Production analysis and permeability evolution of fractured horizontal wells of coalbed methane reservoir

Massive hydraulic fracturing has changed the status of low-productivity and low development efficiency for coalbed methane (CBM) reservoirs. However, the production dynamics of fractured wells and the permeability evolution mechanisms in CBM reservoirs are unclear, which significantly limits the efficient development of CBM reservoirs. Therefore, this study incorporates the total strain evolution under the conditions of gas-adsorption-induced swelling, fracture compression, and unsteady creep, uses the cubic law to establish the permeability model, and obtains the pressure and flow fields via the finite volume method (FVM) and the transient embedded discrete fracture model (tEDFM). Based on the embedded mass exchange law, an adsorbed-free phase multiple-mechanism recovery calculation framework is established to realize production dynamic analysis and productivity calculation. Results show that the production dynamics of CBM fractured wells include five stages: the initial high production stage, desorption-induced productivity increasing stage, mid-time stable production stage, production decline stage, and the final depleted stage. The larger the Langmuir pressure is, the faster sorbed gas production would be. When the Langmuir pressure is 2.6 MPa, after 1 800-day production, adsorbed gas dominates production. When the Langmuir volume is increased to 15 m3/t, desorbed gas’s contribution continuously increases. Adsorbed gas becomes the main gas source after 560-day production. The denser the hydraulic fractures are, the larger the drainage area is, the significantly higher the initial production would be, and the later the production decline occurs. When the fracture spacing is 3 times larger, the maximum gas production decreases by about 48%. When the fracture half-length is increased by 50 m, the initial production would nearly be doubled. The permeability evolution includes three stages: loss, recovery, and enhancement. When the fracture compressibility coefficient is 0.03 MPa−1, and the loss rate is as high as 76% within 800 days. Despite the loss of permeability due to fracture closure, when the methane is desorbed and recovered, the reduction of swelling strain causes the permeability to recover. With fracturing intensity increases, and the permeability recovery becomes faster and stronger, which promotes the long-term recovery of CBM. When the desorption-induced strain is greater than 0.06, the permeability recovers and increases to 1.2 times the initial level in the later production period. The lower the coal viscoelastic modulus is, the more obvious the permeability damage caused by creep would be.