Prediction of Flowing Bottomhole Pressures for Two-Phase Coalbed Methane Wells

A method is proposed to predict the flowing bottomhole pressures （FBHPs） for two-phase coalbed methane （CBM） wells. The mathematical models for both gas column pressure and two-phase fluid column pressure were developed based on the well liquid flow equation. FBHPs during the production were predicted by considering the effect of entrained liquid on gravitational gradients. Comparison of calculated BHPs by Cullender-Smith and proposed method was also studied. The results show that the proposed algorithm gives the desired accuracy of calculating BHPs in the low- productivity and low-pressure CBM wells. FBHP is resulted from the combined action of wellhead pressure, gas column pressure and fluid column pressure. Variation of kinetic energy term, compressibility and friction factors with depth increments and liquid holdup with velocity should be considered to simulate the real BHPs adequately. BHP is a function of depth of each column segment. The small errors of less than 1.5% between the calculated and measured values are obtained with each segment within 25 m. Adjusting BHPs can effectively increase production pressure drop, which is beneficial to CBM desorption and enhances reservoir productivity. The increment of pressure drop from 5.37 MPa2 to 8.66 MPa2 leads to an increase of CBM production from 3270 m3/d to 6700 m3/d and is attributed to a decrease in BHP from 2.25 MPa to 1.33 MPa.

Characteristics and control mechanisms of coalbed permeability change in various gas production stages

According to dimensionless analysis of the coalbed methane （CBM） production data of Fanzhuang block in southern Qinshui basin, the dimensionless gas production rate is calculated to quantitatively divide the CBM well production process into four stages, i.e., drai- nage stage, unstable gas production stage, stable gas pro- duction stage, and gas production decline stage. By the material balance method, the coal reservoir permeability change in different stages is quantitatively characterized. The characteristics and control mechanisms of change in coalbed permeability （CICP） during different production stages are concluded on five aspects, i.e., permeability trend variation, controlling mechanism, system energy, phase state compositions, and production performance. The study reveals that CICP is characterized by first decline, then recovery, and finally by increase and is controlled directly by effective stress and matrix shrinkage effects. Further, the duration and intensity of the matrix shrinkage effect are inherently controlled by adsorption and desorp- tion features.

Desorption hysteresis of coalbed methane and its controlling factors: a brief review

Most coal reservoirs show high gas content with relatively low desorption efficiency,which restricts the efficiency of coalbed methane(CBM)extraction and single-well productivity.This review highlights the desorption hysteresis mechanism and its controlling factors as well as methods and models to reveal desorption hysteresis and potential solutions.Methane adsorption and desorption can be recorded by both gravimetric and volumetric experiments.Although different adsorption models are used,desorption is generally considered with the Langmuir model.Desorption hysteresis is influenced by the petrophysical composition,thermal maturity,pore structure distribution of the coal,reservoir temperature,and moisture and water content.Methods for calculating desorption hysteresis include the area index,hysteresis index and introduction of a hysteresis factor and a hysteresis coefficient.Molecular dynamics simulations of methane desorption are mainly based on theories of kinetics,thermodynamics,and potential energy.The interaction forces operating among coal,water,and methane molecules can be calculated from microscopic intermolecular forces(van der Waals forces).The desorption hysteresis mechanism and desorption process still lack quantitative probe methodologies,and future research should focus on coal wettability under the constraints of liquid content,potential energy adjustment mechanism,and quantitative analysis of methane desorption rates.Further research is expected to reveal the desorption kinetics of methane through the use of the solid–liquid–gas three-phase coupling theory associated with the quantitative analysis of methane desorption hysteresis,thereby enhancing the recovery rate and efficiency of CBM wells.