Cleat Permeability Research Articles

An optimum solution for coal permeability estimation from mesoscopic scale calibrated stochastic and deterministic discrete fracture network models

The efficiency of a coal bed as a reservoir of methane and CO2 sink primarily depends on its fracture permeability. Conventional techniques of permeability estimation from drilled-core plugs in the laboratory and analytical computations are often different from in-situ measurements by multiple orders. Hence, pre-drill estimation of coal bed permeability remains one of the major challenges for the operators. Researchers usually try to solve this issue by building DFN models based on the direct use of µCT images of millimeters scale coal chips. However, such small-scale DFN models also overestimate permeability by many folds. As a solution, we propose the deployment of calibrated Discrete Fracture Network (DFN) modeling technique based on mesoscale coal images. Deterministic and stochastic DFN models are simulated by replicating thirteen coal specimens collected from the Gondwana coal seams in the Raniganj coalfield of India. Different model outputs such as directional permeabilities, permeability anisotropy, connectivity, etc. are used to distinguish and calibrate these models. The derived average permeabilities from these models closely match with available field measurements. Thus, the proposed technique helps in minimizing the huge gap between the laboratory and well-test measurements. In the process, we derive the threshold sample/model size that would sufficiently address the average cleat permeability at the well-drainage scale. Additionally, sensitivity analysis of key fracture attributes is carried out to quantify their effect on coal permeability.

Impact of Stimulated Fractures on Tree-Type Borehole Methane Drainage from Low-Permeability Coal Reservoirs

Tree-type hydraulic fracturing (TTHF) is a promising method applicable to the effective development of methane in low-permeability coal seams. However, a large-scale application of this technique is limited due to the unclear impact of stimulated fractures by TTHF on the effect of post-fracturing methane drainage. To address this issue, a multi-scale methane flow model of coupled thermo-hydro-mechanical (THM) processes in stimulated coal seams by TTHF was developed and verified against laboratory-based measurements. Using this proposed model, a systematic evaluation of the influence extent of hydraulic fractures connecting sub-boreholes in a tree-type borehole on the drainage effect under different fracture apertures, initial permeabilities of the cleat system, and remnant methane pressures was performed. Detailed simulated results showed that the presence of highly permeable fractures induced by TTHF greatly enhanced, as expected, the drainage efficiency of coal seam methane between the ends of adjacent sub-boreholes, and led to a significant increase in the homogeneity coefficient β. Furthermore, increasing the stimulated fracture aperture and initial cleat permeability or reducing the remnant methane pressure also resulted in a larger value of β, but in turn shortened the lead time of the tree-type borehole. The β’s growth rate for different investigated cases compared to identical simulations without stimulated fractures presented an overall trend of increasing at first and then slowly decreasing with sustained drainage time. Meanwhile, large-aperture hydraulic fractures and lower remnant methane pressure are more beneficial to the drainage effect of tree-type boreholes in the initial stages of drainage. These results portrayed herein can be employed to better understand how fractures generated by TTHF play a role in post-fracturing drainage programs and provide theoretical assistance in engineering applications.

Semi-analytic analysis and optimization of stress-dependent permeability model for the coal bed methane gas reservoir

Coal reservoir productions depend entirely on cleat permeability and porosity values. The models used to date to calculate cleat parameters are either stress-dependent or strain-dependent. A study is carried to analyze the individual effects of stress and strain on the reservoir and fluid properties. A one-dimensional cleat for an under saturated low permeable coal bed methane reservoir is semi-analytically solved. Standard stress models, Shi-Durucan and Cui-Bustin, predict a range within which field values lie. A comparative study between strain and stress model behavior is carried out. The strain model considers matrix swell and shrink but ignores stress effects due to them, while a stress model considers them all. The stress-model is dependent on effective horizontal stresses in the reservoir. Stress models are preferred to strain model. Stress-dependent permeability being closer to actual values captures the field in a better manner. Though standard stress models are more accurate than the strain model, yet they cannot determine a specific permeability value at any given point in time. Emphasis is, therefore, laid on developing a new stress-dependent model. An iterative combination study of the standard models provides the new model. An empirical equation to calculate cleat properties, approximate to the field, is developed from the research. The new permeability model is substituted in the fluid production equation to obtain cumulative gas/water produced at any time interval. The results lie in the range predicted by the standard stress models and match the field observations, thus more reliable. The stress model is simple to use and is mathematically easy to formulate. It is flexible and can accommodate reservoir temperature and sorption strain changes.

Enhancing coal bed methane recovery: using injection of nitrogen and carbon dioxide mixture

Nitrogen (N2) and carbon dioxide (CO2) gases injection yield substantially various recovery ways. Here, a one-dimensional mathematical model is applied comprising of mass balances for explaining gas sorption and flow, and a geomechanical relationship to describe the permeability changes during gas injection. That’s why numerical simulations for investigating the performance of injecting flue gas mixture in enhanced coal bed methane (ECBM) recovery are represented. Significant intuitive features are found regarding the gas flow dynamics at the time of displacement and influences of sorption on ECBM operation. The study revealed that the injection of pure CO2 causes much reduction in cleat permeability, but pure N2 is injected to increase the permeability without any loss of injectivity. Therefore, pure CO2 causes more efficient displacement in terms of total CH4 recovery, while the addition of N2 to the mixture assists to make quicker the initial methane recovery.

Multiscale model for flow and transport in CO2-enhanced coalbed methane recovery incorporating gas mixture adsorption effects

In this work we develop a multiscale model for flow and transport problem in CO2-enhanced coalbed methane recovery. The coalbed methane reservoir is characterized by two levels of porosity associated with nanopores in the matrix and cleat network. Mass conservation equations for fluid mixture (CH4 and CO2) in the matrix at the microscale are rigorously derived by using the formal homogenization technique taking into account the gas mixture adsorption in the nanopores. The Density Functional Theory (DFT) is used to compute the gas adsorption isotherms and the solvation force acting on the nanopore wall, showing a much more pronounced adsorption capacity of CO2 compared to CH4. The average transport equations in the matrix together with the multiphase flow problem in fracture network (gas mixture and water) are homogenized giving rise to a macroscopic model ruled by the effective conductivities, partition and transfer coefficients. The cleat permeability evolution due to deformation is taken into account through a three-scale poromechanical model reported in a previous work. Computational simulations illustrate the macroscopic laws underlying the gas pressure distributions, cleat closure phenomena and CH4 production curve enhanced by CO2 injection.

Investigating reservoir stresses and strain effects during production from coal bed methane gas reservoir on reservoir properties and gas production – a numerical study

The effects of stresses and strains, occurring in a reservoir, on the cumulative gas production and the petrophysical properties are studied. Existing strain and stress model equations have been used to analyse the coupled effect of matrix shrinkage and cleat compressibility change on reservoir properties. A coal bed methane (CBM) gas reservoir, low permeable and undersaturated, is semi-analytically modelled. A nearly suitable model from among them is eventually proposed to predict the cleat permeability and gas production existing in the field using a dynamic drainage area (DDA) approach. These results vary with the production time. The strain model predicts gas production values and the stress models predict a range of cleat permeability variation values close to field values. Cleat permeability and cleat porosity both affect gas productivity but in the opposite manner. [Received: December 29, 2018; Accepted: May 14, 2019]

Numerical Simulation Study of Fines Migration Impacts on an Early Water Drainage Period in Undersaturated Coal Seam Gas Reservoirs

Coal fines migration exerts negative impacts on early water drainage of undersaturated coal seam gas (CSG) reservoirs. The complicated migration process results in ineffective and inaccurate forecast of coal fines production. Hence, a robust modelling tool is required to include the mechanisms of fines migration and to predict their impacts on rock and production. In this paper, fines migration in coal is categorized into three stages: generation, migration, and deposition processes. The corresponding models for different stages are established, including (1) a fines generation model, (2) the maximum fines-carrying concentration model and deviation factor of the modified Darcy model, (3) a fines deposition model, and (4) a dynamic permeability and porosity model. The above models are coupled with a water flow model, solved numerically using the finite difference method. Then, two dewatering strategies, including fast and moderate depressurization, are compared using the proposed models to study their effects on coal properties and following production. Finally, the production history of a CSG well in the Qinshui Basin, China, is utilized for history matching in a field case study. The simulation results indicate that new fines will be generated in a fast depressurization process and the water rate decline reduces the cleat permeability significantly. The newly generated fines can enhance the permeability temporarily, but they will block the flow channels and bring serious damage to the permeability when the water rate declines. The moderate depressurization strategy can produce the coal fines in a continuous mode, and the formation damage induced by fines deposition can be reduced to the acceptable level, which is the more reliable way to maintain well productivity. In addition, multiple well shut-in can trigger the irreversible fines deposition, reduce the permeability, and decrease the production rate.

Interaction of Cleat-Matrix on Coal Permeability from Experimental Observations and Numerical Analysis

Gas transport through porous coal contains gas laminar flow in the cleat network and gas adsorption/diffusion in the matrix block. Since permeable capacity of the cleat is greater than that of the matrix, change of the matrix pressure readily lags behind change in the cleat pressure. Such unsynchronized pressure changes can result in a complex compatible deformation of a cleat-matrix system, significantly affecting the coal permeability. In this paper, we investigated the cleat-matrix interaction on coal permeability by using a modified pressure pulse decay method integrated with numerical analysis. The experimental results indicate that the bulk volume of the coal sample rapidly expanded at the beginning of gas injection, and then the volume expansion rate of the coal sample slowed down as the downstream pressure of the coal sample gradually equilibrated with the upstream pressure. During this process, the coal permeability was observed to gradually decrease with time. Numerical analysis results indicate that gas transport from the cleat to the matrix can attenuate the differential pressure between the cleat and the matrix. A smaller ratio of initial matrix permeability to initial cleat permeability can prolong decay duration of the differential pressure inside the cleat-matrix system. Although the coal sample is subjected to a stress-controlled condition, the coal permeability response to gas diffusion is closer to the case using a constant volume boundary. The dynamic change of coal permeability is significantly affected by the cleat-matrix interaction, in cases where the short-term change is mainly attributable to the cleat network and the long-term change is controlled by matrix swelling/shrinkage.

Fully Coupled Multi-Scale Model for Gas Extraction from Coal Seam Stimulated by Directional Hydraulic Fracturing

Although numerous studies have tried to explain the mechanism of directional hydraulic fracturing in a coal seam, few of them have been conducted on gas migration stimulated by directional hydraulic fracturing during coal mine methane extraction. In this study, a fully coupled multi-scale model to stimulate gas extraction from a coal seam stimulated by directional hydraulic fracturing was developed and calculated by a finite element approach. The model considers gas flow and heat transfer within the hydraulic fractures, the coal matrix, and cleat system, and it accounts for coal deformation. The model was verified using gas amount data from the NO.8 coal seam at Fengchun mine, Chongqing, Southwest China. Model simulation results show that slots and hydraulic fracture can expand the area of gas pressure drop and decrease the time needed to complete the extraction. The evolution of hydraulic fracture apertures and permeability in coal seams is greatly influenced by the effective stress and coal matrix deformation. A series of sensitivity analyses were performed to investigate the impacts of key factors on gas extraction time of completion. The study shows that hydraulic fracture aperture and the cleat permeability of coal seams play crucial roles in gas extraction from a coal seam stimulated by directional hydraulic fracturing. In addition, the reasonable arrangement of directional boreholes could improve the gas extraction efficiency. A large coal seam dip angle and high temperature help to enhance coal mine methane extraction from the coal seam.

CO2 adsorption the numerical simulations and the controlling factors to low rank coal

Carbon Capture Sequestration (CCS) in unmineable coal seams questionably gives benefits for the commercial success through potential release of additional methane during the injection of CO2 adsorbs into the coal seams, the process known as enhanced coalbed methane (ECBM) recovery. However, a significant concern lies in the loss of injectivity due to reduction in permeability by coal matrix swelling occurences with CO2 adsorption although this effect can be partially be offset with ‘huff and puff’ scheme of cyclic CO2 injection followed by extraction of the released methane. The paper discusses the results of a numerical simulation study carried out with GEM compositional reservoir simulator to evaluate the effects of uncertainties in various reservoir parameters on the overall volume of CO2 storage and additional methane recovery of low rank coalfield. A 12-15m thick seam at shallow depth, 50-75 m was considered for fluid flow simulation study. While some information on the reservoir setting was obtained through literature and personal communication with the CBM operators, the rest of the information was derived through laboratory studies. The reservoir parameters considered for the study are injection pressure, adsorption capacity, cleat permeability and porosity, and initial gas saturation. A 100-acre drainage area with 5-spot vertical well pattern was considered with one central injector and four producers on four corners of the study area. The maximum allowable injection pressure was estimated to be 7500 kPa at the reservoir setting. The injection pressure was varied from 1000 kPa to 7500 kPa in the simulation. A number of adsorption isotherms were established in the laboratory. The variations in the adsorption parameters observed through the isotherms were considered as uncertainty in the storage capacities. Significant variations were observed due to the variation in adsorption isotherms both for CO2 storage and additional methane recovery. Fracture permeability was varied from 3 md to 200 md, which is the range of permeability observed in the coalfield is around 100-200 mD. The results of simulation indicate a strong influence of porosity on the CO2 storage and ECBM recovery. Fluid flow simulation study shows that variation in sorption time has no significant effect for a low permeability situation while some marginal effect in high permeability situation. Cleat porosity was varied from 1 % to 10 %. Within this range of porosities, enhanced methane recovery varied from 1 % to 10 % relative to the primary recovery but the volume of stored CO2 did not vary significantly. Lastly, the pore pressure, adsorption and gas saturation of CO2 sequestered volume and additional methane recovery were found to increase substantially.

Investigating the relative impact of key reservoir parameters on performance of coalbed methane reservoirs by an efficient statistical approach

The complex and unique production mechanism of CBM has been examined extensively; however, production from such reservoirs requires more investigation to be well-understood, predicted and enhanced. This study is aimed at probing the significance of some controlling parameters on CBM performance by a statistical approach.The relative impact of five CBM reservoir parameters (reservoir pressure, cleat permeability and porosity, Young's modulus and Langmuir pressure) on the performance of natural depletion as well as Enhanced Coalbed Methane (ECBM) were numerically investigated. Recovery factor (RF) for primary depletion and ECBM, original gas in place (OGIP) and CO2 storage were the investigated responses. In order to conduct the research, a synthetic CBM reservoir model was constructed using a commercial reservoir simulator. Since the effects of reservoir parameters on CBM production are quite complicated, it was intended to explore the potential interaction effects between the parameters along with the relative impact of each parameter. Therefore, a professional statistical software, Design Expert, was selected to determine the parameters' effects.The results show that while recovery factor value in primary recovery has positive correlations with all of the five parameters, cleat permeability and Langmuir pressure play the most significant roles. For ECBM by CO2 injection, cleat permeability has the most significant effect on recovery factor measure, followed by cleat porosity. The predicted model for ECBM recovery factor suggests that Young's modulus, opposite to the primary recovery condition, has an adverse relationship with RF and the cleat porosity-permeability interaction has a considerable negative effect on RF measure. The predicted coalbed methane OGIP model proposes that in comparison with pressure and Langmuir pressure, the relative impacts of the other three parameters are negligible. Furthermore, results reveal that CO2 storage is positively affected by cleat porosity and permeability, and negatively affected by Young's modulus and Langmuir pressure.

Theoretical Approach To Model Gas Adsorption/Desorption and the Induced Coal Deformation and Permeability Change

Recovery of coalbed methane (CBM) can trigger a series of coal-gas interactions, including methane desorption, coal deformation, and associated permeability change. These processes may impact each other. A primary objective of this analysis is to simultaneously quantify these interactions and their impacts during CBM recovery. To achieve this and other objectives, a rigorously coupled adsorption–strain–permeability model was developed. Gas adsorption, coal deformation, and cleat permeability characteristics were described using simplified local density (SLD) adsorption theory, a theory-based strain model, and matchstick-based permeability models, respectively. The strain model was verified against measured methane-adsorption-induced coal strain data published during the past 60 years, and the coupled model was tested using well test data measured in the San Juan Basin of New Mexico in the United States. Results suggest that the strain model is very consistent with measured coal deformation data for fluid ...

Enhanced Gas Recovery and CO2 Storage in Coal Bed Methane Reservoirs with N2 Co-Injection

Field demonstrations of carbon dioxide (CO2) storage in coal seam reservoirs have suffered greatly from lost injectivity due to coal swelling. The swelling of the coal matrix, due to preferential adsorption of CO2 as compared to methane (CH4), causes a reduction in the coal cleat porosity and a corresponding exponential reduction in cleat permeability, which negatively impacts CO2 injection and, as a result, CO2 storage. This might be the greatest technical hurdle to the commercial deployment of CO2-enhanced coal bed methane (ECBM) and storage operations. To mitigate the impact of CO2 swelling, this work will demonstrate that the co-injection of nitrogen (N2) may offset the swelling effects presented by the CO2 and consequently improve the utilization of injected gas in terms of incremental methane recovery, and determine the overall storage potential that these coal bed reservoirs may provide.Previous work employing Monte Carlo probabilistic techniques emphasized the impact of pressure dependent permeability which controls the flow, and Langmuir isotherms which control the coal storage capacity, on the storage and ECBM processes. These key parameters were thoroughly studied and defined on a coal rank basis. A parametric study was employed and conducted through reservoir simulation of various injection cases. The results show that low rank coals, thanks to their high porosity, need a minimum amount of N2 to reach optimum conditions. On the other hand, high rank coals, due to their high pore and matrix compressibility, require much more N2 to reach optimum conditions. The optimum methane recovery and sequestration conditions for medium rank coals were found to be highly sensitive to initial porosity values.Economic assessments were conducted only for the medium rank coals to explore the impact of costs on the engineered injection stream. This was done for the low (0.25%) and high (1%) porosity cases. For high porosity cases, the economics confirmed the findings from the simulations requiring less N2 in the optimized injection stream (0 to 20%) to achieve a positive net present value. For the low porosity cases, the gas price had to be set to $6/Mcf to force any case to screen economic. At this setting, only a 60% N2/40% CO2 composition was found to be economic.

Experimental study on physical structure properties and anisotropic cleat permeability estimation on coal cores from China

Knowledge of the natural structure properties of coal seams is essential for the coal bed methane (CBM) production because of their great influence on the inner flow characteristics and permeability features of hydrocarbons and water. In this paper, a series of laboratory tests, including nondestructive low-field nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray computed tomography (CT) and core tests were carried out to characterize the physical structure properties of coal. The pore size distribution, pore type, morphological features and three-dimensional rendering of coal cleat structures are presented. The permeability changes of 7 cylindrical coal cores investigated under a varying effective stress range (0–35Mpa) reveal that coal porosity decreased linearly and coal permeability declined exponentially with the rise of stress; while under higher stress conditions, the occurrence of internal crushing and mechanical damage will result in irreversible change for porosity and permeability. Through the comprehensive analysis based on linking permeability to porosity and CT result, a basic estimation on directional cleat permeability is also realized. This method provides a preliminary but satisfactory estimation for local directional cleat permeability, which reflects that both bedding and non-bedding direction permeability owns great heterogeneity for an individual sample; but face cleat permeability always found to be the largest, and the mean bedding plane permeability value is found usually 3.01–11.68 times lower than face and butt cleat permeability.

Distinct performance characters and inducing mechanisms of CO<SUB align="right">2 and N<SUB align="right">2 enhanced coalbed methane recovery

The distinction of performance characters and inducing multiple mechanisms in CO2 and N2 enhancement are ambiguous, even though the different stimulation performances have been discussed in existing studies. This article establishes an essential mathematical model and a comparison of the recovery dynamics with CO2 and N2 injection is conducted. The results indicate that the distinct characters of CO2 and N2 enhancement derive mainly from the different properties of porosity, permeability and gas sorption. The N2 injection appears more time efficient. The variation of porosity and permeability plays more complex roles when CO2 is injected as the coal seam is divided into two regions of gas concentration, pressure and cleat permeability. The influence of the coal seam and gas properties is discussed. The parameters related to the porosity and permeability variation and gas sorption inflict quite different influences between CO2 and N2 injection, while the coal seam intrinsic properties impact similarly. [Received: November 22, 2013; Accepted: September 7, 2014]

Porosity type analysis and permeability model for micro-trans-pores, meso-macro-pores and cleats of coal samples

Porosity and permeability interact strongly, and the former describes the pore space for fluid storage, whereas the latter is used to describe the ability of fluid to migrate through the rock. Coal pore systems can be commonly classified as micro-trans-pores, meso-macro-pores and cleats, which indicates that the interactions between permeability and porosity in each pore system should be diverse. To investigate this diversity, measurements of nuclear magnetic resonance and routine core analysis are conducted for fifteen coal samples collected from underground mines in China. Then, the total porosity, movable porosity and unmovable porosity of each pore system are calculated, and the scatter diagrams of the three types of porosity versus air permeability are plotted. The results indicate that air permeability is influenced by both movable meso-macro-pore porosity and movable cleat porosity, and is weakly affected by the unmovable porosity of each pore system. According to the relative values of movable meso-macro-pore porosity and movable cleat porosity, coal samples are classified as cleat dominated permeability samples, meso-macro-pore dominated permeability samples, and combined effect samples. The former two types of samples can be used to establish models of cleat permeability and matrix permeability, respectively, based on the modified matchstick and Coates models.

Enhancement of CBM well fracturing through stimulation of cleat permeability by ultra-fine particle injection

Coal permeability declines due to fracture closure during the dewatering stage. A new technique for stimulation of natural coal cleats through ultra-fine and ultra-light high-strength particle injection into a coal fracture system is proposed. Coupling this technique with hydraulic fracturing treatment resulted in particles entering cleats under leak-off conditions. The following optimal water-based coreflood experimental conditions were determined by applying Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to the interaction between glass particles and the coal matrix: stability of a particle-based suspension (no agglomeration); repulsion between particles and the coal matrix; and, immobilisation of coal natural fines. At these conditions, these particles were placed inside cleats and were not attached to the cleat entrance, leading to less external cake formation; no formation damage due to fines migration was observed. The experimental study was carried out on some bituminous coal samples. Micro-sized glass particles were injected into a coal core at minimum effective stress until core permeability decreased to a value predetermined by a mathematical model. An increase of the effective stress to its maximum value by injection of particle-free water resulted in an approximate three-times increase in coal permeability, when compared to the original value. The proposed technique can be used for stimulation of a natural fracture network in conventional and unconventional reservoirs, as well as for the enhancement of conductivity of micro-fractures around the hydraulically induced fractures. These particles can be used as a non-damaging leak-off additive during hydraulic fracturing stimulation treatments leading to long-term fracture conductivity.

Coal cleat permeability for gas movement under triaxial, non-zero lateral strain condition: A theoretical and experimental study

As the permeability of coal seams is mainly determined by the network of natural fractures known as the cleat system, estimation of cleat permeability is of utmost importance for the carbon dioxide sequestration process in deep coal seams. The main objective of this study is to develop a new mathematical model for predicting cleat permeability under non-zero lateral strain conditions such as the conditions encountered in laboratory triaxial experiments. By applying the theory of elasticity to the constitutive behaviour of fractured rocks, a theoretical relationship between permeability and gas injecting pressure, confining pressure, axial load and gas adsorption in triaxial tests is developed. The new model was then verified using experimentally-determined permeability data of two coal samples. Results indicate that the new model can fairly accurately predict the combined effects of effective stress and coal matrix swelling on cleat permeability for both CO2 and N2 injections at various injection pressures. The model also provides quite accurate prediction of the effect of confining pressure on cleat permeability for both CO2 and N2 injections. The model includes parameters for fractured rock properties, namely Poisson’s ratio and Young’s modulus. The model can be applied to predict cleat permeability, regardless of cleat size. When the accuracy of the new model is compared with the existing Gilman and Beckie [5] model, with increasing injecting pressure both models show similar increments of N2 permeability and different reductions for CO2 permeability. This is due to the zero lateral strain assumption of the existing model, which is not applicable to the swelling process under triaxial test condition. The new model is more accurate for the prediction of CO2 cleat permeability under triaxial test condition.

A Field Study on Simulation of CO2 Injection and ECBM Production and Prediction of CO2 Storage Capacity in Unmineable Coal Seam

CO2 sequestration into a coal seam project was studied and a numerical model was developed in this paper to simulate the primary and secondary coal bed methane production (CBM/ECBM) and carbon dioxide (CO2) injection. The key geological and reservoir parameters, which are germane to driving enhanced coal bed methane (ECBM) and CO2 sequestration processes, including cleat permeability, cleat porosity, CH4 adsorption time, CO2 adsorption time, CH4 Langmuir isotherm, CO2 Langmuir isotherm, and Palmer and Mansoori parameters, have been analyzed within a reasonable range. The model simulation results showed good matches for both CBM/ECBM production and CO2 injection compared with the field data. The history-matched model was used to estimate the total CO2 sequestration capacity in the field. The model forecast showed that the total CO2 injection capacity in the coal seam could be 22,817 tons, which is in agreement with the initial estimations based on the Langmuir isotherm experiment. Total CO2 injected in the first three years was 2,600 tons, which according to the model has increased methane recovery (due to ECBM) by 6,700 scf/d.

Permeability increase in Bowen Basin coal as a result of matrix shrinkage during primary depletion

Matrix shrinkage in coal-seam gas reservoirs as a result of production, and the resultant impact on absolute permeability, is a current area of intense interest. A number of publications have documented the existence of this phenomenon. The exponential growth behaviour of coal seam permeability with reservoir pressure depletion has been observed previously at the Fairway wells in the San Juan basin. More recently, an exponential trend has also been confirmed for a group of 10 wells in a region northeast of Fairway. An increase in the absolute permeability of coal seam is a result of matrix shrinkage caused by gas desorption, which becomes a dominant factor on cleat permeability over the effective horizontal stress change during production. Bottom hole pressure recovery during shut-in periods can provide valuable data concerning the impact of coal seam gas production on reservoir permeability. Arrow Energy's historical production data, from the MGP field in the Bowen Basin, contains over 60 instances of data on pressure recovery during shut-in. These time lapse build-up periods have been analysed using the pseudo-pressure method (Kamal and Six, 1991) in an effort to gain an understanding of the extent and effect of this phenomenon within the company's existing assets. The method is based on a pseudo pressure function that uses the adsorption isotherm and the relative permeability relations of the reservoir. The infinite acting radial flow regime was identified and the gradient was recorded on a semi log plot of the pseudo pressure versus dimensionless time. Results from these tests show multi fold increase in absolute permeability as a function of the estimated reservoir pressure.