| Most oil fields in China are found in continental sedimentary basins. They are characterized by complex geologic conditions and high permeability contrasts inside reservoirs. In the absence of active aquifer, these oilfields were developed by water injection at the early stage of their development. However, serious vertical and lateral heterogeneity in formations resulted in a rapid increase in the water-cut of oil wells. A new kind of in-depth diverting agent called Pre-gel particles (PG) has been developed to control conformance of mature reservoirs. These particles do not dissolve in water and can move inside the porous media, and they can be used in injection well to correct in-depth permeability variation. Laboratory study and field application results show that those PG particles can improve conformance control of injected water. This paper mainly studies which factors influence PG property and why PG can improve oil recovery. CNPC and Petro-China sponsor the project of this paper.The main achievements in this paper are as follows:1. A numerical model is derived that can be used to analysis the penetration distance of gelant in different permeability layer. Considering the relevant physical model experiment results of gel, an important conclusion is given that gel treatment only can be applied in those reservoir with fracture or Channel.2. Pre-gelled particles have been successfully developed. They have been designed to overcome the main drawbacks of classic in-situ gelation system-lack of control of the gelation time, lack of control of the stability of the gel formed or ungelation due to adsorbotion, dilution or degradation of the polymer or pH change, temperature and salinity limitation. They are obtained by crushing dry gels that are mainly made from gelation reation in the surface ground. Adjusting the monomer and crosslinker ratio can control the strength of the gel. These particles do not dissolve in water, but they can swell in water. And the swelling ratio ranges from 20 times to more than 100 times of the intial size of the dry particle. The particles can be stable at 120 C in salt water (20,000mg/L) for more that one year.3. PG property can be studied by analysising the influence of acrylamide, crosslinker, initiator, additive, heat stabilizer, salt concentration, pH and temperature on elastic modulus and swelling ability of gel particles. The best formula ratio is given for this kind of gel particle. Assessing method for gel particle property has been built to satisfy the need of oilfields.4. Micro-mechanism of particle motion and plug has been studied through transparent glass micro-model experiments. The experiment results show that gel particle behaves "Ameba" property in porous media and can deform to pass through the pore throat that is smaller than the particle itself. The main reason responsible for the mechanism is the viscoelastic behavior of gel particle. The motion ability of the particle can enlarge the area of action of gel so that it can improve gel treatment application results.5. The matching ratio of gel particle and pore throat size and critical pressure gradient of particle motion have been studied by core test experiments and special structurally visual micro-model experiments. The concept of critical pressure gradient has been given that can explain the mechanism of in-depth diversion of gel particles. The PG particle can move under a high pressure gradient which is higher the critical pressure gradient near wellbore, but it can plug the pore because of a relative low pressure gradient which is lower the critical pressure gradient away from wellbore.6. The mechanism of improving oil recovery has been studied by core test and visual micro-model experiments. The experiment results show that the gel particle can correct vertical and lateral heterogeneity in formations and can displace part of residual oil in water flooding layers. The displacement mechanism is due to the pressure field in pore.7. To satisfy the need of block-wide in-depth fluid...
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