Acid hydraulic fracturing in complex interbedded reservoirs

Background: Preventing contamination of technogenic fractures formed by hydraulic fracturing and by chemical interaction of reactive fluids with the rock is a key condition for increasing reservoir fluid production rates. Reducing the risks of formation and precipitation of insoluble reaction products within the fracture and the near-fracture zone is achieved through experimental research and mathematical modelling of the interaction processes between technological fluids, reservoir rocks, and formation fluids. At the same time, the most important task in preparation for experiments, modelling, and the actual execution of operations can be considered the identification of the principal interacting elements entering into reactions whose products may reduce stimulation efficiency, especially under conditions of limited availability of core material and specialized software for comprehensive chemical molecular modelling. Consequently, theoretical investigation of the main causes of unsuccessful chemical treatments of formations and hydraulic fracturing operations with chemically active technological fluids, as well as assessment of the risks of negative events, becomes particularly important. Aim: Increasing the efficiency of stimulation by acid hydraulic fracturing methods through optimization of technological fluid composition and prevention of the negative impact on the reservoir of reaction products formed by interaction of reservoir rocks or formation fluids with technological fluids. Materials and methods: To evaluate the main causes of insoluble precipitate formation and other factors for reduced stimulation efficiency, a detailed literature review was carried out, and mechanisms increasing and decreasing the corresponding risks were identified. A mechanical and lithological model was constructed in the vicinity of several wells of one of the Central Asian fields, which shows that one of the main reactions leading to negative consequences may occur when several conditions are simultaneously met. The feasibility of such conditions was assessed based on calculations and modelling. Results: Analysis of the geological and mechanical environment at one of the fields and a detailed study of the main reactions with formation rocks made it possible to identify the causes of ineffective acid hydraulic fracturing treatments. A treatment program was prepared aimed at preventing future risks of reduced efficiency of acid hydraulic fracturing. Conclusion: The applied approach is aimed at the justified, targeted selection of hydraulic fracturing technological fluids, which makes it possible to reduce risks or prevent fracture plugging by reaction products, avoid clay migration and swelling, and other negative impacts on the filtration properties of the formation in the vicinity of technogenic fractures. All these measures are aimed at improving the efficiency of stimulation by acid hydraulic fracturing.