Located in the Neogene Shawan formation in the central Block Pai 601 and southern Block Pai 6 of Chunfeng oilfield, the ultra-heavy oil reservoirs are characterized by shallow burial, thin pays, low original formation pressure and high crude oil viscosity. A compound method integrating horizontal well, viscosity reducer, steam with nitrogen is usually used to develop these reservoirs. After several cycles of huff and puff development, the formation pressure dropped, edge and bottom water broke through the oil-water contact and coned upward, resulting in a longer water drainage period, higher cumulative water production, and a shorter effective production period, and consequently relatively low ultimate oil recovery. Therefore, a steam huff and puff test assisted by nitrogen foam was carried out, and indoor experiments and numerical simulation techniques were used to analyze and compare foam applicability and optimize injection parameters and process. Field test results show that after applying steam huff and puff assisted by nitrogen foam in the blocks with edge and bottom water intrusion, the average water drainage period in the oil well was shortened by 8.3 days, the water cut decreased by 32.2% and the cumulative oil production increased by 2 606.0 t. In the blocks after several cycles of huff and puff, injecting nitrogen foam reduced the water cut by 8.6% and increased the cumulative oil production by 1 668.0 t, indicating that nitrogen foam can effectively increase formation energy, block large pore channels, adjust steam absorption profile and play a key role in improving the ultimate oil recovery.
