Specified staged acid fracturing of horizontal well for strong-heterogeneity carbonate gas reservoirs: A case study on the gas reservoir of the fourth member of Upper Sinian Dengying Formation in Gaoshiti-Moxi area of the central Sichuan Basin

The carbonate gas reservoir of the fourth Member of Upper Sinian Dengying Formation (hereinafter referred as “Deng 4 Member”) is characterized by deep burial, high temperature, low porosity, low permeability and strong heterogeneity in the Gaoshiti-Moxi area of the central Sichuan Basin. In order to effectively enhance the well productivity and gas reservoir development benefits, this paper defines the reservoir stimulation factor by comprehensively considering the drilling, logging and well type characteristic parameters, to carry out quantitative reservoir evaluation. Then, the refined segmentation method was developed by fully considering the reservoir stimulation factor, in-situ stress and borehole condition. In addition, the differential acid fracturing technology was prepared for different types of reservoirs. On this basis, the specified staged acid fracturing technology of horizontal well for strong-heterogeneity carbonate reservoirs was developed and then applied in the field. And the following research results were obtained. First, the reservoir stimulation factor (y) can be used to accurately evaluate the reservoir characteristics. The high quality reservoir has y ≥ 1.0, the moderate reservoir has 0.5 ≤ y < 1.0, and the low quality reservoir has y ˂ 0.5. Second, the specified staged acid fracturing technology considering the influence of reservoir stimulation factor, in-situ stresses and borehole condition comprehensively is formed. Stimulation measures are applied specifically in different types of reservoir intervals to give full play to the resource potential in each stimulation segment of a long horizontal section, so as to improve the production rate of natural gas reserves in strong-heterogeneity reservoirs. Third, the acid fracturing technology suitable for different types of reservoirs is also different. It is recommended to adopt acid fracturing with gelled acid or diverting acid in high quality reservoirs, acid fracturing with self-generating acid prepad in moderate reservoirs, and acid fracturing with 2–3 stages alternate injection of self-generating acid prepad and gelled acid in low quality reservoirs. Fourth, 69 wells in the production area of the platform margin of Deng 4 Member gas reservoir in Gaoshiti-Moxi area are stimulated with specified staged acid fracturing. The average absolute open flow is 127.6 × 104 m3/d, which is 46.5% higher than that in the early stage of gas reservoir development. In conclusion, the research results provide remarkable stimulation effects, increase the single well gas production greatly and effectively support the efficient development of Deng 4 Member gas reservoir in the Gaoshiti-Moxi area.