Coal wettability in coalbed methane production: A critical review

Coalbed methane (CBM) has been extracted on a commercial basis for more than six decades. The low permeability of coal and ineffective dewatering are key challenges in some CBM reservoirs. Coal is a naturally fractured rock with an extremely high organic content. Under in situ conditions, the reservoirs are typically saturated with formation water. The majority of gas is stored as sorbed gas in water saturated CBM formations, which leads to complex dual-phase flow during reservoir depletion. A review of the coal wettability characteristics is the focus of this article. Coal wettability directly influences the irreducible water content which can significantly alter the relative permeability of the gas and water during gas depletion. The technical challenge of the hydraulic fracturing operation is how to achieve an effective flowback when the coal is wet, given that the residual water can be hard to remove. During the initial CBM dewatering, water blocking and the Jamin effect are expected to occur more frequently in the pore throats and narrow cleats with hydrophilic surfaces, resulting in permeability damage. Currently, such technical problems may be solved in part by using surfactants to adjust the coal wetting characteristics. The coal wettability performance should be characterized in terms of selection of both the fracturing fluids and CBM formation cleaning to maximize gas production.