A new concept for the extraction of gas from Permian ultra-deep coal seams of the Cooper Basin, Australia: Expanding Reservoir Boundary Theory

2020 ◽  
Vol 60 (1) ◽  
pp. 296
Author(s):  
Erik C. Dunlop
Keyword(s):  
Surface Area ◽  
Host Rock ◽  
High Stress ◽  
Shielding Effect ◽  
Gas Extraction ◽  
Coal Seams ◽  
Extraction Technology ◽  
Matrix Shrinkage ◽  
Coal Matrix ◽  
Cooper Basin

An alternative geomechanical reservoir boundary condition is proposed for ultra-deep coal seams of the Cooper Basin in central Australia. This new concept is embodied within the author’s ‘Expanding Reservoir Boundary (ERB) Theory’, which calls for a paradigm shift in gas extraction technology, diametrically opposed to current practices. As with shale, full-cycle, standalone commercial gas production from Cooper Basin ultra-deep coal seams requires a large stimulated reservoir volume (SRV) having high fracture surface area for gas desorption. This goal has not yet been achieved after 13 years of trials because, owing to the bipolar combination of shale-like reservoir properties and coal-like geomechanical properties, these poorly cleated, inertinitic coal seams exhibit ‘hybrid’ characteristics. Stimulation techniques adopted from other play types are incompatible with the highly unfavourable combination of nanoDarcy-scale permeability, ‘ductility’ and high stress. Nevertheless, gas flow potential counterintuitively increases with depth, contingent upon the creation of an effective SRV. Optimum reservoir conditions occur at depths beyond 9000 feet (2740 m), driven by dehydration, high gas content, gas oversaturation, overpressure and a rigid host rock framework. The physical response of ultra-deep coal seams and the surrounding host rock to pressure drawdown is inadequately characterised. It remains to be established how artificial fracture and coal fabric aperture width change due to the competition between desorption-induced coal matrix shrinkage and compaction caused by increasing effective stress. Studies by the author suggest that pressure arching may ultimately control gas extraction efficiency. Harnessing this geomechanical phenomenon could resolve the technical impasse that currently inhibits commercialisation. Pressure arching neutralises SRV compaction by deflecting stress to adjacent strata of greater integrity. These strata then function as an abutment for accommodating increased stress outside the SRV. This shielding effect allows producing ultra-deep coal seams to progressively de-stress and ‘self-fracture’ naturally, in an overall state of shrinkage-induced tensile failure. An ‘expanding reservoir boundary and decreasing confining stress’ condition is generated by the combined, mutually sustaining actions of coal matrix shrinkage and sympathetic pressure arch evolution. This causes the SRV to steadily increase in size and permeability. Cooper Basin ultra-deep coal seams may be effectively stimulated by harnessing this self-perpetuating, depth-resistant mechanism for creating permeability and surface area. The ultra-deep coal seams may be induced to pervasively ‘shatter’ or ‘self-fracture’ naturally during production, independent of ‘brittleness’, analogous to the manner in which shrinkage crack networks slowly form, in a state of intrinsic tension, within desiccating clay-rich surface sediment.

Gas Storage and Flow in Coalbed Reservoirs: Implementation of a Bidisperse Pore Model for Gas Diffusion in Coal Matrix

2005 ◽  
Vol 8 (02) ◽  
pp. 169-175 ◽  
Author(s):  
Ji-Quan Shi ◽  
S. Durucan
Keyword(s):  
Surface Area ◽  
Gas Transport ◽  
Gas Production ◽  
Dual Porosity ◽  
Coal Seams ◽  
Production Rates ◽  
Free Gas ◽  
Coal Matrix ◽  
The Face ◽  
Reservoir Simulator

Summary Unipore diffusion models are used widely to model gas transport in a coal matrix in conventional dual-porosity coalbed-reservoir simulators. The unipore models implemented in conventional coalbed-reservoir simulators assume that there is a negligible free-gas phase in the coal matrix and that gas exists only in an adsorbed state under hydrostatic pressure. In low-rank coals, however, a substantial amount of free gas may exist in the macropores of the coal matrix. There is strong laboratory evidence that many coals exhibit bi- ormultimodal pore structure. This paper describes the implementation of abidisperse pore-diffusion model in a coalbed-reservoir simulator. In the bidisperse model, gas adsorption is assumed to take place only in the micropores, with the macropores providing storage for free gas, as well astortuous paths for gas transport between the micropores and cleats. Gas-production performance from a sub-bituminous Powder River basin coalbed reservoir has been studied using an in-house coalbed-reservoir simulator. The implementation of the triple-porosity formulation in the simulator overcame the reported inconsistency between field gas-production rates and predicted rates obtained with conventional dual-porosity simulators. With the introduction of an appropriate storage volume of free gas in the macropores, the predicted increase in gas-production rates are consistent with the published field data. Introduction Coal seams may be characterized by two distinctive porosity systems: a well-defined and almost uniformly distributed network of natural fractures(cleats), and matrix blocks containing a highly heterogeneous porous structure between the cleats. The cleat system can be subdivided into the face cleat, which is continuous throughout the reservoir, and the butt cleat, which is discontinuous and terminates at intersections with the face cleat (Fig. 1). The cleat spacing is very uniform and ranges from the order of millimeters to centimeters. Unlike conventional gas reservoirs, methane in coalbeds is stored primarily as a sorbed gas, at near-liquid densities, on the internal surface area of the microporous coal. The surface area of the coal on which the methane is adsorbed is very large (20 to 200 m2/g) and, if saturated, coalbed-methane reservoir scan have five times the volume of gas contained in a conventional sandstone gas reservoir of comparable size. Virgin seams are often saturated with water. During primary recovery by pressure depletion, methane production is facilitated by dewatering the target seams to allow desorption of the adsorbed methane, which then migrates through the coal matrix into the cleats. The transport of gas through a coal seam is considered a two-step process. It is generally assumed that flow of gas and water through the cleats is laminar and obeys Darcy's law. On the other hand, gas transport through the porous coal matrix is controlled by diffusion. As in a fractured conventional reservoir, the permeability of coalbeds comes primarily from the network of natural fractures. Being normal to the bedding plane and orthogonal to each other, the face and butt cleats in coal seams are usually subvertically orientated. Thus, changes in the cleat permeability can be considered to be controlled primarily by the prevailing effective horizontal stresses that act across the cleats, rather than the effective vertical stress, defined as the difference between the overburden stress and pore pressure. Permeability of coal has been shown to be highly stress-dependent.

High Stress Contrast between Different Lithologies and its Implications for Drilling and Stimulation in the Northern Cooper Basin

2016 ◽  
Author(s):  
Feng Gui ◽  
Khalil Rahman ◽  
Duncan Lockhart ◽  
Diogo Cunha ◽  
Dylan Meadows
Keyword(s):  
High Stress ◽  
Stress Contrast ◽  
Cooper Basin

scholarly journals Prospects of "Slate Revolution" and Problem of Power Industry of China

2013 ◽  
pp. 118-124
Author(s):  
M. Scryabina
Keyword(s):  
New York ◽  
Shale Gas ◽  
Energy Security ◽  
Chinese Economy ◽  
Gdp Growth ◽  
Gas Extraction ◽  
Extraction Technology ◽  
Us States ◽  
The Core ◽  
The Usa

The remarkable results of “shale gas revolution” in the USA have provoked a huge interest in Chinese energy circles. Beijing seriously considers the prospects of developing its abandoned domestic shale gas resources, which might result in a second “shale gas revolution”, this time in Asian region. Developing shale gas would help Beijing to bridge the gap between energy consumption and supply, and would also create a viable alternative to coal. However, the technology of shale gas extraction (hydraulic fracturing) is highly controversial, and raises a lot of concerns among environmentalists. “Fracking” has already been banned in a number of European states and there, and is a subject to moratorium in US states of New York, Connecticut and New Jersey. The core question is whether China can successfully adapt the extraction technology to its geologic conditions, and most importantly whether “fracking” of shale gas will help to alleviate the environmental degradation caused by rapid GDP growth, and help to increase energy security of Chinese economy.

scholarly journals Micropore Structure and Fractal Characteristics of Low-Permeability Coal Seams

2018 ◽  
Vol 2018 ◽  
pp. 1-15 ◽  
Author(s):  
Guang-zhe Deng ◽  
Rui Zheng
Keyword(s):  
Fractal Dimension ◽  
Electron Microscope ◽  
Scanning Electron Microscope ◽  
Coal Seam ◽  
Linear Relationship ◽  
Surface Area ◽  
Pore Volume ◽  
Low Permeability ◽  
Coal Seams ◽  
Scanning Electron

With the raw coal from a typical low-permeability coal seam in the coalfield of South Junger Basin in Xinjiang as the research object, this paper examined six kinds of coal samples with different permeabilities using a scanning electron microscope and a low-temperature nitrogen adsorption test that employed a JSM-6460LV high-resolution scanning electron microscope and an ASAP2020 automatic specific surface area micropore analyzer to measure all characteristic micropore structural parameters. According to fractal geometry theory, four fractal dimension calculation models of coal and rock were established, after which the pore structure characteristic parameters were used to calculate the fractal dimensions of the different coal seams. The results show that (1) the low-permeability coal seam in the coalfield of South Junger Basin in Xinjiang belongs to mesoporous medium, with a certain number of large pores and no micropores. The varying adsorption capacities of the different coal seams were positively correlated with pore volume, surface area, and the mesoporous surface area proportions, from which it was concluded that mesopores were the main contributors to pore adsorption in low-permeability coal seams. (2) The raw coal pore fractal dimension had a negative linear relationship to average pore size, a positive linear relationship with total pore volume, total surface area, and adsorption capacity, and a positive correlation with the mesoporous surface area proportion; that is, the higher the fractal dimension, the larger the pore volume and surface area of the raw coal. (3) The permeability of the low-permeability coal seam had a phase correlation with the micropore development degree; that is, the permeability had a phase negative correlation with the pore distribution fractal dimension, and there was a positive correlation between permeability and porosity. These results are of theoretical significance for the clean exploitation of low-permeability coal seam resources.

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

2019 ◽  
Vol 9 (21) ◽  
pp. 4720 ◽  
Author(s):  
Ge ◽  
Zhang ◽  
Sun ◽  
Hu
Keyword(s):  
Coal Seam ◽  
Hydraulic Fracturing ◽  
Hydraulic Fracture ◽  
Scale Model ◽  
Gas Extraction ◽  
Coal Seams ◽  
Directional Hydraulic Fracturing ◽  
Coal Mine Methane ◽  
Multi Scale ◽  
Fully Coupled

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

scholarly journals Field Investigation of Hydraulic Fracturing in Coal Seams and Its Enhancement for Methane Extraction in the Southeast Sichuan Basin, China

Energies ◽  
2018 ◽  
Vol 11 (12) ◽  
pp. 3451 ◽  
Author(s):  
Zuxun Zhang ◽  
Hongtu Wang ◽  
Bozhi Deng ◽  
Minghui Li ◽  
Dongming Zhang
Keyword(s):  
Coal Seam ◽  
Hydraulic Fracturing ◽  
Sichuan Basin ◽  
High Stress ◽  
Fluid Pressure ◽  
Field Investigation ◽  
Stress Sensitivity ◽  
Extraction Rate ◽  
Coal Seams ◽  
Methane Extraction

Hydraulic fracturing is an effective technology for enhancing the extraction of reservoir methane, as proved by field experience and laboratory experiments. However, unlike conventional reservoirs, coal seams had high stress sensitivity and high anisotropy. Therefore, the efficiency of hydraulic fracturing in coal seams needs to be investigated. In this study, hydraulic fracturing was performed at Nantong mine in the southeast Sichuan basin, China. The field investigation indicated that the hydraulic fracturing could significantly enhance the methane extraction rate of boreholes ten times higher than that of normal boreholes in one of the minable coal seams (named #5 coal seam). The performance of hydraulic fracturing in three districts revealed that compared with south flank, the fluid pressure was higher and the injection rate was lower in north flank. The methane extraction rate of south flank was inferior to that of north flank. It indicated hydraulic fracturing had less effect on #5 coal seam in south flank. Moreover, the injection of high-pressure water in coal seams could also drive methane away from boreholes. The methane extraction rate of the test boreholes demonstrated the existence of methane enrichment circles after hydraulic fracturing. It indicated that hydraulic fracturing did act on #5 coal seam in south flank. However, due to the high stress sensitivity of coal seams and the high geo-stress of south flank, the induced artificial fractures in #5 coal seam might close with the decline of the fluid pressure that led to a sharp decline of the methane extraction rate.

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