Alteration of Reservoir Diatomites by Hot Water Injection

Abstract

Abstract Pore microstructure and mineral composition of diatomaceous reservoir core were analyzed in concert with core-scale thermal recovery tests. Samples from two diatomaceous oil reservoirs were subjected to spontaneous and forced imbibition coreflood experiments using hot synthetic steam condensate (180°C to 230°C, pH = 10). The effects of these experiments on the rock fabric were determined by comparing mineralogy, pore structure, and physical properties of material collected before and after the experiments. One set of reservoir samples consist of relatively clean, calcite-rich opal-A and opal-CT diatomites. Samples from the other reservoir are clay-rich opal-A diatomites. The hot alkaline fluids produced porosity channels in samples from both reservoir types. These small channels (10 μm to 2 mm in diameter) form initially at the inlet and grow slowly toward the outlet as experiments progressed. Fines mobilization and perhaps hydraulic action during forced imbibition form the channels. Silica dissolution occurs as shown by ICP analysis of the aqueous effluent; however, signs of pitting and corrosion of diatoms were not observed at magnifications up to 10,000X. The channels improve the homogeneity of the diatomite by penetrating low porosity barriers as shown by the CT-imaged oil saturation profiles with time. As a result of the experiments, permeability may either increase or decrease despite porosity channel formation. Opal-A to opal-CT conversion did not occur during the experiments as determined by scanning electron microscope imaging and X-ray diffraction analyses. An increase in the amount of mixed layer illite/smectite corresponds with a decrease in plagioclase abundance in the clay-rich diatomite samples. In addition, the grain density decreases in these clay-rich samples due to the preferential dissolution of ferroan dolomite and pyrite. In the calcite-rich samples, grain density increases as the relatively low density silica dissolves leaving the samples enriched with calcite. The increase in grain density is much greater in more soluble, and more calcite-rich, opal-A diatomite samples compared to the less soluble opal-CT diatomites.