Abstract

Oil leakage into the upper aquifer is generally considered to be an environmentally-unfriendly issue and shorten production life. Despite not necessarily receiving the most coverage, oil leakage do happen in steam huff-puff production wells and may actually occur more frequently than we perceive. Presently, the oil leakage through annular space around a cemented casing into upper water layer during huff stage has become a problem to be urgently solved in thermal recovery production. The solutions to this difficulty are usage of thermal-stability cement slurry system with high-adsorbability additives and thickened low-permeability surface cement sheath. The well section from 100 metres below the upper aquifer is cemented by thickened cement sheath, and the rest well section from the bottom of oil layer to the wellhead is cemented by a novel high-adsorbability cement slurry system. The objective of usage of high-adsorbability cement slurry system is to absorb spilled oil, and the objective of the thickened low-permeability surface cement sheath is to achieve a stable physical plugging. SEM photographs indicate that the microstructure of thermal-stability cement paste after high-temperature steam is more compact. The performance of hardened cement paste before and after steam vapor curing shows that the compression strength, tensile strength and shear stress had little change. It avoids annular cement paste as a channel for the leakage of crude oil. The scaled 3D oil adsorption physical simulation was employed to examine the effectiveness of cement paste with high-adsorbability additives. It can reduce the migration of oil, and the pollution is limited in the annular space. The results show that permeability of cement paste is about 0.03×10-3 μm3. The thickened low-permeability surface cement paste ensures that spilled oil cannot enter the upper aquifer. These results had been validated on site test. The CBL/VDL test report revealed that acoustic amplitude value of the well is less than 10%. The retest results of CBL/VDL after thermal recovery production for a long time reveal that acoustic amplitude value almost remain unchanged and almost no oil exists in annular space. No spilled oil has been observed in observation wells located in the upper aquifer.

Through usage of physical adsorption and plugging, the volume of oil leakage into annular space is obviously reduced, almost all the spilled oil is absorbed by cement paste, and no spilled oil has been observed in the upper aquifer. This method completely solved the problem of crude oil polluting the upper water layer and extended the production life.

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