XJ6 is a conglomerate reservoir, with oil viscosity 80mPa.s. It is similar to carbonate reservoir that the formation sedimentation does not show obvious rhythm. The pore structure and size varies greatly, with dual and complex porosity and permeability distribution, causing seriously uneven sweeping. Traditional polymer flooding technology is not suitable. A pilot test, conducted in 2013, used a new particle-type polymer, combining several small polymer gel slugs, obtaining obvious oil rate increase and water cut decrease.
Cores were sampled from 8 sealing core drilling wells. The casting core film images were analyzed, and the microscopic structure of formation pores was studied. The pore size and distribution were quantitatively characterized by mercury injection testing data, and also predicted by geology modeling. The macroscopic water flowing channel distribution and volume were quantified through numerical simulation and tracer test, based on which, the combined slug parameters of polymer flooding were designed.
There are 5 injectors and 11 producers in the pilot. Related testing results show that the water flowing velocity between injector and producer varies with big difference, 17 times, and the highest velocity is 27.7 m/d. It is developed with wide range of pore radius, 1.72-354.6μm. To control water cut rising, injection and production rates were kept at low value. The remaining oil distributed in complex porosity and permeability zones was hard to be initiated. The injection plan was alternating injecting the slugs, consisting of big slug of the new particle-type polymer dispersion with particle size in the grade of micrometer, and small slug of crosslinked polymer. The injection liquid amount is 0.2PV. One month after the test started, the water cut of the pilot began to decrease fast and the daily oil rate began to increase fast. The daily oil rate was increase to 62.2 t/d from 16.1 t/d, and the water cut was decreased to 35.8% from 82.0%. The center well got better effect. The average injection pressure of the pilot was increased by 2MPa. The daily water injection rate and daily liquid production rate were increased by 20% gradually.
By the pilot test, the new oil displacing mechanism and slug design method were explored, achieving a success. It was shown that, for conglomerate reservoir with complex formation pore structure and large pore size difference, with the previous slug of highly viscous polymer gel to inhibit the large channels, taking the new submillimeter/micrometer grade particle-type polymer slug can favor displacing the remaining oil.