scholarly journals Influences of Coal Properties on the Principal Permeability Tensor during Primary Coalbed Methane Recovery: A Parametric Study

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-17
Author(s):  
Jie Zang ◽  
Ze Ma ◽  
Yong Ge ◽  
Chengxin Li
Keyword(s):  
Pore Pressure ◽  
Pitch Angle ◽  
Coalbed Methane ◽  
Young Modulus ◽  
Permeability Tensor ◽  
Swelling Ratio ◽  
Coal Permeability ◽  
Coal Properties ◽  
Dip Angle ◽  
Plane Of Isotropy

Coal permeability is intrinsically anisotropic because of the cleat structure of coal. Therefore, coal permeability can be denoted by a second-order tensor under three-dimensional conditions. Our previous paper proposed an analytical model of the principal permeability tensor of coal during primary coalbed methane (CBM) recovery. Based on this model, 18 modeling cases were considered in the present study to evaluate how the principal permeabilities were influenced by representative coal properties (the areal porosity, the internal swelling ratio, and the Young modulus) during primary CBM recovery. The modeling results show that with regard to the influences of the areal porosity on the principal permeabilities, an increase in cleat porosity reduces the sensitivity of each principal permeability to pore pressure change. The magnitudes of the principal permeabilities are positively proportional to the internal swelling ratio. The principal permeabilities thus tend to monotonically increase with a depletion in the pore pressure when the internal swelling ratio increases. Because the internal swelling ratio represents the extent of gas-sorption-induced matrix deformation, an increase in the internal swelling ratio increases desorption-induced matrix shrinkage and thus induces an increase in permeability. The principal permeabilities are positively proportional to the isotropic principal Young moduli and the synchronously changing anisotropic principal Young moduli. On the other hand, the principal Young modulus within the plane of isotropy influences the principal permeabilities within this plane in diverse patterns depending on both the dip angle of the coalbed and the pitch angle of the cleat sets. The principal permeability perpendicular to the plane of isotropy is positively proportional to this principal Young modulus, and this correlation pattern is independent of both the dip angle and pitch angle.

scholarly journals Model development and analysis of coal permeability based on the equivalent characteristics of dual-porosity structure

2019 ◽  
Vol 17 (2) ◽  
pp. 313-327
Author(s):  
Haijun Guo ◽  
Kai Wang ◽  
Yuanping Cheng ◽  
Liang Yuan ◽  
Chao Xu
Keyword(s):  
Coalbed Methane ◽  
Model Development ◽  
Gas Pressure ◽  
Dual Porosity ◽  
Permeability Evolution ◽  
Evolution Law ◽  
Fracture Width ◽  
Coal Permeability ◽  
Porosity Structure ◽  
Gas Seepage

Abstract Mining is a dynamic fracture process of coal and/or rock. The structural failure of coal bodies will change the coal matrix-fracture characteristics and then affect the distribution characteristics of the coalbed methane (CBM). Because of the structural complexity of coal, the coal matrices and fractures will be assumed to the geometries with rule shapes when the gas seepage characteristics in coals are analyzed. The size of the simplified geometries is the equivalent scale of dual-porosity coal structures (i.e. the equivalent fracture width and equivalent matrix scale). In this paper, according to the reasonable assumptions with regarding to dual-porosity coal structures, a new coal permeability evolution model based on the equivalent characteristics of dual-porosity structure (ECDP model) was built and the effect of the equivalent characteristics of dual-porosity structure on the coal permeability evolution law was analyzed. It is observed that if the initial fracture porosity is constant and the equivalent matrix scale increases, the range in which the permeability of coal rises with rising gas pressure increases; if the equivalent fracture width decreases and the equivalent matrix scale is constant, the range in which the permeability of coal rises with rising gas pressure decreases. The ECDP model is more suitable for revealing the evolution law of the coal permeability when large deformations occur in the coal bodies and/or the coal structure is damaged irreversibly, especially during enhancing CBM recovery.

scholarly journals A Mathematical Model to Study the Coupling Effect of Deformation-Seepage-Heat Transfer on Coalbed Methane Transport and Its Simulative Application

2020 ◽  
Vol 2020 ◽  
pp. 1-18
Author(s):  
Jianlin Xie ◽  
Yangsheng Zhao
Keyword(s):  
Heat Transfer ◽  
Mathematical Model ◽  
High Temperature ◽  
Pore Pressure ◽  
Coalbed Methane ◽  
Coupling Effect ◽  
Gas Content ◽  
High Temperature Water ◽  
Heat Injection ◽  
Temperature Water

Injection of high-temperature water or steam into low-permeability coalbed for efficient and rapid extraction of coalbed methane has been studied by our university for many years and will soon be implemented in the field. With comprehensive consideration of coupling of heat transfer, water seepage, desorption of coalbed methane, and coal-rock mass deformation, the paper establishes a more comprehensive mathematical model of the coupling effect of deformation-seepage-heat transfer on coalbed methane transport. Compared with the previous studies, this theoretical model considers the change of adsorbed and free coalbed methane at high temperature and the coalbed methane transport caused by a high-temperature gradient. Using the Tunlan Coal Mine of Shanxi Coking Coal Group to conduct the numerical simulations on the coalbed methane extraction project using heat injection technology, results show that (1) high-temperature water flowed towards the extraction hole along fractured fissures, with seepage towards the coal mass on both sides of the fissure at the same time, gradually heating the coalbed and forming an arcuate distribution of temperature from high to low for an area from the fractured fissure to the coalbed upper and lower boundaries. On the thirtieth day of heat injection, the temperature of the coalbed in the heat injection area ranged from 140°C to 260°C. (2) Under high temperatures, desorption of the coalbed gas was quick, and the adsorption gas content formed an oval funnel from the heat injection hole towards the extraction hole, centered by the fractured fissure, and migrating towards the coalbed upper and lower boundaries. Along with heat injection and extraction, the absorbed gas content rapidly decreased, and on the thirtieth day of injection, the absorbed gas content of the entire heat injection area decreased to 1.5 m3/t, only 7% of the original. (3) During heat injection, the coalbed gas pore pressure rapidly increased and reached 5.5 MPa on the tenth day, about 4.5 times the original, and the pore pressure steadied at 3.5 MPa on the thirtieth day of extraction. Such a high gas pressure gradient promoted the rapid flow and drainage of the gas.

Coalbed Methane Production: Why Coal Permeability Matters

2010 ◽  
Author(s):  
Dong Chen ◽  
Jishan Liu ◽  
Zhejun Pan ◽  
Luke Connell
Keyword(s):  
Methane Production ◽  
Coalbed Methane ◽  
Coal Permeability ◽  
Coalbed Methane Production

Creep: A Neglected Phenomenon in Coal Permeability Evolution and Coalbed Methane Production

2015 ◽  
Author(s):  
N. N. Danesh ◽  
Z. Chen ◽  
S. M. Aminossadati ◽  
M. Kizil ◽  
Z. Pan ◽  
...  
Keyword(s):  
Methane Production ◽  
Coalbed Methane ◽  
Permeability Evolution ◽  
Coal Permeability ◽  
Coalbed Methane Production

A Permeability Model for Undersaturated Coalbed Methane Reservoirs Considering the Coal Matrix Shrinkage Effect

2013 ◽  
Vol 807-809 ◽  
pp. 2413-2420 ◽  
Author(s):  
Jun Long Zhao ◽  
Da Zhen Tang ◽  
Hao Xu ◽  
Yan Jun Meng ◽  
Yu Min Lv
Keyword(s):  
Pore Pressure ◽  
Coalbed Methane ◽  
Well Test ◽  
Dynamic Prediction ◽  
Permeability Model ◽  
Matrix Shrinkage ◽  
New Model ◽  
Shrinkage Effect ◽  
The Matrix

With the analysis of key elements on the strain state of coal, a permeability dynamic prediction model which is divided by the critical desorption pressure for undersaturated coalbed methane (CBM) reservoirs was established on the basis of pore pressure and considering the matrix shrinkage effect of coal. The law between permeability and pore pressure was analyzed during production with the new model. Through case study, the rationality of the model was also verified. The research shows that the degree of permeability changes mainly depends on the relationship between the critical desorption pressure and the rebound pressure which depends on the strength of the matrix shrinkage. Under the condition of equivalent matrix shrinkage, the reservoirs permeability rebounds better with high Young's modulus and low Poisson's ratio. Adjustment factor contributes to improve the influence of matrix shrinkage on permeability and the larger the matrix shrinkage strength is, the higher the permeability rebounds. PM model and CB model are similar to the new model. PM model limits the matrix shrinkage strength, and CB model is a special case of the new model. Comparing with the well test permeability, the new model is more reasonable to characterize the matrix shrinkage effect in the development process.

Modeling Laboratory Permeability in Coal Using Sorption-Induced Strain Data

2007 ◽  
Vol 10 (03) ◽  
pp. 260-269 ◽  
Author(s):  
Eric P. Robertson ◽  
Richard L. Christiansen
Keyword(s):  
Pore Pressure ◽  
Fractured Reservoirs ◽  
Permeability Change ◽  
Coal Seams ◽  
Overburden Pressure ◽  
Change Models ◽  
Coal Permeability ◽  
Coal Matrix ◽  
Strain Data ◽  
The Matrix

Summary Sorption-induced strain and permeability were measured as a function of pore pressure using subbituminous coal from the Powder River basin of Wyoming, USA, and high-volatile bituminous coal from the Uinta-Piceance basin of Utah, USA. We found that for these coal samples, cleat compressibility was not constant, but variable. Calculated variable cleat-compressibility constants were found to correlate well with previously published data for other coals. Sorption-induced matrix strain (shrinkage/swelling) was measured on unconstrained samples for different gases: carbon dioxide (CO2), methane (CH4), and nitrogen (N2). During permeability tests, sorption-induced matrix shrinkage was demonstrated clearly by higher-permeability values at lower pore pressures while holding overburden pressure constant; this effect was more pronounced when gases with higher adsorption isotherms such as CO2 were used. Measured permeability data were modeled using three different permeability models that take into account sorption-induced matrix strain. We found that when the measured strain data were applied, all three models matched the measured permeability results poorly. However, by applying an experimentally derived expression to the strain data that accounts for the constraining stress of overburden pressure, pore pressure, coal type, and gas type, two of the models were greatly improved. Introduction Coal seams have the capacity to adsorb large amounts of gases because of their typically large internal surface area (30 to 300 m2/g) (Berkowitz 1985). Some gases, such as CO2, have a higher affinity for the coal surfaces than others, such as N2. Knowledge of how the adsorption or desorption of gases affects coal permeability is important not only to operations involving the production of natural gas from coalbeds but also to the design and operation of projects to sequester greenhouse gases in coalbeds (RECOPOL Workshop 2005). As reservoir pressure is lowered, gas molecules are desorbed from the matrix and travel to the cleat (natural-fracture) system, where they are conveyed to producing wells. Fluid movement in coal is controlled by diffusion in the coal matrix and described by Darcy flow in the fracture (cleat) system. Because diffusion of gases through the matrix is a much slower process than Darcy flow through the fracture (cleat) system, coal seams are treated as fractured reservoirs with respect to fluid flow. However, coalbeds are more complex than other fractured reservoirs because of their ability to adsorb (or desorb) large quantities of gas. Adsorption of gases by the internal surfaces of coal causes the coal matrix to swell, and desorption of gases causes the coal matrix to shrink. The swelling or shrinkage of coal as gas is adsorbed or desorbed is referred to as sorption-induced strain. Sorption-induced strain of the coal matrix causes a change in the width of the cleats or fractures that must be accounted for when modeling permeability changes in the system. A number of permeability-change models (Gray 1987; Sawyer et al. 1990; Seidle and Huitt 1995; Palmer and Mansoori 1998; Pekot and Reeves 2003; Shi and Durucan 2003) for coal have been proposed that attempt to account for the effect of sorption-induced strain. Accurate measurement of sorption-induced strain becomes important when modeling the effect of gas sorption on coal permeability. For this work, laboratory measurements of sorption-induced strain were made for two different coals and three gases. Permeability measurements also were made using the same coals and gases under different pressure and stress regimes. The objective of this current work is to present these data and to model the laboratory-generated permeability data using a number of permeability-change models that have been described by other researchers. This work should be of value to those who model coalbed-methane fields with reservoir simulators because these results could be incorporated into those reservoir models to improve their accuracy.

scholarly journals Influence of Migration and Plugging of Nanoparticles on Coal Permeability in Coal Reservoirs

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-15
Author(s):  
Xiaopeng Zhai ◽  
Weihong Chen ◽  
Yun Xu ◽  
Yishan Lou ◽  
Shuhong Xu ◽  
...  
Keyword(s):  
Pressure Difference ◽  
Coalbed Methane ◽  
Pressure Oscillation ◽  
Working Fluid ◽  
Low Permeability ◽  
Coal Permeability ◽  
The North ◽  
Theoretical Support ◽  
Coal Reservoir ◽  
Selection Of

The plugging of nanopores in low-permeability coal reservoirs is an important factor that affects productivity reduction. However, the mechanism of plugging of the nanopores in coal reservoirs remains unclear. In this study, the coal samples from the Anze coalbed methane block of the North China Oilfield are used as the research object. Experiments are conducted on the mechanism of nanopore plugging by the variation of nanopore permeability based on the pressure oscillation method and the nanopore (scanning electron microscope) method. The research shows that the foreign working fluid invades a coal sample; the sample changes from being hydrophobic to being water absorbent within a certain period. The instability caused by the expansion of coal clay mineral particles promotes the dispersion and shedding of particles, and the migration of particles is accelerated under the shear stress of the working fluid. In addition, the viscosity and pressure difference of the working fluid are important factors that affect particle plugging. The viscosity of the fluid increased by two times, and permeability decreased by 1.21 times. As the pressure difference increases by two times, permeability can be reduced by up to two orders of magnitude. The findings of this study can help for better understanding of the mechanism of plugging of the nanopores in coal reservoirs and the reasons of production reduction in low-permeability coal reservoirs. Such findings provide theoretical support for the selection of the working fluid, and reasonable production pressure difference can effectively reduce the damage on coal permeability in a low-permeability coal reservoir.

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