Carbon Dioxide Injection and Resultant Alteration of Weber Sandstone (Pennsylvania-Permian), Rangely Field, Colorado

Abstract

Abstract Geologic interpretations made during the current EOR (enhanced oil recovery) project at Rangely field (Rio Blanco County, Colorado), have made interesting connections between alteration of reservoir mineralogy and texture, changes in produced water composition, and increased production problems. Carbon dioxide is being injected into the Weber Sandstone in portions of Rangely field. The carbon dioxide injection is part of a very successful tertiary recovery project initiated in late 1986. The bottom-hole pH of Weber brine has decreased from approximately 7.5 to 4.5 with the addition of C02. At Rangely, the Weber is composed mostly of fine-grained eolian sandstone with intercalated (and nonproductive) fluvial sandstones, siltstones, and shales. Much of the Weber is partially cemented with carbonate minerals, including ferroan calcite and ferroan dolomite. Clay minerals, predominantly illite, are also present. Changes in the chemistry of produced water are associated with alteration of reservoir mineralogy. The C02 flood has caused a substantial increase in the concentrations of iron, calcium, magnesium, and strontium in the produced brine. The amounts of increase is directly related to the volume of C02 produced in each well. This increase resulted from the dissolution of carbonate cements, authigenic clays, and detrital feldspars. An increase in the calculated scaling potential of the produced water is a result of this change in chemistry. Although barium and sulfate concentrations generally have not changed with the C02 injection, produced water from some wells has experienced large changes in barium and sulfate concentrations. The result is an increased tendency for barite scale to precipitate in these wells. An increase in chloride concentration indicates that previously segregated portions of the reservoir, which contained very saline original formation water, have been accessed by the carbonated brine.