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Geologic interpretations made during the current enhanced oil recovery project at Rangely field (Rio Blanco County, Colorado) have shown the relationships between alteration of reservoir mineralogy and texture, changes in produced water composition, and increased problems with oil production.
The bottom-hole pH of Weber brine has decreased from approximately 7.5 to 4.5 since the initiation of CO injection into the Pennsylvania-Permian Weber Sandstone in the central portion of the field. Much of the Weber is partially cemented with carbonate minerals, including ferroan calcite and ferroan dolomite, with authigenic clay minerals, predominately illite and mixed-layer smectite/illite, also present. These minerals are affected by the lowered pH of the CO flood.
Changes in the chemistry of produced water are associated with alteration of reservoir mineralogy. The CO flood has caused substantial increases in the concentrations of iron, calcium, and magnesium in the produced brine from the dissolution of carbonate cements and, perhaps, authigenic clays and detrital feldspars. Produced water from some wells has experienced large changes in barium and sulfate concentrations. An increase in bromide concentration indicates that previously segregated portions of the reservoir, which contained very saline original formation brine, have been accessed by the carbonated water.
Hypotheses based on the water-chemistry changes were confirmed in core-flood experiments. These experiments also indicated that there was no net change in permeability following CO injection.
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