Experimental Analysis of Primary and Secondary Oil Recovery from Matrix by Counter-Current Diffusion and Spontaneous Imbibition

Abstract

Abstract Matrix–fracture interaction is a commonly encountered process in fractured subsurface reservoirs during oil, gas, and geothermal fluid recoveries and contaminant transport. It occurs in counter-current manner if the interaction between matrix and fracture takes place from the same side. In the first part of this paper, the effects of the matrix shape factor, wettability, and interfacial tension on the rate of capillary imbibition and development of residual non-wetting phase saturation were studied experimentally. In the second part, secondary oil recovery by diffusion of a solvent from the samples exposed to capillary imbibition is investigated. Primary diffusion experiments were also performed on selected core sizes for comparison. To achieve all these, experiments were conducted on Berea sandstone samples. Cylindrical samples with different shape factors were obtained by cutting the plugs ½, 1 and 2 inches in diameter and 2, 4, and 6 inches in length. All sides were coated with epoxy except one end. Static imbibition experiments were conducted on vertically and laterally situated samples where the imbibition took place upward and lateral directions, respectively. Brine-kerosene, brine-mineral oil and surfactant solution-mineral oil pairs were used as fluids. The rate of imbibition was evaluated for different matrix size, shape factor, wetting/non-wetting phase types and IFT. Conditions at which the shape factor controls the residual oil development were identified. Diffusion experiments were conducted using n-heptane as a solvent. In addition to the cores exposed to full capillary imbibition, oil saturated (virgin) cores were also tested. To serve as a benchmark, uncoated versions of two different core sizes were used. It was observed that the residual oil after capillary imbibition is controlled by the matrix shape factor for the vertically positioned samples unlike horizontal samples. Surfactant concentration plays a role on the rate of imbibition and the development of residual oil saturation. Primary and secondary diffusion experiment showed that it could be more efficient to begin the oil recovery process by capillary imbibition (primary recovery) then continue with diffusion (secondary recovery) depending on the matrix shape factor.