Water is essential for energy exploitation, and moreover the contradiction between water resources and energy recovery seen in China is more severe than those in other countries. Given this, CO2 waterless fracturing, which improves the production and recovery factor of an individual well and meanwhile serves for water preservation and CO2 underground storage, can contribute to the sustainable development of China's oil industry.
The continuity and reliability of equipment is a key technical aspect for the successful waterless fracturing, in which the operation is required to be done in a sealed, pressurized environment during the whole workflow, and the proppant-carrying capability of fluids is low. Therefore, strict requirements are raised up upon the equipment. On the basis of the dynamic fluid balance combined with the fluid phase evolution during the whole construction workflow and its effects on stimulation treatments, this paper optimized the design of key construction equipment, such as CO2 storage tanks, booster pumps, sealed blender trucks and fracturing pump trucks.
Major improvements can be concluded as: 1) the vertical tank is used for the sealed blender, which enhances the control stability of sand supply process jointly by the pressure difference regulation and auger; 2) booster pump unit with high pump-rate capability are included in the system for liquid supply and fluid phase control; 3) the liquid supply combines the mobile transport tanks and fixed storage tanks to increase the liquid supply capability; 4) the fracturing system is equipped with eight special fracturing pumps for waterless fracturing, fulfilling the construction requirement of 20,000 hydraulic horse power. The whole equipment system has treatment capability of available pump rates up to 12 m3/min, sand volume of 27 m3 and CO2 injection of 1500 m3. In 2017, this equipment system was used in waterless fracturing for six times, with a maximum proppant input of 23 m3. Both the liquid and sand supply processes are found stable, and the production gain after stimulation is considerable.
It is estimated that in tight reservoir, oil production brought by 1 unit volume of CO2 equals to that of 2.4 unit volume of water-based fracturing fluid. Providing that the average CO2 injection of waterless fracturing wells is 630 m3, a single well can save 1512 m3 water resource. This equipment system fully meets the requirement of fracturing in vertical and horizontal wells of unconventional resources, and can effectively support the further development of the waterless fracturing technology.