SPE 113410 Application of Coalbed Methane Water to Oil Recovery by Low Salinity Waterflooding

Development and production of coalbed methane involves the production of large volumes of water. The salinities and sodium adsorption ratios of coalbed methane (CBM) water from the Powder River Basin range from 370 to 1,940 ppm and 5.6 to 69 respectively. Surface discharge of CBM water can create serious environmental problems; subsurface injection is generally viewed as economically nonviable. It has been shown that oil recovery from reservoir sandstones can be improved by low salinity waterflooding for salinities ranging up to 5,000 ppm. There may be both technical and regulatory advantages to application of CBM water to oil recovery by waterflooding. Thin section and scanning electron microscope studies of the mineral constituents and distribution of Tensleep and Minnelusa sandstones show they are typically composed of quartz, feldspar, dolomite and anhydrite cements but have very low clay content. The sands contain interstitial dolomite crystals in the size range of up to about 10 microns. Three sandstone cores from the Tensleep formation in Wyoming were tested for tertiary response to injection of CBM water. The cores were first flooded with high salinity Minnelusa formation brine of 38,651 ppm to establish residual oil saturation. Synthetic CBM water of 1,316 ppm was then injected. Tertiary recovery by injection of CBM water ranged from 3 to 9.5% with recoveries for all but one flood being in the range of 5.9 to 9.5%. Previous studies showed that the presence of clay was needed for response to low salinity flooding. As a test of the recovery mechanism, a Tensleep core was preflushed with 15% hydrochloric acid to dissolve the dolomite crystals. The treated core showed no tertiary recovery or pressure response to CBM water. Introduction Coalbed methane (CBM) is a significant source of energy and now accounts for 7.5% of gas production in the conterminous United States. In Wyoming, CBM production contributes 18% of the total gas production (WOGCC, 2006). The Powder River Basin (PRB) in Wyoming and Montana is one of the most active areas of development. It is estimated to contain 61 trillion cubic feet (Tcf) of natural gas in-place, with 39 Tcf being technically recoverable (ARI, 2002). There were 19,523 CBM wells in the Wyoming portion of the PRB at the end of 2006 (WOGCC, 2006) (Fig. 1). Planners forecast as many as 81,000 additional CBM wells (Fisher, 2003). Development of CBM production first requires dewatering of the coal seams. CBM water presents a serious disposal problem which complicates CBM development. For the last 3 years, CBM water production in the PRB alone has been about equal to one third of the total water associated with oil and gas production in Wyoming (Table 1). The majority of the water has high sodium adsorption ratios. Total dissolved solids (TDS) of water co-produced with coalbed methane in the Wyoming portion of the Powder River Basin range from 370 to 1,940 mg/L with a mean of 840 mg/L (Rice, 2000). The sodium adsorption ratios (SAR, the ratio of sodium to calcium and magnesium) range from 5.6 to 69 (Pierce, 2004). Most of this water has limited suitability for domestic and animal consumption or for agriculture. Commonly practiced surface disposal has a range of adverse effects. Disposal into rivers has been contested. Other problems involved with the extraction of such large volumes of groundwater include impacts on domestic water wells and natural springs, water rights, lowering of water tables, and groundwater recharge issues. Under the Environmental Protection Agency’s Underground Injection Control (UIC) program, subsurface re-injection of CBM water is categorized under the restrictive Class V injection which sometimes makes the disposal uneconomic. To date, only a minimal amount of PRB CBM water has been re-injected (Table 1). Environmentally sound and economically viable disposal of CBM water has become crucial to continuous CBM development in Wyoming. Waterflooding is widely used to improve oil recovery from reservoirs. Application of CBM water to oil recovery has special regulatory advantages. Injection of CBM water into oil reservoirs falls within the less restrictive Class II injection