Horizontal Fracture Research Articles

A novel way to look at the cement sheath integrity by introducing the existence of empty spaces inside of the cement (voids)

Horizontal drilling and hydraulic fracturing enabled the extraction of hydrocarbon from tight formations in countries such as the United States, China, and Argentina. Some factors such as stress anisotropy, formation temperature and quantity of natural fractures are reservoir specific and should be considered in the completion design; otherwise, high rates and pressures applied during hydraulic fracturing in a long horizontal section could induce wellbore integrity issues. Based on field parameters, we studied how the integrity of the cement and casing is affected during these operations using finite element analysis (FEA) to study the stress concentration in casing and cement. The yield criterion of equivalent von Mises stress was applied in order to verify if the stresses were exceeding the casing yield strength and the cement compressive strength. Simulation results revealed that a poor centralization that contributes to the formation of drilling fluid voids in the cement will induce casing failure during hydraulic fracturing operations. The equivalent maximum stress can increase from three to four times in casing and cement from a concentric case to an eccentric case in a cement sheath with voids.

Liberation enhancement and copper enrichment improvement for waste printed circuit boards by heating pretreatment

Crushing is the key part for the recycling technology of waste printed circuit boards (WPCBs). In this study, the breakage and liberation effects of WPCBs was improved by heat pretreatment technology before crushing (HPBC). Based on the test results, 200 °C was found as the safe temperature for the HPBC of WPCBs. The fracture mode, particle size distribution, and enrichment characteristics of WPCBs were systematically studied. The experimental results show that the HPBC changed the breakage mode from longitudinal fracture to horizontal fracture and improved the liberation of metal from non-metal components. During the crushing process, the increase in the heat pretreatment time (30–120 min) and temperature (100–200 °C) can improve the crushing effect of WPCBs and increase the content of crushing products of −0.3 mm by 3.16% and 5.64%, respectively. Compared to the non-metallic components, the metal components have ductility and are difficult to break into −0.3 mm during the crushing process. HPBC can promote copper enrichment to narrow grain size. In the 0.3–1 mm range, the content of copper increased from 47.87% to 57.61%, an increase by 9.74%. The initial enrichment of copper was achieved by adjusting the crushing time. The recovery rate of copper can reach 85.66%, and the enrichment rate is 1.74 when 0.3–2 mm breaking product is used as enrichment. Therefore, HPBC can effectively improve the crushing and liberation effect of WPCBs and improve the enrichment rate, and thus is an effective pretreatment method.

Analysis of Fractured Sections in Shale Gas Wells Based on PCA - Logistic Regression Model

In order to optimize the layout and economic exploitation of horizontal fracturing wells and completion in shale gas reservoirs, we propose a model for evaluating shale gas fractured sections based on an improved principle component analysis (PCA) algorithm with logistic regression. The 229 gas production sections in 22 fractured shale gas wells in the main block of the Fuling Shale Development Demonstration Zone were selected, and PCA is used for dimensionalite reduction. According to the PCA results, 6 key parameters are chosen to determine the productivity of fractured wells. Taking the probability distribution of high production after fracturing as the research objective, a logistic regression discriminant model was constructed using the dichotomy method. The prediction results show that the model has 82.1% accuracy and is reliable. The model can be used to classify and gas wells to be fractured, and it provides guiding significance for reasonable optimization of well sections in the area selected for fracturing.

Shale oil and gas resource evaluation through 3D basin and petroleum systems modelling: a case study from the East Midlands, onshore UK

Technological advances in horizontal drilling and hydraulic fracturing have led to a re-evaluation of the UK Carboniferous sequences for shale oil and gas potential. In the Gainsborough Trough, hemipelagic mudstones known collectively as the Bowland Shale were deposited during the Pendleian Substage (Late Mississippian). In this study the interpretation of heritage 2D and recent 3D seismic data allowed the reconstruction of the tectonic evolution of the basin, which was simulated in a 3D basin and petroleum systems model. The model enabled the first prediction of generated, adsorbed, retained and expelled hydrocarbon volumes. Between 8 and 26 Bbbl of STOIIP, and between 11 and 38 tcf of GIIP have been estimated to lie within the Bowland Shale in the Gainsborough Trough. However, at the present time, there is considerable uncertainty concerning these in-place volumes, and no tests have proven the recoverability of oil and gas from the Bowland Shale in this area. Importantly, the Bowland Shale has been modelled as a single homogeneous layer, and the in situ volume numbers need to be corrected for a net to gross factor, once the criteria required for the definition for net reservoir in this formation are better understood.

Pressure performance of multi-stage fractured horizontal well with stimulated reservoir volume and irregular fractures distribution in shale gas reservoirs

With the development of hydraulic fracturing technology of horizontal well, most unconventional gas reservoirs are developed by horizontal well multi-stage fracturing technology to improve gas well production. In this paper, we present a semi-analytical model of multi-stage fractured horizontal well (MFHWs) with consideration of adsorbed gas desorption and the irregular hydraulic fractures distribution in shale gas with the stimulated reservoir volume (SRV). On the bases of the point source function, Langmuir isotherm adsorption and Fick's law, wellbore pseudo-pressure solution of finite-conductivity MFHWs model is obtained by employing Laplace transform, finite cosine Fourier transform. and the discrete fracture approach in shale gas. The pressure transient responses log-log curves of MFHWs are drawn by Gauss elimination method and Stehfest numerical algorithm. In the light of the characteristics of the pressure derivative curve, the characteristics of each flow stage are identified and analyzed in detail. With the comparison of the commercial well test software numerical solution and the previous literature, the comparison results reach a good agreement under condition of same parameter. The effects of vital parameters, such as hydraulic fractures number and size of the SRV region, hydraulic fractures distribution styles and crossflow coefficient, on pressure transient curve are analyzed in detail. The presented model in this paper not only provides some insights for pressure dynamic analysis of shale gas reservoir development, but can help to engineers understand the pressure dynamics in shale gas reservoirs with SRV and irregular fracture distribution.

An efficient automatic history matching method through the probabilistic collocation based particle filter for shale gas reservoir

Shale gas can be economically developed with the advancement of horizontal well drilling and hydraulic fracturing. The complex multiscale flow mechanisms associated with the shale gas production simulation model can lead to severe model non-linearity, and the model parameters may not have the Gaussian distribution. Particle filter is a sequential data assimilation method derived from Bayesian theorem and can be an effective automatic history matching method for shale gas reservoir without the limitation on the model linearity and Gaussian assumption. However, particle filter may require a large amount of model evaluations to obtain the stable and accurate results. To improve the computational efficiency of the particle filter, we propose the probabilistic collocation based particle filter method (PCPF). An illustrative case study on shale gas production is conducted to validate the efficiency and accuracy of the proposed PCPF method. The results show that the computational efficiency can be greatly improved and it makes the full particle filter analysis computationally affordable. With more measurement data available, the parameter estimations and the corresponding shale gas production predictions can be more accurate and the uncertainties associated with the parameter estimations and shale gas production predictions are significantly reduced during the history matching process.

Optimization of horizontal well fracturing in shale gas reservoir based on stimulated reservoir volume

Optimization of multi-stage fracturing horizontal wells is with great significance in shale oil and gas reservoir production to enhance the gas recovery. The stimulated reservoir volume (SRV) is an important factor for fracturing horizontal well design and gas reservoir development. However, very few research has studied horizontal well optimization considering the effect of SRV. This paper proposes a mathematical model for SRV estimation during hydraulic fracturing. The model couples both rock and fluid mechanic effects. The displacement discontinuity method (DDM) is adopted to calculate the formation stress field. Then a natural fracture failure criterion based on formation pressure and stress fields is used to estimate the SRV. Additionally, a stress dependent permeability (SDP) algorithm is applied to estimate permeability change within the SRV region. Based on SRV and SDP models, an automatic optimization framework is established, with the net present value (NPV) employed as the optimization objective function, in which the overall cost and the ultimate income are involved simultaneously. A Modified-PSO algorithm is proposed and adopted to realize the optimization process, which can significantly reduce the calculation time while maintaining the accuracy of the result. In the end, two cases with shale gas reservoirs are presented, and different types of hydraulic fracturing wells are optimized to validate this optimization framework. Parameters including hydraulic fracturing spacing and half-length, well spacing and well length are optimized to achieve the optimum well design. The optimization results are compared and discussed, which show that this approach can contribute to obtaining the optimal shale gas recovery and provide a new insight into hydraulic fracturing well design under the effect of SRV.

The velocity-stress finite-difference method with a rotated staggered grid applied to seismic wave propagation in a fractured medium

To enable a mathematical description, geologic fractures are considered as infinitely thin planes embedded in a homogeneous medium. These fracture structures satisfy linear slip boundary conditions, namely, a discontinuous displacement and continuous stress. The general finite-difference (FD) method described by the elastic wave equations has challenges when attempting to simulate the propagation of waves at the fracture interface. The FD method expressed by velocity-stress variables with the explicit application of boundary conditions at the fracture interface facilitates the simulation of wave propagation in fractured discontinuous media that are described by elastic wave equations and linear slip interface conditions. We have developed a new FD scheme for horizontal and vertical fracture media. In this scheme, a fictitious grid is introduced to describe the discontinuous velocity at the fracture interface and a rotated staggered grid is used to accurately indicate the location of the fracture. The new FD scheme satisfies nonwelded contact boundary conditions, unlike traditional approaches. Numerical simulations in different fracture media indicate that our scheme is accurate. The results demonstrate that the reflection coefficient of the fractured interface varies with the incident angle, wavelet frequency, and normal and tangential fracture compliances. Our scheme and conclusions from this study will be useful in assessing the properties of fractures, enabling the proper delineation of fractured reservoirs.

Traumatic Atlanto-occipital Dislocation

Retrospective case analyses. The aim of this study was to investigate the incidence and characteristics of associated upper cervical spine injuries in 15 survival cases of traumatic atlanto-occipital dislocation (AOD). Traumatic AOD is a rare and generally fatal injury. Information regarding associated upper cervical spine injuries that may affect treatment methods, outcomes, and prognosis is limited. Fifteen patients (11 patients with posterior-type AOD and four patients with vertical-type AOD) who survived traumatic AOD were included in this study. Plain radiographs, computed tomography, magnetic resonance imaging, and medical records were reviewed. The incidence and characteristics of associated upper and lower cervical spine and thoracolumbar spine injuries and brain injuries were evaluated. Thirteen patients with traumatic AOD (11 patients with posterior-type AOD and two patients with vertical-type AOD) showed associated upper cervical spine injuries; the overall incidence was 87% (100% in posterior-type AOD and 50% in vertical-type AOD). In posterior-type AOD, eight (72.7%) patients had C1 fractures (four patients had three-part fracture, three patients had two-part fracture, and one patient had four-part fracture), four (36.4%) patients had C1 lateral mass fractures, two (18.2%) patients had transverse atlantal ligament injuries. In vertical-type AOD, two (50%) patients had C1-C2 vertical subluxation with C1 anterior arch horizontal and sagittal split fractures. In posterior-type AOD, two (18%) patients had lower cervical spine injuries and one (9%) patient had brain injury. In vertical-type AOD, one (25%) patient had thoracic spine injury, and three (75%) patients had brain injuries additionally. Survival cases with traumatic AOD showed a high incidence of associated upper cervical spine injuries and brain injuries. High index of suspicion and careful radiologic examination are needed to investigate the presence of associated upper cervical spine injuries and brain injuries in traumatic AOD, which affects treatment, outcome, and prognosis. 4.

Improvement of oil production in low permeability deposits with a system of horizontal wells

The article describes an approach to the production of difficult to extract oil, confined to deposits with low poroperm properties, using a system of horizontal wells and hydraulic multistage fracturing. The main problems that arise during the development of the well are analyzed. A conclusion is reached on the possibility of continuing to apply the method based on the analysis of pilot tests.

Unmanned aerial vehicle observations of cold venting from exploratory hydraulic fracturing in the United Kingdom

Unmanned aerial vehicle (UAV) surveys allow for rapid-response near-field sampling, downwind of emission sources, such as gas extraction sites, without the need for site access. UAVs can be used in emission source identification alongside instantaneous flux estimation. A UAV was used to sample downwind of the UK’s first and only gas extraction site to use exploratory onshore horizontal hydraulic fracturing (fracking) of shale formations, in Little Plumpton, Lancashire. In-situ calibrated UAV methane mole fraction measurements were made from a neighbouring field on five sampling days between October 2018 and February 2019, during fracking, flow-back and flow testing. Methane emissions were identified on one of the five sampling days (14 January 2019), associated with known cold venting, following fluid unloading using a nitrogen lift. A near-field Gaussian plume inversion approach was used to calculate four instantaneous fluxes on this day (from four separate intermittent UAV flight surveys) with lower and upper uncertainty bounds of between 9–80 g s−1, 23–106 g s−1, 16–82 g s−1 and 34–156 g s−1, respectively. The cold venting emissions observed on this single day were at least an order of magnitude higher than UAV methane fluxes calculated for nearby dairy farm buildings, also presented here. Identifying and quantifying these methane emission sources are important to improve the national emissions inventory and to regulate this developing UK industry.

Daily Satellite Observations of Methane from Oil and Gas Production Regions in the United States

Production of oil and natural gas in North America is at an all-time high due to the development and use of horizontal drilling and hydraulic fracturing. Methane emissions associated with this industrial activity are a concern because of the contribution to climate radiative forcing. We present new measurements from the space-based TROPOspheric Monitoring Instrument (TROPOMI) launched in 2017 that show methane enhancements over production regions in the United States. In the Uintah Basin in Utah, TROPOMI methane columns correlated with in-situ measurements, and the highest columns were observed over the deepest parts of the basin, consistent with the accumulation of emissions underneath inversions. In the Permian Basin in Texas and New Mexico, methane columns showed maxima over regions with the highest natural gas production and were correlated with nitrogen-dioxide columns at a ratio that is consistent with results from in-situ airborne measurements. The improved detail provided by TROPOMI will likely enable the timely monitoring from space of methane emissions associated with oil and natural gas production.

Transient pressure behavior for unconventional gas wells with finite-conductivity fractures

Horizontal wells and hydraulic fracturing technologies are the only way that makes commercial production from the low and ultra-low permeability unconventional plays possible. The development of analytical and semi-analytical tools to analyze transient behavior of multi-fractured horizontal gas wells have been largely biased towards solving the simplified problem of one-dimensional (1D) Cartesian gas flow in matrix domain, which are only applicable to the production from infinite-conductivity fractures. This study presents a novel semi-analytical method for transient behavior analysis of horizontal gas wells producing from finite-conductivity fractures. Unlike the available transient analysis tools for finite-conductivity fractures which are developed based on empirical and approximate solution, the proposed solution is rigorously derived by solving the governing gas diffusivity equations written for gas flow in two contiguous regions of matrix and fracture simultaneously. Nonlinear, pressure-dependent gas properties remaining in pseudopressure-based diffusivity equations—namely viscosity and compressibility—are rigorously and straightforwardly captured in both fracture and matrix systems. The validity of proposed solution is verified against commercial numerical simulator (COMSOL Multiphysics®) for a series of synthetic cases considering constant and variable -rate production constraints. The case studies presented in this paper also showcase the applicability of proposed solution to the analysis of transient behavior of multi-fractured horizontal gas wells in shale plays under the following two state-of-the-art interpretations: 1) production from finite-conductivity hydraulic fractures in a single-porosity matrix; and 2) production from infinite-conductivity hydraulic fractures in a naturally-fractured matrix.

Experimental Study of Proppant Transport in Complex Fractures with Horizontal Bedding Planes for Slickwater Fracturing

Summary The use of hydraulic fracturing has taken off over the past decade, and the importance of efficient proppant placement in fractures has increased accordingly. Connection with weak-formation bedding planes brings about distinctive fracture propagation and development. This might hinder hydraulic fractures from extending in a vertical direction, yet they might instead propagate in a horizontal direction after the existing bedding planes. The bedding plane introduces additional hydrocarbon flow path during the recovery process. This naturally leads to proppants traveling in the horizontal section of the fractures. Optimal proppant delivery and conductivity in bedding planes directly relate with effective fracture height and well performance. This paper focuses on understanding horizontal movement and delivery of proppants and their distribution in vertical and horizontal fractures on the basis of experimental work. The size of proppants is one of the key determining parameters when calculating uplift and settling velocity. They travel farther into the fracture because they settle slower with less gravitational effect. Fine mesh is highly recommended to effectively deliver proppant to the horizontal bedding plane. Large proppants have to rely on the critical bed height to move into the bedding plane sitting on top of the vertical fracture. A higher injection rate promoted bedding-plane proppant delivery in the inverted T-shape case because it opened up a longer and larger area for particles to settle. Excessively strong turbulent flow generated by two inlet cases prevented proppants from settling and seeping into the bedding plane. Continuous mixing and agitation disturb proppants from settling. They limited additional coverage area expansion in the bedding plane. Moreover, inlet design shows little effect in the T-shape, yet having more injection points improves proppant placement in the bedding plane. Direct injection to the bedding plane surprisingly did not affect much in the T-shape case. Overall, only a small percentage of proppant can travel into the bedding plane for both T-shape and inverted T-shape-fracture geometries.

Renormalization group study on connectivity of fracture network of overlying strata in deep coal mining

This paper focuses on the analysis of the connectivity of fracture network of overlying strata in deep coal mining based on the renormalization group method. Firstly, the probability formula of transfixion fracture formation in fractured overlying strata is derived out. The critical probability of transfixion fracture formation presents a negative exponent relation with the Weibull shape parameter. Secondly, on the basis of geological conditions of No. 15-22060 coal mining face in No. 8 Mine of Pingdingshan Coal Mining Group, the physical model under lateral pressure is constructed. During the process of coal mining face advancement, fracture networks of different advancement distances are extracted and binarized. These fracture networks are then transformed into matrixes of “0” and “1” elements. After applying the renormalization group method to these matrixes, the in-plane, horizontal, and vertical connectivities of fracture networks in deep coal mining are studied when the coal mining face is advanced 180 m. As to in-plane fracture networks of different classes, the last four matrixes obtained with the renormalization group method are zero, indicating that these networks are not connected, which is induced by the low occupancy of fracture areas of fracture networks of different classes. The distribution shape of “1” representing fractures is consistent with that of strata-separating fractures, and it is possible that the horizontal fracture is connected only if strata-separating fractures are present in the whole area of interest. The fracture networks of different classes are not connected in the vertical direction. However, when the area of interest is decreased, the proportion of elements “1” reflecting the vertical connectivity of fracture networks in the matrix is increased, and the rank of the zero matrix is decreased. These results may serve as a reference to the reasonable methane exploitation in deep coal mining.

Study of Oil-Water Two-Phase Stratified Flow in Horizontal Fractures

The relative permeability of oil-water two-phase flow is an important parameter in fractured petroleum reservoirs. It is widely accepted that the sum of relative permeabilities is less than 1. In this study, a series of experiments have been conducted on six rectangular fractures for oil-water two-phase flows. Analytical investigations of the effects of flow rate, aspect ratio, and fracture size on the relative permeability of oil-water two-phase are analysed. Basic fluid flow equations are combined to develop a new analytical model for water-oil two-phase flow in a horizontal fracture. The simulation results predicted by this model are in good agreement with the experimental data. The relative permeability is a function of flow ratio, viscosity ratio, aspect ratio and saturation. It increases as aspect ratio increases if the fracture depths are the same, while it decreases as aspect ratio increases if the fracture widths are identical. Both experiment and model indicate that the sum of relative permeabilities of oil and water is greater than 1 in some cases, different from the accepted view.

Effect of Coalbed Methane Well Fracturing on Slope Stability of Open‐Pit Coal Mine: A Case Study of Shengli East No. 2 Open‐Pit Coal Mine

The impact of hydraulic fracture from CBM well fracturing on slope stability of the Shengli East No. 2 open‐pit coal mine is analyzed by numerical simulation and limit equilibrium methods. The interference effect of coalbed methane (CBM) well fracturing on slope stability of the open‐pit coal mine promotes the coordinated development of CBM, and open‐pit coal is discussed before and after coal mining. It shows that nearly horizontal fractures are formed in the coal seam due to CBM well fracturing, of which the dynamic facture length and propping fracture length are 137.2 m and 105.2 m, respectively. Moreover, the dynamic fracture height is 137.2 m and propping fracture height is 130.6 m. At the location of perforation, the dynamic fracture width is 0.873 cm and average propping fracture width is 0.111 cm. The CBM well fracturing barely imposed any effect on slope stability before open‐pit coal mining. The maximum vertical displacement at the toe of slope induced by fracturing is 0.293 mm. In situations with and without CBM well fracturing, vertical stress distributions in the toe, top, and interior of slope have no obvious difference. There is some extent of vertical stress increase within the interior of slope, which is merely 0.2 MPa higher than that in the condition of initial in situ stress equilibrium. The presence of hydraulic fractures has little effect on the overall displacement of slope during coal mining; and there is no obvious difference between the slope stability during coal mining and the slope stability impacted by fracturing. According to the results of limit equilibrium method and numerical simulation, the overall slope stability coefficient is 1.5–1.97, which accords with the requirements of the Design Code for Open-Pit Mine of Coal Industry (GB50197-2015). Therefore, more attentions should be paid to the ways of excavation and sloping during coal mining, avoiding slope instability caused by excavation.

Accuracy of digital image enhancement in detection of vertical and horizontal root fracture

Two-dimensional intraoral radiography is the most common tool for recognizing root fractures. Improving the quality of images by means of enhancement tools can increase the recognition power of them. The aim of this study is to evaluate the effect of digital image enhancement on vertical and horizontal root fractures (HRFs) diagnostic accuracy. In this in vitro study, 100 human extracted teeth, involving 50 mandibular premolars and 50 maxillary incisors, were investigated. In total, 25 premolar teeth were vertically fractured and other 25 sound teeth served as testing group. According to the verified methods, 25 incisor teeth were fractured and other 25 teeth of this group served as testing ones. Following, by using the charge-coupled device sensor, preapical digital images were recorded. The original images were altered using reverse-contrast and colorization enhancement tools. Two different observers independently investigated all of the images. Receiver operating characteristic analysis was used to calculate the area under the curve (AUC) and sensitivity and specificity of all images. Data analyzde using receiver operating characteristic (ROC) analysis. Two-ways variance analysis was used to assess differences in the values (P = 0.05 ). AUC and sensitivity and specificity related to the original, reverse-contrast, and colorized images were calculated (0.84, 0.64, 0.99), (0.84, 0.64, 0.96), and (0.82, 0.64, 0.92) respectively, for vertically root fractured images. AUC and sensitivity and specificity related to the original, reverse-contrast, and colorized images were calculated (0.49, 0.44, 0.56), (0.50, 0.44, 0.60), and (0.48, 0.48, 0.48), respectively, for horizontally root-fractured images. The results of the present study revealed that reverse-contrast and colorized enhancement filters cannot be used as critical methods in detecting in vitro vertical and HRF.

Investigating the Potential Toxicity of Hydraulic Fracturing Flowback and Produced Water Spills to Aquatic Animals in Freshwater Environments: A North American Perspective.

Unconventional methods of oil and natural gas extraction have been a growing part of North America's energy sector for the past 20-30years. Technologies such as horizontal hydraulic fracturing have facilitated the exploitation of geologic reserves that were previously resistant to standard drilling approaches. However, the environmental risks associated with hydraulic fracturing are relatively understudied. One such hazard is the wastewater by-product of hydraulic fracturing processes: flowback and produced water (FPW). During FPW production, transport, and storage, there are many potential pathways for environmental exposure. In the current review, toxicological hazards associated with FPW surface water contamination events and potential effects on freshwater biota are assessed. This review contains an extensive survey of chemicals commonly associated with FPW samples from shale formations across North America and median 50% lethal concentration values (LC50) of corresponding chemicals for many freshwater organisms. We identify the characteristics of FPW which may have the greatest potential to be drivers of toxicity to freshwater organisms. Notably, components associated with salinity, the organic fraction, and metal species are reviewed. Additionally, we examine the current state of FPW production in North America and identify the most significant obstacles impeding proper risk assessment development when environmental contamination events of this wastewater occur. Findings within this study will serve to catalyze further work on areas currently lacking in FPW research, including expanded whole effluent testing, repeated and chronic FPW exposure studies, and toxicity identification evaluations.

Investigation of the factors causing the coalbed methane production differences in Shizhuangnan Block, Southern Qinshui Basin, North China

The geology and engineering characteristics are investigated in the Zaoyuan area of the Shizhuangnan Block, Southern Qinshui Basin to determine variations in the water/gas production not only amongst regions, but wells within the same region. Using geophysical logging, hydraulic fracturing and production data, it appears that a roof/floor with low shale content may be the cause of excessive water production when a coal reservoir predominantly develops horizontal fractures due to hydraulic fracturing. Maintaining a slowly bottom hole pressure decline rate before gas production and stable after producing gas is favourable for the extension of the pressure drop funnel. This is also more likely to help wells have high gas production over their lifetime. Whether or not a coal reservoir depressurises fully is crucial for gas production in both north and south regions. [Received: October 19, 2017; Accepted: July 5, 2018]