Preformed Particle Gel for Conformance Control: Transport Mechanism Through Porous Media

Abstract

Abstract Preformed particle gel (PG) has been successfully synthesized and applied to control excess water production in some mature water-flooded oilfields in China. Investigations show that PG is strength- and size-controlled, environment-friendly, stable over long periods of time, and very likely capable of overcoming some drawbacks inherent in gel treatments based on in-situ gelling. Its thermostabilization is not sensitive to reservoirs minerals and formation water salinity. To support its future applications, this paper describes experiments that investigate the mechanisms for PG propagation through porous media. Visual observations in etched-glass micro-models demonstrate that PG propagation exhibits six patterns of behavior: direct pass, adsorption and retention, deform and pass, snap-off and pass, shrink and pass, and trap. Which pattern is dominant is related to the diameter ratio of swollen PG and pore throat, PG strength and the driving force. In macroscopic scale, PG propagation through porous media can be described by three patterns: pass, broken and pass, and plug. Which kind of pattern is dominant can be determined by pressure change with time at different tap, particle size of effluent and residual resistance factor at different segment of a core. Measurements from core-flooding and micro-model experiments show that a swollen PG particle can pass through a pore throat whose diameter is smaller than its diameter due to the elasticity and deformability of swollen PG. PG strength is a principle parameter to determine the diameter ratio of a PG particle and a pore throat that PG can pass through a porous medium. A PG particle can move through a porous medium only if a driving pressure gradient is higher than a threshold pressure gradient. The threshold pressure depends on PG strength, the diameter ratio of particle and average pore size. Further work will investigate the potential for PG to improve oil recovery and the optimization method to design PG treatments. Introduction Reservoir heterogeneity is a principle factor responsible for low sweep efficiency of injected water or gas. To control conformance in water or gas flooding, many technologies have been applied, such as polymer flooding, foam flooding, surfactant flooding and so on.[1–3] Injecting large volumes of gel to correct in-depth permeability for those reservoirs with fracture or channel has become an attractive technology.[4–8] In this paper, "channel" means an open, linear-flow structure. It does not mean flow through matrix. In recent years, the study of preformed gel for conformance control has gained more interest among gel-based enhanced oil recovery processes. Seright studied some properties of preformed bulk gel through fractures and proved that preformed gel had better placement than in-situ gel and could effectively reduce gel damage on low permeability unswept oil zones.[9–11] Chauveteau et al. synthesized preformed microgels which were cross-linked under shear,[12–14] and Feng et al. proved that the microgels could be easily injected into porous media without any sign of plugging and these micro-gels should be good candidates for water shutoff and profile control operations. [15] PG is another kind of preformed gel in which bulk gel synthesized in surface facilities is cut into particles and is dried to form xerogel particle at a higher temperature. This process provides a product that is more easily packed and saves transport cost.[16–18] Some advantages of PG can be summarized as following:PG, synthesized on surface facilities, can overcome some distinct drawbacks inherent in in-situ gelation systems, such as lack of gelation time, uncertainness of gelling due to shear, degradation, chromatographic of gelation compositions, dilution by formation water;PG is strength- and size-controlled, environment-friendly, and its thermostability not sensitive to reservoirs minerals and formation water salinity;PG can resist high temperature (120 °C) and high salinity (300,000 mg/L)