“…permeability, coal seam thickness, etc. (Zhou, 2012). Figure 10.Comparison of gas (a) and water (b) production for models with different desorption times but the same connection Type A. Inset: a portion of the figure zoomed.…”
This paper presents a study on the impact of well integrity failure on coalbed methane (also known as coal seam gas) production and potential shallow water contamination using numerical simulations with a finite-difference method. Two connection types and 12 cases were simulated: Type A-leakage through cement sheath and Type B-impaired zonal isolation at the aquifer interval. The effect of the distance between the aquifer and the coal seam, drainage area and desorption time on gas and water production was also inspected. Results show that both connection types have strong effects on the gas and water production; the cumulative water and gas production increases with increasing drainage radius; the distance between aquifer and the coal seam has a negative effect on the water production but a negligible effect on the gas production; desorption time, ranging from 5 to 30 days, has a negligible effect on the water and gas production. Connection Type A yields a potential water contamination but connection Type B does not. Gas concentration in the shallow aquifer decreases sharply with an increase of distance away from the producer and the unsafe area are within an area with a radius ranging from approximately 50 m to 90 m away from the producer in this study.
“…permeability, coal seam thickness, etc. (Zhou, 2012). Figure 10.Comparison of gas (a) and water (b) production for models with different desorption times but the same connection Type A. Inset: a portion of the figure zoomed.…”
This paper presents a study on the impact of well integrity failure on coalbed methane (also known as coal seam gas) production and potential shallow water contamination using numerical simulations with a finite-difference method. Two connection types and 12 cases were simulated: Type A-leakage through cement sheath and Type B-impaired zonal isolation at the aquifer interval. The effect of the distance between the aquifer and the coal seam, drainage area and desorption time on gas and water production was also inspected. Results show that both connection types have strong effects on the gas and water production; the cumulative water and gas production increases with increasing drainage radius; the distance between aquifer and the coal seam has a negative effect on the water production but a negligible effect on the gas production; desorption time, ranging from 5 to 30 days, has a negligible effect on the water and gas production. Connection Type A yields a potential water contamination but connection Type B does not. Gas concentration in the shallow aquifer decreases sharply with an increase of distance away from the producer and the unsafe area are within an area with a radius ranging from approximately 50 m to 90 m away from the producer in this study.
“…Laboratory experiments have been conducted to investigate the permeability of different coal with steady-state and unsteady-state methods (Durucan et al, 2013;Shen et al, 2011). In another approach, relative permeability has been determined by history matching with field production data (Zhou, 2012). Dynamic relative permeability models have also been proposed with consideration of the combined effects of stress changes and desorptioninduced shrinkage of the matrix system (Xu et al, 2014).…”
This study presents development and application of a fully coupled two-phase (methane and water) and poromechanics numerical model for the analysis of geomechanical impact on coalbed methane (CBM) production. The model considers changes in two-phase fluid flow properties, i.e., coal porosity, permeability, water retention, and relative permeability curves through changes in cleat fractures induced by effective stress variations and desorptioninduced shrinkage. The coupled simulator is first verified for poromechamics coupling and simulation parameters of a CBM reservoir model are calibrated by history matching against one year of CBM production field data from Shanxi Province, China. Then, the verified simulator and calibrated CBM reservoir model are used for predicting the impact of geomechanics on production rate for twenty years of continuous CBM production. The simulation results show that desorption-induced shrinkage is the dominant process in increasing permeability in the near wellbore region. Away from the wellbore, desorptioninduced shrinkage is weaker and permeability is reduced by pressure depletion and increased effective stress. A sensitivity analysis shows that for coal with a higher sorption strain, a larger initial Young's modulus and smaller Poisson's ratio promote the enhancement of permeability as well as the production rate. Moreover, the conceptual model of the cleat system, whether dominated by vertical cleats with permeability correlated to horizontal stress or with permeability correlated to mean stress can have a significant impact on the predicted production rate. Overall, the study clearly demonstrates and confirms the critical importance of considering geomechanics for an accurate prediction of CBM production. (1) A fully coupled two-phase flow and poromechanics model for methane recovery (2) Simulation parameters are calibrated by history matching against field data (3) The geomechanics behaviors significantly affect the prediction of CBM production
“…In CMG simulations, the mass conservation equations are based on the following conditions: there are only two phases (water and gas) in the coal seam; the temperature is constant; coal cleats are isotropically distributed; the gas desorption obeys the Langmuir equation; and the gas is not dissolved in water [44]. In this study, the coal shrinkage [30,36] after gas desorption is not considered.…”
A surrogate model approximates a computationally expensive solver. Polynomial Chaos is a method used to construct surrogate models by summing combinations of carefully chosen polynomials. The polynomials are chosen to respect the probability distributions of the uncertain input variables (parameters); this allows for both uncertainty quantification and global sensitivity analysis.In this paper we apply these techniques to a commercial solver for the estimation of peak gas rate and cumulative gas extraction from a coal seam gas well. The polynomial expansion is shown to honour the underlying geophysics with low error when compared to a much more complex and computationally slower commercial solver. We make use of advanced numerical integration techniques to achieve this accuracy using relatively small amounts of training data.
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