Several investigators proposed the use of precipitates (or other products of phase transitions) as blocking agents for fluid diversion in oil recovery processes (see Refs. 92-107 in Ref. 10). Typically, these processes involve forming a blocking agent in situ by mixing two incompatible chemical solutions in the formation. Alternatively, chromatographic separation in a formation can be exploited to form a blocking agent from a stable mixture. Llave and Dobson74 described a recent example of the latter process. In their process, a low-viscosity surfactant-alcohol blend was injected. In the formation, the blend chromatographically separated, with the alcohol propagating more rapidly than the surfactant. After the alcohol was removed from the surfactant, the surfactant formulation became very viscous and restricted flow.
We surveyed the petroleum and patent literature to investigate whether blocking agents formed in situ from phase transitions have potential advantages over gels.10 In most cases, the flow properties of the proposed materials (before the phase transition) are no different from those of gelants. Therefore, their placement characteristics are similar to those of gelants. Specifically, for a given distance of penetration into a high-permeability zone, the distance of penetration into a less-permeable zone will be no less for a precipitate or phase-transition product than for a gelant with a water-like mobility. Certainly, the mechanism of forming the blocking agent can be different for a gel versus a phase-transition product. This difference could allow one blocking agent to penetrate deeper overall into a formation than another blocking agent. However, it will not change the relative placement properties (i.e., the distance of penetration in one zone relative to that in a nearby zone during unrestricted injection).1 Thus, placement of these materials is not better than that achieved using a low-viscosity gelant.
Very little work has been reported on the permeability dependence of the permeability-reduction properties of precipitates. We suspect that they usually will be the same as those for particles. As mentioned earlier, residual resistance factors tend to increase with increasing ratio of particle size to pore size.69-71 (Particles that enter porous rock reduce the flow capacity of low-permeability rock by a greater factor than in high-permeability rock.)
Thompson and Fogler73 investigated the use of "reactive water-blocking agents" to plug water zones in preference to oil zones in production wells. These chemicals are dissolved in oil and then injected. They react upon contact with water to form a precipitate or solid barrier. Ideally, watered-out zones will be restricted by blocking agents formed at the front between the displaced water bank and the injected bank of reactive chemicals, while no blocking agent should form in zones with high oil saturations. To maximize formation of blocking agents in water zones, Thompson and Fogler proposed using a relatively viscous oil as a carrier fluid for the reactive chemicals. When the well is returned to production after injecting the reactive chemicals, water should finger through the bank of reactive chemicals—thereby promoting mixing and formation of the blocking agent. One of the main challenges in using these materials is that reaction with residual water in the oil-bearing zones could damage oil productivity. More work is needed to assess the potential of reactive water-blocking agents, especially their effects on oil productivity.