Coal Seam Gas Production Research Articles

Multiple tracers for dis-connectivity of shallow aquifers, alluvium, and coal seam gas wells in the Great Artesian Basin

The potential for connectivity between water supply aquifers and gas reservoirs raises community, government, and scientific concerns. Methane can occur naturally, making it difficult to determine whether water bore methane levels are being influenced by nearby gas operations. This poses a challenge in the Surat Basin, where coal seam gas production operates alongside groundwater using industries (including feedlots, agriculture, mines). Water and gas samples were taken from water bores and coal seam gas (CSG) wells in the Walloon Coal Measures and from overlying aquifers (nominally, the Springbok, Gubberamunda, Orallo, and Mooga sandstones) and the Condamine Alluvium, for stable isotopes of gases, groundwater and dissolved inorganic carbon, as well as strontium isotopes. Most of the sampled water bores had isotopic signatures distinct from CSG wells, though a minority from gassy Springbok Sandstone and Walloon Coal Measure water bores could not be distinguished from CSG wells. In those few cases, neither connectivity or dis-connectivity could be confirmed. Alluvium and shallow aquifer samples have higher R36Cl values distinct from the older CSG production waters, as is the case with most 14C measurements. Waters from the Condamine River indicate potential surface water connectivity with the alluvium. The use of multiple tracers has shown that groundwater in some aquifers can be differentiated from groundwater in the coal seam gas reservoir and hence are useful tools in identifying where groundwater connectivity occurs. Understanding this connectivity forms another line of evidence to improve impact prediction models on a regional scale as well as providing information on connectivity in local groundwater investigations.

The Reids Dome beds and its coal seam gas characteristics – Queensland’s latest coal seam gas target

Queensland’s coal seam gas production industry began in 1996 with production from the Permian Baralaba Coal Measures and Bandanna Formation located in the eastern and western Bowen Basin, respectively. It went to another level when production commenced from the Jurassic Walloon Coal Measures in 2005. Production in 2018/2019 amounted to nearly 1500 PJ with nearly 80% coming from the Walloon Coal Measures. The remaining 20% is sourced from the Late Permian Bandanna Formation, Baralaba Coal Measures and Moranbah Coal Measures. A minor volume comes from the Early Permian Cattle Creek Formation. The Early Permian Reids Dome beds are located in the Denison Trough, a graben complex located in the southwest Bowen Basin. The Reids Dome beds were deposited as the basal unit of variably sized grabens and half-grabens. The unit varies in thickness from 9 to >2770 m but the unit hasn’t been fully intersected where it is thickest. Net coal within the formation is also highly variable (<1 to >65 m). At present there has been no commercial production from the coals of the Reids Dome beds, but a number of small fields have reported commercial production from coals of the Early Permian Cattle Creek Formation. By definition, the Cattle Creek Formation was deposited in a marine environment and is contemporaneous with the upper Reids Dome beds. It is here suggested that coals intersected at these producing locations are located in the Reids Dome beds. The interpreted depositional centres for the Reids Dome beds underlie or are immediately adjacent to major Late Permian field developments with access to infrastructure and transmission. Ongoing exploration and appraisal work is continuing across the Denison Trough to fully understand the potential of these coals. Coals from the Reids Dome beds average >10 m3/tonne, >90% saturation >70% methane and up to 20% ethane.

Experimental study on ultrasonic irradiation for enhancing coalbed methane recovery

The present study proposes the use of a new ultrasonic irradiation method to enhance permeability and desorption for gas recovery from low-permeability coal reservoirs. A triaxial stress ultrasonic irradiation test apparatus was developed specifically for coal, considering the properties of gas adsorption, migration, and sound intensity, and providing a simultaneous measurement of gas flux, to investigated the deformation and temperature of coal samples obtained from the Fuxin coal field by permeability and desorption experiments. With the ultrasonic irradiation duration, the permeability of coal improved gradually with unequal variation, accompanied by the Klinkenberg effect where it decreased rapidly and then increased slowly with increasing gas pressure. The ability to desorb coal was enhanced by higher sound intensity ultrasound irradiation, and the volume of gas desorption was much greater than that of the sample without mange, the temperature and strain were demonstrated as a “J shaped” curve. An X-ray computer tomography (CT) technique was used to visualise the meso- or macro-cracks in the coal sample at pre- and post- ultrasonic irradiation, consequently, fractures expanded under the irradiation of ultrasonic waves. A permeability and desorption model was developed to describe the improvement of coal seam gas production capacity under ultrasonic irradiation, which introduced effective sound pressure.

An improved capillary pressure model for coal seam gas reservoirs

Coal cleats are the main flow path for gas and water during coal seam gas (CSG) production, and the two-phase flow behaviour in coal cleats is sensitively affected by the capillary pressure. Cleat aperture is a key factor controlling the magnitude of capillary pressure. The deformation of coal cleats is significantly affected by the reservoir depressurization and desorption-induced matrix shrinkage, but there does not appear to be any existing capillary models explicitly considering this effect. In this work, an improved capillary pressure model is developed, the impact of the cleat aperture change on capillary pressure value and residual saturation is considered, which is a key contribution of this work compared to the existing models. The model is then extended to reservoir scale to understand the impact of cleat aperture change on capillary pressure and CSG production performance.The results show that the cleat aperture change can alter the capillary pressure and gas production rate considerably. Larger Young's modulus and Langmuir volumetric strain constant can lead to smaller capillary pressure and higher gas rate as the cleat aperture rise more. Higher Poisson's ratio delivers larger capillary pressure and smaller gas rate as it gives less cleat aperture increase. Influence of Langmuir pressure constant on capillary pressure and production rate transit varies along the production process due to its complex impact on the change of cleat aperture. For the two newly introduced parameters in improved model, the cleat geometry factor can reduce capillary pressure and raise the production rate, while a larger residual saturation fitting factor gives smaller capillary pressure and production rate. Sensitivity analyses are also conducted to evaluate the relative importance of various factors. The findings from this study can help better understanding the impact of cleat aperture change on capillary pressure during CSG production, and is expected to deliver more reliable production estimation.

Sensitivity analysis on the microwave irradiation enhancing coal seam gas recovery: A coupled electromagnetic-thermo-hydro-mechanical model

Microwave irradiation (MI) technology is a potential effective method to improve the efficiency of coal seam gas recovery. However, the research on how to increase coal seam gas production by microwave heating is rare. In this paper, microwave stimulation technology is introduced in coal seam gas extraction area for the first time. Based on the gas extraction technology with directional long borehole, a new coupled electromagnetic-thermo-hydro-mechanical model for coal seam gas recovery is established. The heat and gas transfer characteristics of coal seam under MI heating is analyzed. What's more, the impacts of key parameters on gas production, temperature distribution, gas pressure distribution and permeability ratio are discussed. Under the MI power 1000 W, numerical simulation results show that the cumulative gas production increased by 80%, which indicating the great effective of microwave heating for improving gas extraction rate. Furthermore, it reveals that the daily gas production will decrease rapidly with the attenuation coefficient λ. And with the greater of attenuation coefficient λ, the faster the permeability ratio decays. Comparing with the case of λ = 1 × 10−8, the permeability ratio decreases by 26% when λ = 5 × 10−8. And we also found that the initial permeability k0 is negatively correlated with the peak pressure in matrix. Overall, microwave heating can effectively reduce the methane adsorption capacity of the coal matrix and significantly increase the gas extraction production from coal seam.

The effects of sorbing and non-sorbing gases on ultrasonic wave propagation in fractured coal

The acoustic wave propagation in geomaterials at different frequencies (ultrasonic, sonic, and seismic) has been extensively studied in the past to relate geomaterials' physical parameters to wave features. The ultrasonic analysis, commonly used for laboratory scale measurements, has been mainly employed to investigate wave propagation in non-sorbing rocks. The ultrasonic response of gas sorptive sediments particularly coal and its involved physical processes have however attracted limited attention to date. In this study, we use a combination of ultrasonic transmission and X-ray micro-computed tomography (XRCT) along with numerical simulation to investigate the mechanisms influencing the wave propagation in fractured coals. The simultaneous ultrasonic transmission and XRCT imaging are performed on stressed coal specimens saturated with non-sorbing (He) and sorbing (CO2) gases at different pore pressures. We further simulate ultrasonic wave propagation on segmented XRCT images and compare the numerical results with those of the experiments. A further numerical simulation is conducted on synthetic coal geometries to investigate the cleat and matrix property variation on wave propagation. Analysis of experimental and numerical results reveals that the closure of fractures (cleats) by matrix swelling (induced by CO2 adsorption) predominantly controls the change in wave velocities and the effect of change in matrix properties by gas adsorption on wave propagation is rather trivial. The numerical analysis on synthetic coal geometries further confirms that the effect of changes in coal matrix properties (density and stiffness) by gas adsorption on wave velocity is insignificant. In addition, the results show that the fracture aperture and density influence the wave velocities more significantly at low fracture porosity indicating that wave velocities are more sensitive to fractures when swelling is present (reduction in fracture porosity). These observations have significant implications in the characterization and monitoring of coal seam gas production and gas drainage.

Differential Strain Index-Based Multiphysics Model for Coal Seam Gas Production

Conventional multiphysics for coal seam gas production is generally coupled with poroelasticity-based coal permeability models, which are derived under the assumption that gas pressure is distributed uniformly within the representative elementary volume (REV). Under this assumption, the pore swelling/shrinking strain is equal to the bulk one. However, this assumption is considered as the primary reason for the inconsistency between experimental data, field data, and model results. In this study, a new concept of differential strain index (DSI) is proposed to theoretically define the relation among desorption-induced strains of the coal bulk, pores and solid grains. DSI is a function of the gas pressure, and its magnitudes are regulated by the Langmuir constants of both the solid grains and the coal bulk. Furthermore, a DSI-based coal permeability model is incorporated into the coupled multiphysics model for coal seam gas production. The model results of both laboratory-scale and field-scale show that coal permeability changes over a wide range during gas production. These changes are controlled by the DSI, and the magnitudes of change are defined by the Langmuir strain ratio of solid grains to coal bulk, while the trends of change are defined by the Langmuir pressure ratio of solid grains to bulk coal.

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.

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.

History, Geology, In Situ Stress Pattern, Gas Content and Permeability of Coal Seam Gas Basins in Australia: A Review

Coal seam gas (CSG), also known as coalbed methane (CBM), is an important source of gas supply to the liquefied natural gas (LNG) exporting facilities in eastern Australia and to the Australian domestic market. In late 2018, Australia became the largest exporter of LNG in the world. 29% of the country’s LNG nameplate capacity is in three east coast facilities that are supplied primarily by coal seam gas. Six geological basins including Bowen, Sydney, Gunnedah, Surat, Cooper and Gloucester host the majority of CSG resources in Australia. The Bowen and Surat basins contain an estimated 40Tcf of CSG whereas other basins contain relatively minor accumulations. In the Cooper Basin of South Australia, thick and laterally extensive Permian deep coal seams (&gt;2 km) are currently underdeveloped resources. Since 2013, gas production exclusively from deep coal seams has been tested as a single add-on fracture stimulation in vertical well completions across the Cooper Basin. The rates and reserves achieved since 2013 demonstrate a robust statistical distribution (&gt;130 hydraulic fracture stages), the mean of which, is economically viable. The geological characteristics including coal rank, thickness and hydrogeology as well as the present-day stress pattern create favourable conditions for CSG production. Detailed analyses of high-resolution borehole image log data reveal that there are major perturbations in maximum horizontal stress (SHmax) orientation, both spatially and with depth in Australian CSG basins, which is critical in hydraulic fracture stimulation and geomechanical modelling. Within a basin, significant variability in gas content and permeability may be observed with depth. The major reasons for such variabilities are coal rank, sealing capacity of overlying formations, measurement methods, thermal effects of magmatic intrusions, geological structures and stress regime. Field studies in Australia show permeability may enhance throughout depletion in CSG fields and the functional form of permeability versus reservoir pressure is exponential, consistent with observations in North American CSG fields.

A process-based coal swelling model: Bridging the gaps between localized swelling and bulk swelling

Injection of CO2 and coal seam gas production induce coal swelling/shrinkage. Laboratory tests indicate that coal swelling may go through several stages from localized swelling at fracture surfaces to bulk rock swelling. The coal Langmuir swelling strain constant normally ranges from 0 to 0.04. This study provides a process-based coal swelling model that covers the whole swelling procedure. We account for three swelling strains, videlicet, the matrix swelling strain, fracture swelling strain, and bulk rock swelling strain. Dynamic partial separation coefficients are employed to quantify the contributions of the matrix swelling strain to bulk swelling and the fracture (cleat) aperture reduction, bridge the gaps between localized swelling and bulk swelling, and link the three types of strains. This model is inserted into a permeability model and verified against permeability measurement and rock swelling data. The three samples used for verification are bituminous coals from the Illinois Basin (depth of ~ 200 m) in the US and Henan Province in China (depth of ~ 310 m). In essence, the swelling model is also applicable to other coal types with a linear relationship between the matrix swelling strain and adsorbed gas content. By matching with long-term permeability measurement data, multiple permeability evolution stages are observed due to the swelling transition. Sensitivity analyses show that the matrix block size, matrix swelling properties, and diffusivity control the duration, occurrence, and magnitude of matrix swelling’s contribution to local and bulk swelling during swelling transition, while rock bridge swelling properties have a marginal influence on coal swelling.

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.

Gas sources and concentrations in Surat Basin shallow aquifers: a field sampling method comparison, and isotopic study

The Surat Basin hosts various industries that extract groundwater including coal seam gas (CSG), feedlots, and agriculture. With water drawdown, gas has been observed in some bores drawing groundwater from different aquifers across the Basin. While methane can occur naturally in aquifers, biogenic CSG has been extracted from the Walloon Coal Measures raising questions on the sources of gas in overlying aquifers. Current standard monitoring uses a direct fill approach to measure dissolved methane concentrations in vials; however, this approach may lose gas present above solubility levels. Water and gas sampling was performed on bores in the Springbok, Gubberamunda, Mooga, Hutton, and Precipice Sandstones, the Orallo Formation, and the Condamine Alluvium. Water bores and CSG production wells from the Walloon Coal Measures were also sampled. We compared direct fill with a closed sampling method for dissolved gas, plus a method for sampling the total free and dissolved gas in the field. Higher dissolved and total methane concentrations were measured using closed sampling methods, especially in gassy bores. The majority of our sampled aquifer gases and waters have stable isotopic signatures distinct from CSG, where methane had likely been formed insitu in shallow aquifers by primary microbial CO2 reduction or fermentation processes. In several gassy bores, the source of the methane could not be clearly identified. This study indicates that (1) current monitoring methods may be underestimating methane concentrations above identified thresholds especially in gassy bores and (2) a combination of isotopic techniques may distinguish methane sources and interaquifer disconnectivity in the majority of cases.

Adherent microbes in coal seam environments prefer mineral-rich and crack-associated microhabitats

Biogenic coal seam gas is a bridge fuel that aids in the global transition to renewable resources. Since microbial activities form a significant portion of contemporary gas reserves, research into this area has increased over the past decade. As the initial attachment of early colonisers is considered a crucial step in the production of biogenic coal seam gas, investigation into biofilm formation has enhanced our understanding in generating gas reserves for energy security. Observations across multiple studies of Australian and Chinese coal have reported biofilm formation centred around cracks on the coal surface. Examination of scanning electron micrographs revealed lighter-coloured areas on the coal surface containing higher densities of adherent microbes. In this study, a replicated investigation into these phenomena was conducted. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and image analyses were used to calculate microbial densities. The light-coloured regions were identified as mineral-rich regions. It was calculated that microbial density was significantly higher on crack-associated and mineral-rich areas. This study has demonstrated the selective preference for microbial attachment to the surface of coal where crack-associated and mineral-rich areas are present. Further investigation to ascertain the mechanisms behind these phenomena would provide further insight to this type of behaviour and enhance our understanding of subsurface microbial catabolism.

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.

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.

Laboratory Measurements of the Relative Permeability of Coal: A Review

The relative permeability of coal to gas and water is an essential parameter for characterizing coalbed methane (CBM) reservoirs and predicting coal seam gas production, particularly in numerical simulations. Although a variety of studies related to the relative permeability of coals have been conducted, the results hardly meet the needs of practical engineering applications. To track the dynamic development of relative permeability measurements in the laboratory, discover the deficiencies, and discuss further work in this field, this paper investigates the relative permeability measurement preparation work and laboratory methods and summarizes the development of techniques used to determine the water saturation during experimentation. The previously determined relative permeability curves are also assembled and classified according to coal rank and the absolute permeability. It is found that the general operations in the relative permeability measurement process are still not standardized. The techniques applied to determine the water saturation of coal in experiments have been refined to some extent, but no optimal technique has been recognized yet. New techniques, such as the incorporation of high-precision differential pressure gauges, can be used to determine the water production during relative permeability measurement. In addition, the existing relative permeability data are limited, and no study has focused on supercritical carbon dioxide-water and mixed gas (methane and carbon dioxide)-water relative permeability measurements. To meet the requirements of actual projects, further research on this topic must be conducted.

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.

Complete coal permeability models from initial to ultimate equilibrium

Coal permeability is a crucial property for coal seam gas production. In previous studies, it was assumed that both matrix blocks and coal bridges share an average equalized gas pressure. This assumption results in an equalized swelling strain within the REV (representative elementary volume). As a result of this assumption, no change in fracture aperture is induced by gas sorption. However, this assumption may contradict the conceptual model under stress-controlled conditions. We hypothesize that this contradiction is due to the inability of current permeability models to consider the impact of non-uniform swelling on the evolution of permeability from initial to ultimate equilibrium. In this study, the aperture change is represented by an averaging method after obtaining the distribution of matrix swelling within the REV. The approach shows the matrix swelling initially reduces the fracture aperture, and the aperture recovers from initial to ultimate equilibrium. This approach has led to a new set of non-equilibrium permeability models under different distributions of matrix deformation. The model results clearly demonstrate that coal permeability evolves even under both constant confining pressures and constant effective stresses. Equilibrium permeability models represent the bounds of permeability change while new non-equilibrium models can fill the gap from the initial equilibrium state (pre-gas injection/extraction) to the ultimate equilibrium state (post-gas injection/extraction) to generate a map of permeability change. Therefore, our non-equilibrium permeability models represent complete evolutions of coal permeability.