Practices and Understandings of Acid Fracturing of Ultra/Extra-Deep Exploratory Wells in Sichuan Basin and Its Periphery

Compared with that of ordinary deep exploratory wells, the acid fracturing of ultra/extra-deep wells in the Sichuan Basin and its periphery faces the working environments featuring deeper wells, higher temperature and pressure, and more complicated fluid and lithology, and it has the problems of large operating friction, low flow rate, small scale, short effective distance of acid solution, poor fracturing result, unreliable tools, low success rate of the first operation execution, difficult security guarantee, expensive investment, and unsatisfactory yield. Therefore, the exploration progress of such wells is slow. By applying the latest achievements of the oil and gas industry and manufacturing industry in China and abroad, the technical research was conducted based on actual experiences and lessons, and the new large-diameter and high-strength air-tight seal pipe string was used to replace the traditional pipe string to reduce the friction and increase the flow rate. The bi-directional bimetal seal which could be motivated simultaneously was used to replace the bi-directional single-metal seal to test the sealability and maintain the long-term stability, so as to increase the wellhead control capacity; and the bi-directional anchored expansion packer was used to replace the bottom-anchored rotary pressure setting packer, so as to reduce the probability of cylinder movement and seal failure caused by pipe string expansion during acid fracturing. Multiple sets of shock absorbers were connected in series to reduce the adverse effects of detonation force, and the “four-valve-one-packer” method was adopted to replace the “two-valve-one-packer” method. In addition, the backup valves were connected in series to improve reliability, and the corrosion-inhibiting and velocity-retarding working fluid suitable for temperature of ≥220 °C and pressure of ≥180 MPa was developed. Equal emphasis was placed on simultaneous two-step operation and simultaneous three-step operation instead of the traditional emphasis on simultaneous two-step operation, and thus the optimum selection method of the acid fracturing process was established. The field application was progressing smoothly, and the safety guarantee ability, fracturing scale, fracturing effect, and success rate of the first operation execution were significantly improved, providing a reference for acid fracturing of similar wells.