The role of viscoelasticity of alkali/surfactant/polymer solutions in enhanced oil recovery

Studies have been done in order to treat alkali scale related problems encountered in the alkali/surfactant/polymer (ASP) flooding pilot area of Daqing Oilfield. The interfacial tensions (IFT) of ASP solutions are tested using TX-500 type interfacial tensimeter (made in the USA). The effect of alkalis and surfactants on the viscoelasticity of ASP solutions are studied using HAAK RS-150H type rheometer (made in Germany). Macro-coreflood tests are performed using natural cores, 2-D vertically-heterogeneous artificial cores without restraining barriers, and 2-D vertically-heterogeneous artificial cores with restraining barriers. Blind ends' residual oil sweeping tests are performed with simplified blind-end models. The studies indicate that alkali has adverse effect on the viscoelasticity of an ASP solution. A high concentration of alkali can cause the HPAM molecular chains curl up, and can form alkali scales, although it is beneficial to an ultralow IFT. Ultralow IFT has a pronounced effect on the oil recovery of a homogeneous reservoir, while the viscoelasticity of an ASP solution contributes more to the oil recovery of a heterogeneous reservoir. The stronger the homogeneity is, the greater the effect of IFT on the oil recovery; on the contrary, the stronger the heterogeneity is, the greater the effect of viscoelasticity on the oil recovery. Laboratory "co-injection and separated-layer production" experiment also shows that a reasonable (for example 0.6%) rather than too high an alkali concentration is more beneficial to starting-up layers with medium and low permeability, and washing out the residual oil in the swept layers, and a relatively high viscoelasticity is more beneficial to sweeping the residual oil in blind ends. As far as a heterogeneous reservoir is concerned, ultralow IFT is not a "have-to" condition for enhancing oil recovery, while the viscoelasticity of an ASP solution is more important to effectively sweeping oil. The capillary number theory is of great practical importance for homogeneous reservoirs, but it has limitations and cannot be readily applied in heterogeneous reservoirs. For example, the trend of capillary numbers versus IFTs (corresponding to different alkali concentrations), which are based on the theory, does not match with the trend of oil recovery efficiencies. Therefore, for a heterogeneous reservoir, without an ultralow IFT (for example with an IFT of 10(-2) mN/m), i.e. with a reasonably lower alkali concentration, a satisfying oil recovery can still be achieved as long as the viscoelasticity is high enough. This not only can solve the problems caused by alkali scales, but also can enlarge the selecting range of surfactants and reduce the cost of ASP flooding. (c) 2005 Elsevier B.V. All rights reserved.