Abstract

The heavy oil block field Z70 has oil viscosity of 165mPa.s and high temperature of 93°C, and is highly heterogeneous. After water injection, the water cut rose quickly and achieved 83% when oil recovery was only 7.6%. In order to control the quick rising of water to oil rate ratio, from 2001 to 2003, crosslinked polymer gel was injected as field test, but the effect was limited. Besides, the highly viscose polymer damaged the low permeability layers or zones, leading to hard water injection on some injectors. Since 2010, A new type of polymer flooding (NPF) technology has been applied and greatly improved oil recovery.

The new polymer is particle-type polymer with various particle sizes. When working, it is dispersed in injected water, and has low apparent viscosity, 1–4 mPa.s, so it is easy to be injected into formation. While the polymer particle moves into formation, the size may increase by water absorbing, and it may temporarily plug the prevailing filtration throats, making the carrying water be diverted into the relatively small pores/throats and displace the remaining oil there. The polymer particle has high deformation capacity, so it can deform and pass through the throat under the follow-up pressure, and then it will go to plug another throat. The process will be repeated again and again, so the technology can inhibit water production and enhance oil production.

The particle size and slug sequence were designed for Z70 field by physical simulation on physical models. Sub-millimeter grade of particle was suitable for some wells, and micrometer grade of particle was suitable for some other wells, while some wells need both of them. Totally 0.086PV of the dispersion fluid was injected in the whole field with the concentration of 3000mg/L. After one month injection, the water cut started decreasing quickly and the oil rate started increasing quickly. During the new polymer injection, the water cut decreased to 74.8% from 81.1%, and the oil rate doubled. During the follow-up water injection period, the water cut continued decreasing to 75%. Until now, the water cut has returned to more than 77%, but the NPF keeps taking effect, and the oil recovery is increased by 5.8%.

Summing up the lab studies and field application, NPF works through decreasing water effective permeability and overcoming the unfavorable mobility ratio. It is different from the conventional polymer flooding, which works through increasing the viscosity of the displacing phase. The new technology is more efficient, and the mechanism deserves further studying.

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