Signature of coproduced water quality for coalbed methane development

Coalbed methane is an unconventional natural gas resource with large reserves across the globe. In China, more than four billion cubic meters of coalbed methane are produced per year with large volumes of coproduced water. The various chemical constituents of coproduced water can be used as an exploration tool for coalbed methane development. This study investigated the composition of coproduced water from the lower Cretaceous Fuxin Formation in the Tiefa basin to determine its chemical signature. Water samples were collected from 12 wells in the Beier Mining Area and analyzed for chemical composition. The quality of the coproduced water after hydraulic fracturing is significantly chemically different from pristine formation water. Interplay is present between the gas production rate and the quality of the coproduced water. Wells with high gas production rate coproduce higher total dissolved solids waters that are enriched in sodium, potassium and bicarbonate but depleted in magnesium. This chemical signature may result from more methane generation, more gas production rate, long drainage time and more water production. Infill wells are not always good for the gas recovery of the existing wells because the fracturing fluids can be harmful to the existing wells in terms of the extent of depressurization and the flowing ability. The concentrations of chloride, sulfate, bicarbonate, carbonate and sodium in the fracturing fluids used in the infill wells are different from pristine formation water; therefore, they can act as indicative ions to explain the impact of the hydraulic fracturing of the infill wells on the existing well. Higher levels of chloride, sulfate and sodium and less bicarbonate and carbonate in the coproduced water from the existing wells means that more fracturing fluid from the infill wells, more proppant carried by the fracturing fluid and more pulverized coal produced by fracturing have entered the radius of pressure drop, which is harmful to the depressurization and the flowing ability of the existing wells and represents more harm to the gas production rate of the existing wells. Similarly, a smaller change of the indicative ion concentrations in the existing wells after the fracturing of infill wells means that the fracturing had less of an impact on the gas production rate. (C) 2017 Elsevier B.V. All rights reserved.