Novel method for optimizing the dewatering rate of a coal-bed methane well

Abstract

<jats:p> The reasonable dewatering rate in the single-phase water flow plays an essential role in pressure propagation and coal-bed methane production. However, current fluid velocity sensitivity experiments cannot provide an optimum dewatering rate for field coal-bed methane production. This study proposes a new method to optimize the dewatering rate for coal-bed methane wells by assuming the investigation distance reaches the well boundary when the bottom hole pressure declines to the critical desorption pressure. The effect of the stress sensitivity and fluid velocity sensitivity on pressure propagation was first simulated with COMSOL Multiphysics software. The results showed that the expansion area considering the stress sensitivity is shorter than that neglecting the stress sensitivity when the bottom hole pressure reached to the critical desorption pressure at 200 days. The expansion area with high dewatering rate will be shorter about 35 m than that with low dewatering rate at 200 days. The relationship between the maximum investigation distance and required time was established to optimize the dewatering rate by combining the pressure profile considering the influence of stress sensitivity with material balance equation. The new model indicates that the initial permeability, porosity, and cleat compressibility have an important effect on investigation distance. The simulation of these parameters’ sensitivity suggests that the bigger the ratio of initial permeability and porosity, the longer the investigation distance is, and the smaller the cleat compressibility is, the longer the expansion area is. According to this model, we need to take more than 600 days at 0.58 m/d constant dewatering rate to reach the maximum investigation distance of 0.67 mD initial permeability. This work can be conducive to choose reasonable dewatering rate in single-phase water flow for coal-bed methane well production. </jats:p>

Dewatering Optimization of Coal Seam Gas Production Wells: A Review and Future Perspectives

“…A key dewatering objective at the single-phase water flow stage is to determine the optimal BHP drawdown rate to maximize the water drainage radial expansion (i.e., reach the well drainage boundary) before reaching the critical gas desorption pressure, − as presented by the solid blue line in Figure . Based on this principle, the optimal dewatering rate can be calculated using the pressure difference between initial reservoir pressure, p i , and gas desorption pressure, p cd , as well as the time required to reach the maximum expansion, as expressed in eq − …”

“…The assumptions made in eq are constant permeability and porosity, which may be invalid for the actual dewatering process. A new correlation between the production time ( t i ) and the radius of investigation is proposed by incorporating the stress-dependent reservoir parameters as where r w and r e denote the borehole and outer boundary radii, respectively; k̅ and denote average permeability and porosity, respectively, where permeability and porosity variations are reflected at each depressurising point.…”

“… where p b and p bhp denote the outer boundary and BHP pressure, q the water production rate, and G the total water production. Under a constant water production rate, (see the detailed derivation in Lu et al).…”

“…Input parameters in Lu et al’s work are obtained from the well testing in Zhengzhuang Block in China. Their results show that neglecting stress-dependent permeability and porosity barely affects the pressure propagation in the early dewatering stage.…”

“…Methods to optimize the dewatering process are currently in a very primitive development stage, and CSG operators rely on empirical and instinctive approaches to deal with the problem. The critical points for optimizing the dewatering rate are to (i) minimize the rapid loss of effective permeability, considering both absolute permeability and relative permeability, and (ii) accomplish a smooth and even pressure propagation to increase the radius of the drawdown funnel and maximize the depressurization area before the occurrence of gas–water two-phase flow. , Although a few attempts have been conducted in the literature trying to optimize the dewatering strategy, they generally only focus on the effect of stress-dependent permeability. A systematic approach to integrating the key affecting factors (e.g., borehole instability, absolute and relative permeability evolution, capillary trapping, and coal fines generation) does not appear to have been performed yet. − The primary goal of this work is therefore to review the dewatering optimization methodology applied in the literature, with a focus on the incorporated factors, physics, and processes in each of the existing methods and to recommend a systematic approach for better physical-based CSG reservoir dewatering optimization.…”