Multiple thermal fluid huff and puff, which uses nitrogen, carbon dioxide and hot water as thermal medium, has achieved a high increase of oil production rate in BN reservoir, a heavy oilfield locating at Bohai Bay. Six wells have completed two cycles of thermal huff and puff. Comparing to initial conventional production using natural energy, the periodic well production in first cycle increased to 1.6 times and the annual production rate of the whole reservoir increased from 0.4% to 0.7%. However, due to high formation oil viscosity (413∼741mPa•s) and strong heterogeneity, the injected fluid breakthrough happened three times in the second cycle. On one hand, the well production rate didn't gain an obvious increase because of the heat loss, on the other hand, the surrounding wells which were broken into had to be shut down. Consequently, the annual reservoir production rate was only 0.5%, which was much lower than project design. There is an urgent need to find a suitable solution to the injected thermal fluid breakthrough.
The most effective way to restrain the fluid breakthrough is injecting the thermal fluid into all the thermal recovery wells simultaneously, which is not realizable due to the offshore platform space limit. Combinatorial method with thermo-gel treatment have been optimized for more cycles of thermal huff-puff. Thermo-gel polymer and cross-linking agent spontaneously gel when they reach a critical temperature, which is configured between original reservoir temperature and reservoir temperature during thermal huff period. Thermo-gel was injected ahead and entered the high permeability channels mainly. With the plugging effects, the fluid breakthrough was prevented. Meanwhile, the thermal fluid permeated into low permeability zone where remaining oil concentrated, resulting to higher efficiency of injected heat.
Thermo-gel treatment was carried out for the rest three wells in the second cycle. Around each of the three thermal injection wells, there were 2∼3 wells that could have fluid breakthrough and only one surrounding well happened. Compared with the first three wells in the second cycle, the periodic oil increment of the last three wells increased to 2.6 times and the average effective period extended 105 days. Field practice showed a positive synergistic effect between thermal huff-puff and thermo-gel treatment. In the help of thermo-gel treatment, incremental oil production for single well has reached 1.58×104m3 in the past few years.
Our experiences show that such thermo-gel treatment assisting technology for thermal recovery is effective for enhancing oil recovery of heavy oil fields and meets the demand of high speed development for offshore oil fields.