Abstract: The preliminary estimated resources of carbonate oil reservoirs in Qianjiang Sag are 3.7×10⁸ t, which are important replacement resources for the exploration and development of Jianghan Oilfield. However, due to the diverse lithologies and well-developed interlayers of oil reservoir in Qianjiang Sag, the early acid fracturing technology had poor implementation results, obtaining an average oil rate of 1.6 t/d after fracturing, and the effective rate of only 42.9%, which was unable to achieve effective resource utilization of the block. Therefore, it is necessary to study the geological–engineering characteristics of carbonate rocks with different lithologies and research differentiated measures and technologies. By analyzing and processing data such as well logging, mechanical testing, and mineral composition, some important relevant indicators are obtained, and the multiple linear regression method is used to establish the fracability evaluation model. In addition, two sub-members with good fracability are optimally selected as high-quality sweet spot layers, i.e., the fourth sub-member of the third member of the Paleogene Qianjiang Formation and the lower sub-member of the fourth member of the Paleogene Qianjiang Formation. The basic experimental research such as mechanical testing is conducted on core samples with various lithologies, which enables to identify fracture propagation laws of different carbonate rocks, determine the concept of differentiated reservoir reconstruction, and develop two sets of fracturing technologies, namely the acid fracturing composite sand addition technology for granular carbonate rocks and complex network fracturing technology for micritic carbonate rocks. The former technology has been applied for granular carbonate rocks in four wells, with an effective rate of 100%, and an average oil rate of 8.4 t/d, forming a good scene of increasing reserves with an amount of nearly 20 million tons. The complex network fracturing technology has successfully been applied in Well Z99X, with an oil rate of 137.6 m³/d by blowout testing with a 4 mm oil choke, marking a major breakthrough in new layers in new areas.

Key words: Qianjiang Sag, carbonate, fracability, acid fracturing, fracture network, fracturing technology