Heterogeneous porous-fractured-cavernous carbonate oil reservoirs are the most challenging targets for acidizing, because of heavy contrasts in permeability, which might be a few orders of magnitude higher in fractured zones, as compared with oil-saturated porous matrix blocks. Conventional technologies to divert acid do not work in these reservoirs; neither quasi-viscous hydrophobic emulsions nor high-concentration polymer systems are able to plug cavernous-fractured zones, and the injected acid will not divert into the oil-saturated matrix intervals. The efforts were made to develop structure-forming colloidal systems to divert low-viscosity acid systems while selective matrix acidizing of heterogeneous porous-fractured carbonate reservoirs.
Acid diversion process can be improved by polymer fibers that are able to temporarily plug the reservoir zones. As soon as the diverting system enters the fracture, the self-destructing disperse fibers and particles accumulate and aggregate, thus, preventing further moving of the system. The subsequently injected acid is diverted into the target zones. The self-destructing plugging polymer fibers and particles are dissolved within the predetermined time interval, which depends on the formation temperature and the pH value.
The project was aimed to solve two main objectives: selection of structure-forming disperse filling agents with controllable plugging effect and predetermined period of self-destruction; and development of visco-elastic carrier systems to deliver disperse filling agents into fractures.
In the role of structure-forming disperse filling agents we have studied granules and fibers of polylactic acid (lactide). The laboratory research involved the following experiments: selection of structure-forming disperse filling agents and development of visco-elastic carrier systems, analysis of rheological properties under different physical and chemical conditions, evaluation of plugging efficiency of the system; flow studies and flooding tests under simulated selective matrix acidizing conditions; testing of physical and chemical properties of the systems, ways to control destruction, optimal compositions of structure-forming colloidal systems.
The anticipated effect is enhancement of oil production due to improved effectiveness of selective acid treatment of fractured-cavernous carbonate reservoirs of the main oil fields of Tatneft PJSC.
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