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With the rise of volume fracturing concept and its application in new wells, it has been shown that increasing fracturing volume is an effective measure to improve production and achieve long-term stable production of ultra-low permeability reservoirs in Ordos Basin. However, refracturing requires running liner, which makes the flow passage smaller and can't reach the volume of new wells. The challenge is how to design the tubing combination and develop matching tools to further improve refracturing volume in 5.5″ casing. The objectives of this study are to design a combined mechanical isolation liner and carry out fine-staged large-volume refracturing with a rate of 8 m3/min in 5.5″ casing. In this study, researchers have designed the liner, optimized refracturing tools and wellhead equipment by comprehensively considering reservoir pressure, wellbore, available tubing combinations and friction of fracturing fluid. Downhole microseismic technology is used to monitor refracturing microseismic events, and high-precision 3D scanning technology is used to detect and analyze the wall wear of liners after refracturing. Field tests have been carried out in Ordos ultra-low permeability horizontal wells and achieved perfect effect. This research provides a reliable solution and technical reserve for volume refracturing of ultra-low permeability reservoir in horizontal wells, and verifies that large-volume plays an important role in improving refracturing effect in the test area.

Plugs have been widely used for staged fracturing in horizontal wells. But in wells with long lateral or low reservoir pressure, plug milling are far more complex due to debris accumulation and fluid loss. This study proposed a new multistage fracturing solution for horizontal wells using one kind of ball seat. Researchers have designed mechanical model of the seat. Metal material with great flexibility has been selected out and processed into seats according to the model. The key tools such as elastic seat, large-diameter soluble ball and the matching tools such as cylinder and launching tool were developed. The results of indoor tests and large-scale simulation tests of real parameters have shown that the seat has the capability of repeated rebounding and the capacity of pressure bearing and packing reaches 70MPa. Field tests of 12 horizontal wells, containing totally 115 stages, have been carried out and achieved the technological goals of infinitely large-scale staged fracturing, no milling, and nearly full-bore access. The fracturing cycle has been shortened by more than 30% and the operation cost of single well has been reduced by more than 15%.