Technology to increase oil recovery from a porous formation beyond that obtained by conventional means. Conventional oil recovery technologies produce an average of about one-third of the original oil in place in a formation. Conventional technologies are primary or secondary. Primary technologies rely on native energy, in the form of fluid and rock compressibility and natural aquifers, to produce oil from the formation to wells. Secondary technologies supplement the native energy to drive oil to producing wells by injecting water or low-pressure gas at injection wells. The target of enhanced recovery technologies is that large portion of oil that is not recovered by primary and secondary means. See also Petroleum engineering.
Many of the challenges encountered by secondary technologies are identical to those encountered by enhanced recovery technologies. Those challenges include reducing residual oil saturation, improving sweep efficiency, fitting the technology to the reservoir heterogeneities, and minimizing up-front and operating costs.
Residual oil remains trapped in a porous rock after the rock has been swept with water, gas, or any other recovery fluid. The residual oil saturation is the percentage of the pore space occupied by the residual oil. The residual oil saturation depends on the pore size distribution and connectivity, the interfacial tension between a recovery agent and the oil, the relative wettability of the rock surfaces with respect to the recovery agent and the oil, the viscosity of the fluids, and the rate at which the fluids are moving through the rock.
The sweep efficiency specifies that portion of a reservoir that is contacted by a recovery fluid. Sweep efficiency increases with volume of injected fluid. It also depends on the pattern of injection and production wells in a formation, on the mobility of the oil and the recovery fluid, and on heterogeneities in the formation.
A wide variety of processes have been considered for enhancing oil recovery: thermal processes, high-pressure gas processes, and chemical processes. Specifically, low residual oil saturation can be obtained by selecting a recovery fluid that provides a very low interfacial tension between the oil and the fluid. With very low interfacial tension, the capillary number is large. And high sweep efficiency can be obtained by selecting a recovery agent with low mobility or by increasing the mobility of the oil.
