Evaluating the filtration property of fracturing fluid and fracture conductivity of coalbed methane wells considering the stress-sensitivity effects

The filtration property of fracturing fluid and fracture conductivity are among the key parameters for stimulation in coalbed methane (CBM) reservoirs, and ultimately play important roles in the production of CBM wells. This study evaluates the overall filtration property of fracturing fluid in high-rank coal reservoirs, and proposes a model considering the stress-sensitivity effects to estimate the overall filtration coefficient. This study also investigates the effects of effective stress, porosity, permeability, viscosity of fracturing fluid, and reservoir pressure on the overall filtration coefficient of fracturing fluid. The results show that, with the increases of effective stress and fracturing fluid viscosity, the overall filtration coefficient of fracturing fluid in CBM reservoirs shows a monotonous downward trend; with the increase of the porosity/permeability and reservoir pressure, the overall filtration coefficient of fracturing fluid increases. Based on the Carman-Kozeny formula, this study established a model to estimate facture conductivity under different conditions. During the hydraulic fracturing of CBM reservoirs, comparative analysis suggests that increasing proppant size and number of paved layers increases fracture conductivity. The flow conductivity of fractures filled with multiple-layer proppants is significantly larger than that of fractures with single-layer proppants; with the increasing closure pressure, fracture conductivity decreases, and fracture width reduces due to the embedding of the proppant, and thereby the fracture conductivity declines. In the early stage of fracturing, small-size proppants with low emission are typically employed, which is conducive to the extension of the hydraulic fracture. In the later stage of fracturing, the flow conductivity of near wellbore can be improved by using large-size proppants.