Coal Seam Gas Reservoirs Research Articles

Flowing Material Balance and Rate-Transient Analysis of Horizontal Wells in Under-Saturated Coal Seam Gas Reservoirs: A Case Study from the Qinshui Basin, China

Two phase flow and horizontal well completion pose additional challenges for rate-transient analysis (RTA) techniques in under-saturated coal seam gas (CSG) reservoirs. To better obtain reservoir parameters, a practical workflow for the two phase RTA technique is presented to extract reservoir information by the analysis of production data of a horizontal well in an under-saturated CSG reservoir. This workflow includes a flowing material balance (FMB) technique and an improved form of two phase (water + gas) RTA. At production stage of a horizontal well in under-saturated CSG reservoirs, a FMB technique was developed to extract original water in-place (OWIP) and horizontal permeability. This FMB technique involves the application of an appropriate productivity equation representing the relative position of the horizontal well in the drainage area. Then, two phase (water + gas) RTA of a horizontal well was also investigated by introducing the concept of the area of influence (AI), which enables the calculation of the water saturation during the transient formation linear flow. Finally, simulation and field examples are presented to validate and demonstrate the application of the proposed techniques. Simulation results indicate that the proposed FMB technique accurately predicts OWIP and coal permeability when an appropriate productivity equation is selected. The field application of the proposed methods is demonstrated by analysis of production data of a horizontal CSG well in the Qinshui Basin, China.

Geochemical influences on methanogenic groundwater from a low rank coal seam gas reservoir: Walloon Subgroup, Surat Basin

Hydrochemical data responds at a much slower rate to changes in groundwater conditions than does the propagation of hydraulic pressure, and therefore may provide more insight to groundwater flow paths. In low rank coal measures, where gas is biogenic, it is important to understand the fluid-rock and microbial interactions that affect the spatial and temporal distribution of groundwater composition. Pressure data may not reflect true groundwater conditions pre-anthropogenic influence, nor does it provide information on the main drivers of groundwater composition, actual aquifer behaviour or even prove groundwater flow.This study uses a process-based approach to interpret a combination of tracer (36Cl, 14C, 87Sr/86Sr, 18O/16O) and hydrochemical data obtained from coal seam gas production wells to identify the main geochemical processes and thus controls on the groundwater composition in different coal seam producing areas of the Walloon Subgroup, Surat Basin, Australia. This is arguably one of the largest coal seam gas producing regions in the world. Tracer data measured in this study show that the Walloon Subgroup behaves as a stagnant aquitard, as indicated by the almost total loss of cosmogenic tracers over relatively short groundwater flow distances (~15 km), suggestive of very low ground water flow velocities. The range of 36Cl is 9.0 to 23.8 (x 10−15) while the 36Cl values across the Undulla anticline in the eastern edge of the basin, are essentially the same (12.2–14.7) within analytical error. It is argued that these isotopic values represent secular equilibrium for the Walloon Subgroup. Radiometric carbon (14C) levels across all three production areas (Roma, Undulla Nose, Kogan Nose) are also too low (range = 0.12–1.95 pMC) for viable field interpretation largely owing to the long residence time of the groundwater and the local activity of methanogens. Groundwater flow velocity was estimated to be <0.1 m/y, which is significantly less than the 0.7 m/y recently reported for the underlying Hutton Sandstone.As a result of the low groundwater flow velocities, trends in geochemistry are visible only in production regions proximal to the subcrop. At flow distances greater than 10–15 km from subcrop, several low-temperature interactions (cation exchange, silicate weathering, matrix diffusion and hyperfiltration) start to influence groundwater composition. Shallow subsurface chemical and microbial reactions may initially dominate the geochemical composition of the meteoric groundwater, but this is then overprinted by the actions of sulfate reducers and methanogens, resulting in groundwater with the typical geochemical characteristics similar to other coal bed methane groundwater in basins across the world (low SO4, Ca, Mg and high HCO3, Na, Cl). As distance and depth increase further, low temperature fluid-rock interactions then begin to influence the groundwater composition.This holistic, process-based approach applied to a combination of cosmogenic and stable isotopes, and standard hydrochemical data interpreted against basin lithology has enabled a more comprehensive picture on the behaviour of the groundwater of the Walloon Subgroup and is applicable to the study of other sedimentary basins.

Effects of moisture, salinity and ethane on the competitive adsorption mechanisms of CH4/CO2 with applications to coalbed reservoirs: A molecular simulation study

Injection of carbon dioxide (CO2) into coal seam gas reservoirs has been considered one of the most popular approaches to enhance methane (CH4) production. However, many geological and engineering factors influence the efficiency of CH4 displacement by CO2. These factors include depth-induced changes in pressure and temperature, accompanied by gas compounds, moisture content, and salinity of the underground water. Thereby, the mechanisms of CH4 adsorption and the competitive adsorption processes between CH4 and CO2 in coalbeds at the microscope level remain to be explored. In this paper, a hybrid of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) method is conducted to probe the adsorption properties of CH4 and competitive adsorption behavior between CH4 and CO2 at 0–5 wt% moisture content, 0–6 mol/L saline of NaCl, and 0–5 wt% C2H6. Gas molecules adsorption processes are undertaken at 308, 338, and 368 K with pressure up to 30 MPa. Effects of Moisture, salinity and C2H6 effects on gas adsorption characteristics are discussed in detail. Based on the simulation results, moisture impedes the CH4 adsorption isotherms, which leads to a 17.1 % reduction in CH4 adsorption amount at 5 wt% moisture content. Moreover, salinity further decreases the CH4 adsorption capacity, which drops 6.1 % at 6 mol/L NaCl within 5 wt% moisture content. In addition, C2H6 is another negative factor in the CH4 adsorption process, which reduces the CH4 adsorption amount up to 36.6 % at 5 wt% ethane content. These mentioned factors influence preferential selectivity SCO2/CH4 shows the competitive adsorption behavior is also analyzed. Moisture content promotes the preferential selectivity SCO2/CH4 owing to CH4's more sensitivity to the presence of H2O molecules. Furthermore, high salinity hinders the CH4 solubility in water, whereas an inverse adsorption characteristic of CO2, thereby, high salinity improves the preferential selectivity SCO2/CH4. Besides, preferential selectivity SCO2/CH4 is also developed with the presence of C2H6. The observations of this study have important implications for a more accurate estimation of gas in unconventional resources and help with the displacement efficiency of CH4 by CO2.

Effective application of proppants during the hydraulic fracturing of coal seam gas reservoirs: Implications from laboratory testings of propped and unpropped coal fractures

Commercial production of coal seam gas is a significant proportion of the total Worldwide natural gas supply due to its high gas storage capacity at shallow extraction depths. Coal seam gas recovery may be enhanced by pure fluid-based or proppant based hydraulic fracturing – the decision whether or not to use proppants depends on coal seam characteristics – in particular stress-permeability characteristics. Although proppant application has been common in deep reservoir fracturing, its application on soft rocks such as coal should be carefully investigated. Higher concentrations of proppant are known to minimise proppant damage in deep reservoirs, with stiffness and flow tortuosity also impacting the response. We explored the impact of proppant loading, coal type and fracture roughness on fluid flow characteristics in propped fractures by conducting a series of triaxial permeability experiments combined with micro-CT imaging. The results reveal that proppant application in coal is only effective for deep coal seam gas reservoirs with effective stress greater than 6 to 8 MPa (~700 to ~900 m). Further, proppant application may cause up to 1–2 folds of fracture conductivity improvement in coal seam gas reservoirs located at over 1.2–1.4 km depths (effective stresses > 12 MPa). The impact caused by proppant embedment on fracture conductivity degradation is much significant compared to other proppant damage mechanisms. The effect, however, depends on the stiffness of the coal rock matrix and the proppant type implemented.

Simulation of Effects of Incident Angles and Frequencies of Ultrasonic Waves on Wave Energy Propagation and Variation of Cleat Width Induced by Ultrasonic Waves

Ultrasonic wave stimulation shows potential in enhancing the permeability and production rate of low-permeability coal seam gas (CSG) reservoirs, however, the mechanisms of mechanical vibration induced by ultrasonic waves in coal is still unclear. In this paper, the effects of physical attributes of ultrasonic wave signals, including the incident angles and frequencies, on the propagation/attenuation of wave energy and cleat width variation are numerically analyzed. In the numerical modelling, the propagation direction of ultrasonic waves to the orientation of cleats varies from 0° to 90° by an 10° increasing range, and the frequency range of signals varies between 2 and 10 MHz with an increment of 2 MHz. To evaluate the changes of mechanical vibration near coal-cleats induced by the changes of physical attributes of ultrasonic waves, four parameters are discussed, including shear wave energy (SE), accumulation of wave energy (WE), cleat-width variation at each moment (D1(t)), and accumulation of cleat-width variation during a certain period (D2). Numerical simulation results show that: (1) wave signals with larger incident angles to the orientation of cleats contribute to larger SE, longer SE accumulation near cleat interfaces, and larger D1(t) and D2; (2) the ultrasonic waves at higher frequencies contribute to larger SE near cleat interfaces, which attenuates more easily during propagation. Meanwhile, the D1(t) becomes larger during the initial period when waves firstly arrive at the cleat. However, in the case of smaller frequencies, comparatively larger D1(t) lasts longer after waves leave from cleat, and D2 is larger during the overall stimulation.

Relationship between coal composition, fracture abundance and initial reservoir permeability: A case study in the Walloon Coal Measures, Surat Basin, Australia

The Surat Basin, which is a major eastern Australian Coal Seam Gas (CSG) reservoir, produces gas from its Middle to Late Jurassic sub-bituminous Walloon Coal Measures, and shows variability in initial reservoir permeability across the basin. For a given lithostatic stress, the presence of vitrinite group or telovitrinite macerals within the coal can influence the fracture abundance within the coal reservoirs of similar rank, which in turn potentially impacts the initial reservoir permeability of the CSG reservoirs. The predictive relationship between megascopic banding character or lithotype, microscopic maceral composition and fracture intensity within CSG reservoirs is well explored in bituminous coals, but not so much in sub-bituminous coals. Here, we analyse wellbore image log data along with the conventional geophysical logs, maceral analysis data, core descriptions validated by core photos from the well completion reports and interpreted permeability data from well tests to systematically investigate the relationship between lithotype, maceral composition with a focus on vitrinite/telovitrinite, and fracture intensity in the sub-bituminous rank Walloon coals, to test their influence on permeability trends across the basin.Permeability-depth trends in the Walloon coals vary regionally across the basin, and show different inclinations depending on coal measures stratigraphy, coal lithotype composition, fracture intensity and stress orientation between local anticlines, monoclines and synclines. Depth normalised permeability and thickness-weighted telovitrinite content data from 55 wells were analysed. Furthermore, analysis of 3.8 km of borehole image logs, along with 156 maceral composition data points and thickness-weighted average lithotypes derived from the core description across 10 vertical wells, revealed a moderately positive relationship between the coal type and fracture abundance. This indicates some coal compositional control on the fracture abundance at this lower rank. The derived average total fracture intensity (cleat and sinusoidal coal fracture) from borehole image logs and thickness-weighted average telovitrinite content from the core analysis also exhibited an exponential increase, with the increasing depth normalised permeability derived from the well test data within the 10 analysed wells. This relationship suggests the influence of the variation in coal composition and fracture intensity on the rheological behaviour of the CSG reservoir within the Walloon Coal Measures. Spatial distribution of depth normalised permeability from the 55 wells across different domains and their relationship with the maceral composition within the study area also suggests the influence of geological structures on the depth normalised permeability. Therefore, though coal composition controls the fracture intensity, under favourable stress and fracture conditions the relationship can be substantially modified in parts of the Surat Basin, and altogether these factors control the rheological behaviour of the Walloon Coal Measures.

Numerical investigation of the effects of proppant embedment on fracture permeability and well production in Queensland coal seam gas reservoirs

This paper introduces the development of a new predictive model in support of proppant injection in naturally fractured coal seam gas (CSG) reservoirs. In the proposed model, the finite element method (FEM) is used for the prediction of proppant embedment and elastoplastic deformation of the coal. The lattice Boltzmann method (LBM) is applied to the modelling of fluid flow through propped fractures, in which the modified partially saturated method (MPSM) is implemented to characterise the fluid–solid interactions. Permeability diagrams of the fractures are then generated as functions of particle packing ratio and effective stress. Finally, these results are incorporated into a radial Darcy flow analytical solution to predict the productivity index of the CSG wells under various proppant injection pressures and cleat compressibilities. The developed model is applied to selected coal samples from the Bowen and Surat Basins in Queensland, Australia. Modelling results indicate that proppant injection leads to increased fracture permeabilities and enhanced well productivity indexes. The elastoplastic deformation results in smaller permeability increase and less production enhancement when compared to the traditional linear elastic models. Although focused on Australian coals, the developed workflow can be broadly applicable to the assessment of potential stimulation efficacy in other unconventional reservoirs. In addition, a better understanding and implementation of the proppant injection scheme can effectively improve the post-fracturing performance, particularly in low-permeability coal intervals, which benefits the CSG industry.

Induced stress evolution of hydraulic fracturing in an inclined soft coal seam gas reservoir near a fault

Hydraulic fracturing is an efficient technique for enhancing coal seam gas production. Induced stress (or stress shadow) of hydraulic fracturing has been popularly studied recent years. In this study, induced stress was measured in a field during the process of hydraulic fracturing. It was found that the induced stress evolution was closely related to the increase of injection volume. The relation between the average induced stress value and the injection volume was fit by the Michaelis-Menten equation well. When carried out the injection operation, we found that the induced stress increased significantly within the radius of 70 m around the hydraulic fracturing borehole. The induced stress also dropped occasionally during the injection process, which indicated some large fractures’ propagation. When the injection operation was finished, the induced stress began to decrease slowly. The subsequent drilling of gas extraction could release the induced stress. In addition, the induced stresses caused by adjacent fracturing boreholes (spacing was 50 m) substantially influenced each other. The induced stress acted earlier in the upper part of the coal seam than the lower part influenced by the steep incidence of the coal seam. Furthermore, the orientation of the induced stress distribution was eventually parallel to the maximum horizontal principal stress direction controlled by a nearby large fault, although the initial orientation was arbitrary. The results are helpful for hydraulic fracturing evaluation and optimization and numerical simulation research.

Improved modelling of pressure-dependent permeability behaviour in coal based on a new workflow of petrophysics, hydraulic fracturing and reservoir simulation

Many coal seam gas (CSG) reservoirs (also known as coalbed methane) can have low permeability, require stimulation to produce economic rates and often exhibit pressure-dependent permeability (PDP) behaviour. Defining PDP behaviour in coal using reservoir simulation is a non-unique problem based on the uncertainty in coal properties and input parameters. Recent research demonstrated that an integrated analysis coupling of a diagnostic fracture injection test analysis, hydraulic fracture modelling and reservoir simulation can better characterise PDP behaviour in order to evaluate stimulation effectiveness in coals (Johnson et al. 2020). The present work aims to improve the recently developed model by including multilayer and permeability anisotropy effects. A reservoir model with multiple coal layers is established in a pressure-dependent reservoir simulator, based on the image log interpretations. Permeability anisotropy in the formation is realised by introducing heterogeneous distribution of permeability in different directions. Modelling results indicate effects of aspect ratio between multilayers on the pressure distribution and production history. A lower permeability anisotropy ratio yields better well productivity, and higher stimulation is required to increase the stimulated reservoir volume to maximise gas recovery. The improved model and workflow are applicable to other CSG fields for defining key variables where hydraulic fracturing performance has been unable to overcome limitations based on pressure dependency, often accompanied by low-permeability behaviour. This workflow has applications in Australia and many areas (e.g. China and India) exhibiting low-permeability and PDP behaviour and where only typically collected field data is available.

Coupling of transient matrix diffusion and pore network models for gas flow in coal

Processes of sorption and diffusion play a critical role in the analysis and simulation of coal seam gas (CSG) reservoirs since most of the gas reserves are held by the coal matrix. Thus, it is essential to understand the displacement of gas in coal matrix and fractures. This can be formulated in terms of sorption and diffusion processes inside the matrix and advective flow in the cleats. The multi-scale porous structure and multi-physics nature of gas flow in coal complicate numerical modelling of these phenomena. A novel approach is developed to solve this problem by coupling conventional pore network modelling (PNM) with transient diffusion flow. The coupling is undertaken for each pore network throat separately by utilising the Finite Volume Method and Fick's second law of diffusion. The Langmuir isotherm is employed to describe sorption processes within the coal matrix and to link the concentration and pressure values when coupling the diffusion model with PNM. The proposed approach provides the possibility to account for the transient nature of gas transport across the different scales, while limiting the number of unknowns in the simulation to an effective diffusivity parameter. Our results demonstrate that steady-state pore network models can be effectively coupled with a transient phenomenon such as Fick's diffusion allowing to estimate the diffusive constitute of the matrix to the total gas flow in coal over time. The obtained gas sorption capacity and its release rate can then be used to accurately estimate the time-dependent production profile of CSG reservoirs.

Energy and Mineral Resources Mining Contradictions in Coal-Uranium Coexisting Basin and the Countermeasure Suggestions

It is common that sandstone-type uranium deposits co-exist with coal and oil & gas in the same sedimentary basin, and the output of uranium deposits is closely related to the coal seam or oil & gas reservoir. An “upper uranium and lower coal” ore-bearing pattern is formed in Ordos Basin and Erlian Basin in China. Due to the absence of unified resource mining plan and policy coordination mechanism, the coal-uranium resource mining contradictions have become increasingly prominent in coal-uranium coexisting areas in recent years. It is difficult to realize in-situ leaching exploitation of the coal-uranium resources in the Ordos coal-uranium coexisting area because of disorderly mineral resource mining, radioactive contamination risks are posed to the ambient environment of the mining area, and consequently, coal mine enterprises have to stop work and production, which gives rise to huge economic loss. Given this, the current resource contradictions in this area and their influence paths were comprehensively expounded through a systematic survey on a coal-uranium superposition area, and then the corresponding countermeasure suggestions were proposed.

Investigating the proppant damage mechanisms expected in a propped coal fracture and its effect on fracture flow

Although mixing of proppants with the fracturing fluid can significantly improve the productivity of many unconventional gas reservoirs, application of this technique to coal seam gas reservoirs has been challenging. This is particularly due to the unique characteristics of coal, including its highly soft nature that promotes proppants damage mechanisms in coal. Therefore, correctly understanding the fracture conductivity of proppant supported fractures under coal seam gas reservoirs' conditions is essential in enhancing the coal seam gas (CSG) extraction. A series of advanced laboratory experiments combined with an X-ray CT analysis was thus conducted under in-situ reservoir stress conditions to understand the behaviour of two widely using proppants types in coal (sand, and ceramic proppants). According to the results, although sand proppants are more effective compared to stronger ceramic proppants at shallow depth-conditions, in-efficient at deeper depths, at which ceramic proppants are more effective. Importantly, the fracture width variation is strongly influenced by the shape of the proppants, where sand propped fractures can be subjected to a significant fracture width variation, even under a low confining stress condition available at a shallow depth due to the high angularity and associated particles edges’ disintegration happens in the sand. At deeper depths (>6 MPa confinement), sand proppants are more vulnerable to being crushed under higher reservoir pressures, and ceramic proppants are more susceptible to embed into the coal surface. Overall, the study delivers a fundamental understanding of the proppant damage mechanisms in propped coal fractures under various types of proppants and their effect on the fracture flow, during the CSG production process. These findings would be essential benchmarks for the related future field applications.

A semi-analytical model for the transient pressure behaviors of a multiple fractured well in a coal seam gas reservoir

Multiple radial fractures can be generated in unconventional gas reservoirs after stimulation treatments such as hydraulic refracturing, high-energy gas fracturing, and explosive fracturing. This study presents a semi-analytical model to simulate the bottom-hole pressure (BHP) performances of a fractured well with multiple radial hydraulic fractures (MRHF) in a stress-sensitive coal seam gas reservoir. The adsorption–desorption and diffusion for fluid in reservoirs are considered. The continuous line-source function associated with discretization method in a composite coalbed gas reservoir is applied in Laplace domain. By means of Stehfest numerical inversion algorithm and Gauss elimination, the transient pressure responses in the well bottom are obtained and discussed.The results demonstrate that, from type curves, the main flow regimes for the proposed model in the coalbed gas reservoir are bilinear flow between adjacent radial hydraulic fractures, linear flow between adjacent radial hydraulic fractures, pseudo radial flow in stimulated region, and radial flow in un-stimulated region. Moreover, an obvious “hump” appears between the bilinear flow regime and the linear flow regime in stimulated region, which represents the interference between adjacent radial hydraulic fractures. The impacts of the ratio of permeability in stimulated region to that in un-stimulated region, the radius of stimulated reservoir volume (SRV), the permeability modulus, and the properties of hydraulic fractures on the transient pressure responses in well bottom are analyzed. The findings obtained in this study can pave the way for the quantitative understanding of the transient performances of multiple fractured vertical wells in unconventional reservoirs.

Modeling the Contribution of Individual Coal Seams on Commingled Gas Production

SummaryIn coal-seam-gas (CSG) fields, where single wells tap multiple seams, it is likely that some of the individual seams hardly contribute to gas recovery. This study aims to examine the contribution of individual seams to the total gas and water production considering that each seam can have different properties and dimensions. A sensitivity analysis using reservoir simulation investigates the effects of individual seam properties on production profiles.A radial model simulates the production of a single CSG well consisting of a stack of two seams with a range of properties for permeability, thickness, seam extent, initial reservoir pressure, coal compressibility and porosity. The stress dependency of permeability obeys the Palmer and Mansoori (1998) model. A time coefficient (α) relates seam radius, viscosity, porosity, fracture compressibility, and permeability. It is used to aid interpretation of the sensitivity study. Finally, two hypothetical simulation scenarios with five seams of different thicknesses and depths obtained from producing wells are explored. The range in properties represents conditions found in the Walloon Coal Measures (WCM) of the Surat Basin, relevant to the Australian CSG industry.Each seam in the stack achieves its peak production rate at different times, and this can be estimated using α. Seams with lower α reach the peak gas rate earlier than those with higher α-coefficient. The distinct behavior of gas-production profiles depends on the combination of individual seam properties and multiseam interaction. At a αratio &amp;gt; 1 (i.e., αtop/αbottom &amp;gt; 1), the bottom seam peaks first but achieves lower gas recovery than the top seam. An increasing αratio is associated with the inhibition of less-permeable seams and reduced overall well productivity. For αratio &amp;lt; 1, the top seam experiences fast depletion and total gas-production rates decrease drastically. This outcome is confirmed by a more realistic scenario with a higher number of coal layers. Poor combination of seams leads to severe production inhibition of some coal reservoirs and possible wellbore crossflow. The contrast of the seam-lateral extent in the stack and fracture compressibility play an important role in well productivity in the commingled operation of a stack of coal seams. Unfortunately, the lateral extent of individual coal seams is difficult to estimate and poorly known and, therefore, represents a major uncertainty in gas-production prognosis. The αratio analysis is a useful tool to gain understanding of modeled well productivity from commingled CSG reservoirs.

Numerical Simulation of Cracking Behavior of Precracked Rock under Mechanical-Hydraulic Loading

The cracking behavior of precracked rocks under mechanical-hydraulic loading is of great significance in underground openings or petroleum engineering. In this study, an advanced in-house finite element code PANDAS proved to be effective in simulating coupled fracturing processes under complex geological conditions was used to simulate the cracking propagation of the precracked rocks under mechanical loading and mechanical-hydraulic loading with different strength parameters. The simulation results demonstrated that (1) the cracks initiate by the induced stresses, and multiple types of tensile cracks originate from the preexisting flaws; (2) crack propagation patterns under mechanical-hydraulic loading were studied with different strength parameters, and the multiple patterns of pure tensile, main tensile, tensile-shear, main shear, and pure shear were observed; and (3) the timing of hydraulic loading has a significant impact on the fracturing process: when hydraulic loading was carried out in the phase of main crack propagation, the tensile fracture was promoted and the shear fracture was inhibited; when hydraulic loading was carried out in the phase of shear crack propagation, the shear fracture and tensile fracture were stimulated. The numerical simulation results are in good agreement with the experimental results by previous studies. The research on the cracking behavior of precracked rocks under mechanical and hydraulic loading will expand the application prospect in the fields of coal seam gas reservoir and tunnel water inrush.

Transient Pressure Analysis of a Multiple Fractured Well in a Stress-Sensitive Coal Seam Gas Reservoir

This paper investigates the bottom-hole pressure (BHP) performance of a fractured well with multiple radial fracture wings in a coalbed methane (CBM) reservoir with consideration of stress sensitivity. The fluid flow in the matrix simultaneously considers adsorption–desorption and diffusion, whereas fluid flow in the natural fracture system and the induced fracture network obeys Darcy’s law. The continuous line-source function in the CBM reservoir associated with the discretization method is employed in the Laplace domain. With the aid of Stehfest numerical inversion technology and Gauss elimination, the transient BHP responses are determined and analyzed. It is found that the main flow regimes for the proposed model in the CBM reservoir are as follows: linear flow between adjacent radial fracture wings, pseudo-radial flow in the inner region or Stimulated Reservoir Volume (SRV), and radial flow in outer region (un-stimulated region). The effects of permeability modulus, radius of SRV, ratio of permeability in SRV to that in un-stimulated region, properties of radial fracture wings, storativity ratio of the un-stimulated region, inter-porosity flow parameter, and adsorption–desorption constant on the transient BHP responses are discussed. The results obtained in this study will be of great significance for the quantitative analyzing of the transient performances of the wells with multiple radial fractures in CBM reservoirs.

Analisis Model Kecepatan Gelombang-P pada Coal-Seam Gas Studi Kasus Cekungan Sumatera Selatan, Indonesia

The rock physics model is one effective yet challenging way to investigate the coal-seam gas potential in Indonesia. However, because of the complex conditions of the Coal-Seam Gas Reservoirs, it is difficult to establish models. Despite the scarce modeling, this study aims to estimate the relation of gas-saturated within pores of coal seam to the elastic properties of rock, which is P-wave velocity. First, the coal seam minerals are applied to quantify matrix moduli using the Voigt-Reuss-Hill Average method. Pride’s simple equation is used to estimate the elastic properties of the coal seam at dry condition (zero gas saturation). Finally, Biot-Gassmann’s theory is applied to determine the elastic properties of coal seam with fully gas saturated. As the result, the proposed model showed that there is a significant negative correlation between gas content with both density and P-wave velocity of the coal seam. Finally, this P-wave velocity model of gas-saturated coal seams should be properly useful as the quick look for identifying coal seam gas potentials.

The effects of cross-formational water flow on production in coal seam gas reservoir: A case study of Qinshui Basin in China

In coal seam gas (CSG) reservoirs, hydraulic connectivity usually exists between the coal and overlying permeable formation, which results in the delay of the dewatering process and a large volume of produced water. The cross-formational water flow can dramatically influence the production of CSG reservoirs, reduce the economic viability of production and increase the impact on the environment.To assess the cross-formational water flow of a CSG well, this study investigates the effects of cross-formational water flow from the overlying permeable formation on gas production. Simulation models are constructed to represent the water crossflow in four connection scenarios, including contact zone with and without penetrated fracture, and no-contact zone with and without penetrated fracture. The penetration ratio, defined as the ratio of fracture height in permeable formation to the formation thickness, is employed to characterize the extension degree of hydraulic fracture in the overlying formation. The effects of cross-formational water flow on CSG production are analyzed using dimensionless production curves and relative permeability curves calculated from the production history. The field application of this study is demonstrated by the production history of fractured CSG wells in the Qinshui Basin, China.The simulation results show that if the overlying formation is directly connected to the coal seam (contact zone scenario), the hydraulic fracture leads to the decrease of the peak gas production rate with the cumulative water production increasing dramatically. If the overlying formation is connected to the coal seam by hydraulic fractures only (no-contact zone scenario), gas production increases in two stages. Initially, the gas production rate exhibits a slow increase, followed by a fast increase before reaching the peak rate. When the hydraulic fracture penetrates the overlying formation, a 10% increase of the penetration ratio results in 0.55% and 1.82% increase of cumulative water production in the indirect and direct connection scenarios, respectively. In both scenarios, 10% penetration ratio increase leads to 7.41% reduction of gas production rate. The methods and simulation results presented in this work can be used to evaluate the impacts of cross-formational water flow on gas production and identify the cross-formational water flow in CSG wells from production history.

Stable Dispersion of Coal Fines during Hydraulic Fracturing Flowback in Coal Seam Gas Reservoirs—An Experimental Study

In subterranean coal seam gas (CSG) reservoirs, massive amounts of small-sized coal fines are released during the production and development stages, especially during hydraulic fracturing stimulation. These coal fines inevitably cause mechanical pump failure and permeability damage due to aggregation and subsequent pore-throat blockage. This aggregation behavior is thus of key importance in CSG production and needs to be minimized. Consequently, such coal fines dispersions need to be stabilized, which can be achieved by the formulation of improved fracturing fluids. Here, we thus systematically investigated the effectiveness of two additives (ethanol, 0.5 wt % and SDBS, 0.001 and 0.01 wt %) on dispersion stability for a wide range of conditions (pH 6–11; salinity of 0.1–0.6 M NaCl brine). Technically, the coal suspension flowed through a glass bead proppant pack, and fines retention was measured. We found that even trace amounts of sodium dodecyl benzene sulfonate (SDBS) (i.e., 0.001 wt %) drastically improved dispersion stability and reduced fines retention. The retention was further quantified by fractal dimensional analysis, which showed lower values for suspensions containing SDBS. This research advances current CSG applications and thus contributes to improved energy security.

Effect of matrix shrinkage on wellbore stresses in coal seam gas: An example from Bowen Basin, east Australia

Coal matrix is subjected to shrinkage as a result of gas desorption. This can contribute to the stress distribution changes around the wellbore during the exploitation of Coal Seam Gas (CSG) resources. The knowledge of these stresses is essential for coal shear failure prediction and assessing coal fines production risks, particularly in horizontal wells which nowadays are becoming common in CSG fields.This study introduces an analytical model to estimate the stress distribution around the wellbore by coupling the effects of depletion, matrix shrinkage, and wellbore trajectory. The model is applied for coal failure analysis in both vertical and horizontal wells in Moranbah Coal Measures, in the Bowen Basin eastern Australia. The results reveal that the stress path value in CSG reservoirs, is not constant during production and it can even be more than one due to the matrix shrinkage. The shrinkage effect can significantly alter the effective horizontal stresses which accordingly can cause a considerable change to the near-wellbore stress distribution. It is shown that the stress differential may increase or decrease, depending on shrinkage/swelling magnitude and wellbore trajectory. In a normal fault stress regime, the matrix shrinkage decreases stress differential on the wellbore wall in vertical wells and consequently, reduces the coal failure potentials. However, in highly deviated and horizontal wells, the matrix shrinkage causes an extra increase of the tangential stress and reduction of radial stress on the wellbore wall (i.e., greater stress differentials on the wellbore wall). This increases the coal failure risks with depletion. The model was verified versus the observed coal failure pressure in the San Juan Basin, USA.