related to the competition between the secondary bridging blockage of pulverized coal particles (PCP) at the pore shrinkage of coal caused by the change in water flow seepage velocity and backwashing, and the pulverized coal's unblocking, redistribution, and capture caused by the local pressure build-up.

Determination of Key Technical Parameters in the Study of New Pressure Sealing Technology for Coal Seam Gas Extraction.

Coal is affected by the concentrated stress disturbance of mining, the disturbance of drilling hole formation, and the concentrated stress of coal shrinkage and splitting of gas desorption from the hole wall; these result in a large number of secondary cracks that collect and leak gas. As a result, it is difficult for the coal seam sealing process to meet engineering quality sealing requirements, which results in problems such as low gas concentration during the extraction process. In this paper, based on the analysis of coal pore and fissure characteristics, and in view of the current situation of gas drainage and sealing in this coal seam, combined with the existing grouting and sealing technology, it is proposed to use pressure grouting and sealing to realize the sealing of deep coal bodies in the hole wall. According to the field conditions, the experimental pressure sealing parameter index is as follows: theoretical sealing length L1 = 9.69 m, the sealing length L2 = 13.98 m is verified, and the final sealing length is determined to be 15 m; the sealing radius is determined to be 0.6 m; the cement slurry was prepared on site with a water: cement ratio of 2:1; PG = 0.43 MPa was calculated; the range of the slurry diffusion radius R was 93.4–176.6 cm; the grouting pressure was determined to be 0.516 MPa. Field application practice has proved that: (1) Under the same drilling parameters and sealing parameters, the gas drainage effect of drilling with pressure sealing is 2.3 times higher than that without pressure sealing; (2) Using traditional sealing technology for drilling holes, the gas extraction concentration is far lower than the sealing operation effect of using the pressure sealing process; (3) Reasonably extending the length of the gas extraction drilling and sealing is a basic guarantee for realizing a substantial increase in the gas extraction concentration; (4) Sealing with pressure leads to a reliable and stable hole process.

Poroelastic solution of a wellbore in a swelling rock with non-hydrostatic stress field

The stress distribution around a circular borehole has been studied extensively. The existing analytical poroelastic solutions, however, often neglect the complex interactions between the solid and fluid that can significantly affect the stress solution. This is important in unconventional gas reservoirs such as coalbeds and shale formations. In order to address this limitation, this paper presents the development of a poroelastic solution that takes into account the effect of gas sorption-induced strain. The solution considers drainage of the reservoir fluid through a vertical wellbore in an isotropic, homogenous, poroelastic rock with non-hydrostatic in situ stress field. The sorption-induced shrinkage of coal is modelled using a Langmuir-type curve which relates the volumetric change to the gas pressure. The redistributed stress field around the wellbore after depletion is found by applying Biot's definition of effective stress and Airey's stress functions, which leads to a solution of the inhomogeneous Biharmonic equation. Two sets of boundary conditions were considered in order to simulate the unsupported cavity (open-hole) and supported cavity (lined-hole) cases. The implementation was verified against a numerical solution for both open-hole and lined-hole cases. A comparative study was then conducted to show the significance of the sorption-induced shrinkage in the stress distribution. Finally, a parametric study analysed the sensitivity of the solution to different poroelastic parameters. The results demonstrate that the developed solution is a useful tool that can be employed alongside complex flow models in order to conduct efficient, field-scale coupled hydro-mechanical simulations, especially when the stress-dependent permeability of the reservoir is of concern.

Investigations on the mechanism of the microstructural evolution of different coal ranks under liquid nitrogen cold soaking

ABSTRACT Liquid nitrogen (LN2) is an important means to improve the permeability of coal reservoirs. The LN2 soaking method has a wide range of application prospects. However, not enough studies exist to highlight the effect of increasing the permeability of different coal samples using LN2. Hence, this investigation was carried out to examine the effect of cold soaking in LN2 using different coal ranks such as fat coal, lean coal and anthracite coal, on their pore structures and fractal characteristics. Frenkel-Halsey-Hill (FHH) fractal theory was used to understand the fractal dimension characteristics of pores in these different coals before and after cold soaking in LN2. Furthermore, the relationship between the fractal dimensions and pore parameters was also studied. The results show that low-temperature nitrogen adsorption curves of fat coal, lean coal and anthracite before and after cold soaking in LN2 are “S” type. An increase in the total pore volume and pore surface area of coal after LN2 cold soaking is noted. LN2 cold soaking is found to transform the internal pore structure of different coals effectively. Moreover, the heterogeneity of the porous structure of coal is enhanced after LN2 cold soaking along with an increase in the fractal dimension. The growth also decreases with an increase in the metamorphism. The uneven shrinkage of coal occurred during LN2 cold soaking, and the thermal stress generated is greater than the tensile strength of coal, which promotes the development of the porous structure of coal. These results are beneficial in revealing the macroscopic and microscopic pore fissure space expansion and connectivity law of coal seam during the process of LN2 cracking.

Gas seepage laws based on dual porosity and dual permeability: Numerical simulation and coalbed methane extraction practice

AbstractCoalbed methane (CBM) is the very important unconventional energy resource. Enhancing extraction is the main method to improve the utilization of CBM and prevent coal mine gas disasters. To reveal the laws of gas migration during the underground gas extraction, the paper established the solid‐gas coupling model, regarding coal as the homogeneous elastic medium with dual pore‐fracture structure and dual permeability and considering the gas dynamic diffusion coefficient. Then, the regulations of gas migration in the in‐seam borehole were simulated by the COMSOL Multiphysics software and verified via the field trials of gas extraction. The results revealed that the permeability increased with time due to the coupling of matrix shrinkage and volume compression of coal, in which the matrix shrinkage played a leading role. The gas seepage velocity at the observation points can be divided into three stages: the rapid rise stage, the stable decline stage, and the stable and invariant stage. The gas extraction rate continued decreasing with time and finally tended to a fixed value. In the coal seam #4 of Zhongxing coal mine, the measured flow rates of three in‐seam drilling holes were in good agreement with the simulation results, which verified the correctness of the theoretical coupling model.

An experimental study on the effect of nitrogen injection on the deformation of coal during methane desorption

Gas-injection to enhance coalbed methane recovery (ECBM) is a hot issue at present. Several studies have pointed out that nitrogen injection can improve the permeability of coal reservoirs. However, there is still a lack of research on coal deformation induced by nitrogen injection during methane desorption. In this study, five coal samples from Shanxi Province, China were collected to conduct a comparative study on the deformation of coal during methane desorption with/without nitrogen injection, and the influence of nitrogen injection on the deformation of coal was analyzed. Through this study, the reason for the increase the permeability of coal caused by nitrogen injection was analyzed in combination with the deformation characteristics of the coal. The results show that the strain-time curves during adsorption are similar to the Langmuir curves. The swelling of coal during adsorption decreases first and then increases with the increase of random reflectivity of vitrinite (Ro, ran). The shrinkage is less than swelling for the same sample after the adsorption-desorption cycle without nitrogen injection. There is a certain residual deformation since part of methane is unable to become a free state after being adsorbed by coal. The residual strain of the samples is between 39 and 402 uε in the two desorption processes. After nitrogen injection, in the process of desorption with successively decreasing outlet pressure (simulated the change of bottom hole pressure in the field CBM production) and with constant outlet pressure (atmospheric pressure), the accumulated shrinkage of the sample is higher than the corresponding swelling during adsorption. On the one hand, nitrogen injection can reduce partial pressure and promote the desorption of methane. On the other hand, nitrogen injection can induce the shrinkage of coal, which may be one of the reasons that lead to the increase of permeability. The strain induced by nitrogen injection is between 199 and 853 uε, which has a weak negative relationship with the ash yield. The strain induced by nitrogen injection decreases first, then increases and then decreases again with the increase of Ro, ran.

A comparative study on the deformation of unconfined coal during the processes of methane desorption with successively decreasing outlet pressure and with constant outlet pressure

The shrinkage of coal induced by methane desorption during coalbed methane (CBM) production will change the porosity and permeability of reservoirs. It is of great significance to analyze the deformation characteristics of coal in the process of desorption with successively decreasing outlet pressure, which is conducive to the continuous production of CBM. In this study, five cylindrical samples in Shanxi Province, China were used to reflect the deformation and desorption characteristic in the process of methane desorption with decreasing outlet pressure (simulated the change of bottom hole pressure in the field CBM production) by independently designed instruments. And the results were compared with that in the process of desorption with constant outlet pressure (atmospheric pressure). The results show that the coal swells (shrinks) when methane is absorbed (desorbed). There is a linear relationship between the strain and adsorption/desorption volume. Moreover, the shape of desorption rate-time curves is similar to that of the Langmuir curves in two desorption processes. However, the cumulative desorption rate of desorption with successively decreasing outlet pressure is higher than that of desorption with constant outlet pressure due to the smaller attenuation effect on permeability, which leads to the shrinkage during desorption with successively decreasing outlet pressure is also higher. The shrinkage of the samples is always less than the swelling. There is a negative correlation between the strain differences and the desorption rate. The strain differences in the process of desorption with successively decreasing outlet pressure are lower than that of desorption with constant outlet pressure due to the higher desorption rate.

Kinetic behavior of heterogeneous sorption deformation on coal: Effect of maceral/micro-lithotype distribution

The greenhouse effect has attracted increasing attention globally. The geologic sequestration of CO2 is considered as an effective method for reducing the amount of CO2 in the atmosphere. Carbon-dioxide enhanced coalbed methane (CO2-ECBM) recovery is extremely attractive owing to its ability to store CO2 in coal seams while simultaneously enhancing the production of methane. However, the injection of CO2 can induce a deformation of the coal matrix, which has a significant effect on the fluid flow and stability of the coal seams. In this study, the sorption kinetics experiments indicated the high anisotropy and heterogeneity of local sorption deformation in coal samples and a unique kinetic behavior of sorption deformation which was rarely reported and analyzed in detail in previous studies. This kinetic behavior demonstrated a rapid increase of sorption strain followed by a slight or significant falloff until the equilibrium state reached. The characteristics of component distribution in coal samples were investigated using CT scanning and micro-lithotype and EDS analysis. An extensive analysis of the experiment results indicated the following: 1) Carbon dioxide dissolved during the first sorption process induced a rearrangement of the coal structure, leading to a drastic variation of deformation behavior between the first and subsequent sorption process. 2) The unique kinetic behavior of sorption deformation could be induced by either the loss of moisture in coal or the interaction of different coal components. The mechanism of these two factors induced coal shrinkage was different and had a distinct effect on the global behavior of sorption deformation. 3) The quantitative relationship between compressional and dilative components directly verified the significant compression of inertinite-rich regions by the slow swelling of vitrinite-rich regions. 4) The kinetic behavior of sorption deformation was affected by the combination of the macroscopic bedding structure and the heterogeneous component distribution in coal.

The relevance between water release behavior and pore evolution of hard lignite during the thermal-drying process

As known, the shrinkage of coal and the pore collapsed during the drying process due to water removal. The relevance between water release behavior and pore evolution of lignite during the thermal-drying process was studied in this work. From N2 adsorption of traditional pretreated samples and freezing wet samples, most of moisture was in the pores in the range of 26–100 nm, and results from low-field nuclear magnetic resonance (LFNMR) confirmed this conclusion that the removed moisture was in the pores with pore size smaller than 200 nm, while from the mercury intrusion porosemitry (MIP) results, the most obvious reduction of macropores was in the range of pore dimeter larger than 1 μm and there was almost no change of pore volume in the range of pore diameter smaller than 200 nm, which was completely different from traditional ideas that the pore shrunk or collapsed due to water removed. The macropores were more prone to collapse than mesopores and micropores, maybe due to its structural mechanical characteristics. And the effect of pore evolution on moisture release behavior was related to the specific drying methods and mass transfer processes.

Coal permeability: Gas slippage linked to permeability rebound

The main factors/mechanisms that influence coal permeability are effective stress, swelling, shrinkage, deformation and gas slippage. After an extended period of gas production, coal can have a rebound phenomenon where permeability increases with increasing effective stress. This rebound can have a significant impact on gas recovery during the late stages of a reservoir life cycle. This paper aims to characterise coal permeability by combining laboratory measurements with a simple gas slippage model that explains the rebound phenomenon. Gas and Klinkenberg corrected permeabilities of coal are measured at (1) constant confining pressure and (2) constant effective stress. We estimate the length scales relevant to gas flow using mercury intrusion, a permeability slip model, and the kinetic theory of gases, which allows us to estimate the Knudsen number for gas flow. Results show a linear relationship between slip length and the mean free path of gas for all of the tested mean pore pressures. This result suggests that a first order slip boundary condition is sufficient to explain the momentum exchange at the gas/solid boundary during flow under normal reservoir conditions. A correlation between Knudsen number and increased permeability is developed, which further demonstrates that slippage cannot be neglected in coals when Knudsen number is greater than 0.1. Overall, we present a simple model that explains permeability rebound in coal by considering only gas slippage. We do not discredit the mechanism of coal shrinkage, which could also influence coal permeability. We confirm that gas slippage should be considered in coal permeability models.

How moisture loss affects coal porosity and permeability during gas recovery in wet reservoirs?

Moisture in coal seams changes gas adsorption capacity, induces coal deformation, and affects coal porosity. However, fewer studies have investigated the dynamic process of moisture loss. In this study, a fully coupled multi-physical model for coal deformation, gas flow and moisture loss was implemented. It validated the coal-gas-moisture interactions of the decay of gas adsorption capacity and the coal shrinkage. Subsequently, the proposed model was applied to a simulation of coal seam gas recovery from wet reservoir and solved using the finite method in COMSOL Multiphysics 3.5. Analyses of the component factors and the sensitive parameters of moisture loss on coal porosity and permeability were comprehensively studied at last. The results reveal that moisture loss enhances coal porosity and permeability. The decay of gas adsorption capacity decreases coal permeability while the coal shrinkage promotes it. The decrease of the adsorption decay coefficient and the increase of the initial density of saturated water vapor and water evaporation constant can enhance the permeability of wet coal seams.

Numerical investigation on the coupled gas-solid behavior of coal using an improved anisotropic permeability model

Coalbed methane (CBM) extraction uses coal seam depressurization to extract methane adsorbed into the coal matrix. The process involves complex interactions between solid and gas, and between mechanical and chemical effects. Permeability of coal is one of the most important parameters to evaluate the production of CBM reservoirs. Several models have been proposed to establish the relationship between permeability and poromechanical response, gas desorption, and coal shrinkage. However, there have been only a few attempts in describing the anisotropic permeability of coal, which is an important characteristic of gas flow in coal. This paper focuses on the anisotropic properties of coals and its effects on the coupled gas-solid behavior. We develop an improved anisotropic permeability based on linear elastic poromechanical theory and then implement this model into a finite element package to study the effects of the anisotropic permeability on the gas transport in coal. Cases with three boundary conditions including uniaxial strain, constant volume, and constant confining stress are analyzed in details. Numerical simulation results show that the proposed model can well represent the anisotropic permeability of coal. Permeability is closely related to the mechanical properties of coal. We find that modulus reduction ratio is a key parameter to control the anisotropic characteristics. Different modulus reduction ratios alter directional permeability significantly. Anisotropic permeability changes with time and pressure during gas desorption and gas flow. Further, the magnitude of directional permeability variations depends on both the modulus and the boundary conditions. Permeability evolution is controlled by the change in effective stress and matrix shrinkage due to desorption. The effects of anisotropic permeability on the coupled gas-solid behavior of coal during gas desorption and flow processes are demonstrated in this work.

Utilization of raw and dried Victorian brown coal in the adsorption of model dyes from solution

Being plentiful and readily available in many parts of the world, brown coal is probably one of the most potential substitutes for activated carbon. Many reports include the brown coal drying stage prior to adsorption despite the fact that it may lead to coal shrinkage and reduction of its pore volume, thus diminishing its adsorption capacity. In this study, various types of raw (wet) brown coal were investigated as adsorbents for anionic and cationic dyes in aqueous solution. The adsorption capacities of these raw brown coals were compared with those of oven dried brown coals and also with activated carbon. It was found that raw brown coals exhibit high adsorption capacities for cationic dye, but very low capacities for anionic dyes, suggesting the involvement of a cation-exchange mechanism. Additionally, the cation exchange capacities correlate positively with its cation adsorption capacity highlighting the major role of strongly acidic groups in the adsorption process. Drying the brown coal significantly reduces its adsorption capacity for cationic dye, supporting the hypothesis of coal shrinkage and the consequent reduced pore volume upon drying. These findings suggest that raw (wet) brown coal may perform as well as activated carbon for the removal of positively charged species in aqueous solution. Regeneration of the used adsorbent may also not be necessary, since the used brown coal can be simply burnt as fuel or composted.

Methane adsorption-induced coal swelling measured with an optical method

In order to quantify the effect of matrix shrinkage on reservoir permeability during coalbed methane production, coal samples from Huozhou, Changzhi and Jincheng areas in Shanxi province (classified as high-volatile bituminous coal, low-volatile bituminous coal and anthracite, respectively) were collected, and adsorption-induced coal swelling in methane were determined by an optical method at 40°C and pressure up to 12MPa. All three coals showed similar behavior-that swelling increased as a function of pressure up to about 10MPa but thereafter no further increase in swelling was observed. Swelling in the direction perpendicular to the bedding plane is greater than that parallel to the bedding plane, and the differences are about 7.77–8.33%. The maximum volumetric swelling ranges from 2.73% to 3.21%-increasing with increasing coal rank. The swelling data can be described by a modified DR model. In addition, swelling increases with the amount of adsorption. However, the increase shows a relatively slower stage followed by a relatively faster stage instead of a linear increase. Based on the assumption that sorption-induced swelling/shrinkage of coal in methane is reversible, the permeability increases induced by coal shrinkage during methane desorption was analyzed, and the results indicate that the permeability change is larger for higher rank coal in the same unit of pressure depletion.

Coal permeability model on the effect of gas extraction within effective influence zone

In situ gas extraction is the main measure adopted by mines to achieve mining safety standards. Coal permeability is significantly influenced by coal shrinkage during gas release. Based on effective stress and additional damage that resulted from gas extraction, a unique model was proposed to describe the spatial and temporal distribution of coal permeability within the effective influence zone. Calculation models for the conditions of one and two extracting boreholes were developed. A parametric study on both two models was also conducted, in which the effect of extraction time, extraction pressure, interval between two boreholes, and extraction pressure combination were qualitatively analyzed. The results demonstrated that with the increase of extraction time, gas extraction can result in less permeability reduction in the beginning but can enhance permeability. Coal permeability obviously increases when extraction time increases but gradually stabilizes, presenting “high middle and low around” surrounding the extracting borehole. Meanwhile, a decreasing relative amplification implies that permeability ratio does not increase linearly with the increase in extraction pressure. Both intervals between two extracting boreholes and different combinations of extraction pressure significantly influence the distribution of coal permeability. A comparison with some previous permeability studies was conducted to validate the proposed model, and the predicted evolution results matched the permeability testing data. In conclusion, the proposed model is consistent with the phenomenon in real engineering, and the model results can provide preliminary guidance for the design of extracting borehole distribution.

Experimental studies on spalling characteristics of Indian lignite coal in context of underground coal gasification

Abstract Underground Coal Gasification (UCG) is considered to be a clean coal technology primarily intended to utilize deep underground (>300 m) coal deposits. In this process, a mixture of reactant gases like air/oxygen and steam are injected directly to an ignited portion of underground coal seam. UCG involves complex interactions of different processes like drying, pyrolysis, chemical reactions and spalling. Spalling is detachment of small coal particles from the coal seam due to interconnection of cracks developed in it. It plays an important role by offering higher surface area to give improved performance. The mechanism of spalling and its characterization are not well understood. Furthermore, there are no well established experimental techniques to measure the spalling rates. This paper studies spalling behavior of a lignite coal, which is characterized by high moisture and volatile matter, and suggests a possible underlying mechanism. The rate of spalling was measured using an experimental setup under the UCG-like conditions. In this setup, a reacting coal block was attached to a load cell and suspended in a UCG-like environment. When the experiments were repeated under similar conditions with different blocks of same coal, it was found that there were variations in the rates of spalling. This might be due to the heterogeneity in coal blocks in the form of originally present fissures or weak regions. A UCG process model was used to explain these experimental results and also to investigate the effect of spalling rate on product gas calorific value. We believe that spalling happens due to formation and extension of cracks. Hence a microscopic crack pattern on a heated coal monolith was examined in different stages of heating to understand the mechanism of spalling. It is concluded that cracks are first formed during the initial stage of drying due to the capillary stresses developed due to removal of moisture from the pores and were further extended due to shrinkage of coal during pyrolysis. The detachment of coal particles happens due to horizontal linking of vertical cracks, which might result out of either horizontal cracks, if any, or available fissures and weak regions or relatively weak interlayer bonding at the bedding planes.

Development of a material balance equation for coalbed methane reservoirs accounting for the presence of water in the coal matrix and coal shrinkage and swelling

The effects of water presence in the coal matrix and coal shrinkage and swelling phenomena are often ignored in the production performance predictions of coalbed methane reservoirs. This paper presents the development of a new material balance formulation for coalbed methane reservoirs that accounts for water presence in the coal matrix and coal shrinkage and swelling phenomena. The development entails the governing gas and water flow equations in dual-porosity, dual-permeability coalbed methane reservoirs. Various comparative studies are conducted to investigate the capabilities of the proposed and existing material balance equations using the production data generated from a robust two-phase, dual-porosity, dual-permeability coalbed methane simulator developed at Penn State. The results show that exclusion of the two aforementioned phenomena in coalbed methane material balance formalisms reduces the estimated reservoir production capacity resulting in under-predictions of reservoir size. In addition, iterative methods for predicting production performance and average reservoir pressure using the proposed material balance formulation are developed and successfully tested against the simulation model.

Impacts of coal shrinkage, permeability and petrography on gas regime in mines Case study: Tahmoor coal mine, NSW, Australia

The volumetric changes in the coal matrix (Coal Shrinkage), permeability under various gas environment conditions as well as perographical properties were studied in the laboratory. The shrinkage and permeability of coal were examined with respect to changing gas type and confining pressures. The shrinkage tests were carried out in high-pressure bombs while the permeability study was conducted in a specially constructed high-pressure chamber. Methane (CH4), carbon dioxide (CO2), nitrogen, (N2) and a 50% -50% volume mixture of CO2/CH4 gas were used in the study. The tests showed that under different pressure levels gas type affected permeability and shrinkage characteristics of coal. This paper presents a case study of Tahmoor Colliery, NSW, Australia and an overall discussion on coal shrinkage, permeability and coal petrography data of Tahmoor that permits a better understanding of the gas regime in this mine. The results are important to the further understanding of the inter-relationship between gas flow, the coal matrix and permeability in ‘normal’ and ‘tight’ coal conditions (locally referred to as disturbed coal).

Identifying sweet spots and moving CBM reservoirs into production

C oalbed methane (CBM) is well recognised as an important natural gas resource. However, producing it commercially has proven to be a challenging process, primarily due to the complex nature of its gas storage and flow mechanisms. Although CBM reservoirs are naturally fractured due to the presence of face and butt cleats, gas stor­age and saturation depend on the adsorption capacity of the coal matrix. The ability to predict permeability variations and its evolu­tion in a CBM reservoir, which is affected by several dynamic CBM properties and operating conditions, also define the design, optimization, and analysis of the gas recovery process. As the permeability is naturally low, interventions are necessary in order to achieve commercially viable gas production rates. CBM are also dual-porosity media where the vast majority of the gas is stored in the low permeability coal matrix by sorp­tion. The flow to production wells, however, occurs through the coal’s natural fracture system, that stores relatively small amounts of gas, because coal matrix practically has no perme­ability. The result is that properties of the coal matrix have the greatest effect on the estimates of gas in place and recovery. It is well understood that the key parameter controlling gas flow in CBM reservoirs are cleat networks, gas storage, desorp­tion and permeability anisotropy (coal shrinkage and swelling). While the role of reservoir simulation is well under­stood for conventional oil and gas reservoirs and its related techniques for prediction and production optimisation, for unconventional reservoirs (CBM, Shale, etc.) current reservoir simulation software needs to be further developed and adapted to represent the performance of these types of fields and gener­ate more acceptable results. For example, the basic governing fluid flow regime in conventional reservoir simulations is limited to Darcy flow. For unconventional reservoirs, however, another flow regime term comes into the equation that reflects the flow of gas in the coal matrix and stands for diffusion. This new flow regime is not very well defined in the current simulators and needs further studies to appropriately address the production performance of a reservoir. Furthermore, the concept of multi-phase flow in reservoirs is well defined by the introduction of relative permeability equations in the flow equations. However, measuring relative permeability in coal is relatively complex (Ham and Kantas, 2008) due to its friability, heterogeneity, stress dependence and porous morphology. Young et al. (1992), for example, noticed that the derived relative permeability from a history match does not resemble the laboratory measurement. This phenomenon could be due to the change of matrix structure on phase flow in addition to wettability and capil­larity. In other words, both phases may flow through different passages over time due to heterogeneity. Thus dynamic rela­tive permeability (phase saturation) is required to accurately predict recovery from a CBM reservoir. As the focus on CBM has increased in the past decades, so has the quest for more effective technologies to help bring the reserves in such complicated conditions into production and to identify sweet spots. Reservoir modelling and reservoir simulation are technologies that are likely to play a key role in achieving this. Firstly, however, it is necessary to look at the challenges of CBM property variations.