CO2 waterless fracturing is a novel waterless fracturing technology. CO2 exists in the reservoir with supercritical state, and its fracturing stimulation mechanism is very different from that of water-based fracturing. This paper studies the physical and chemical properties of supercritical CO2 and reservoir adaptability of CO2 waterless fracturing.

Supercritical CO2 has the advantages of good fluidity and strong penetrability, which avail to form a complex network fractures. Through miscible phase with crude oil, absorption gas displacement, and reservoir energy enhancement, production and ultimate recovery are further improved. While the liquid CO2 has the disadvantages of poor proppant carrying capacity, high friction and low fracture opening. Based on CO2 waterless fracturing practices in Jilin oilfield, this paper summarizes physical parameters, operation effect and production situation of all wells, analyzes the main factors influencing productivity, and puts forward a set of well and layer selection methods of waterless CO2 fracturing.

Under the condition of existing CO2 thickening and resistance reducing technology, the selection of wells and layers is mainly carried out in 6 aspects. (1) Because the filtration of CO2 fracturing fluid is strong, the permeability of target reservoir should be lower than 5md in order to ensure stimulation effect of remote area. (2) CO2 can react with water and divalent metal ions to produce carbonate sediments to block existing pores and reduce reservoir permeability, so it is better for low water-bearing reservoirs. (3) Frictional resistance of CO2 is 1.9 times as that of conventional guar gum, so the target layer should be 3000m or shallower to reduce frictional pressure drop. (4) Energy increasing effectiveness of unit volume of CO2 is 1.9 times as that of slick-water, which is more suitable for stimulating undercompacted reservoirs. (5) There is no water phase in CO2 fracturing fluid, suitable for stimulating strong water-sensitive reservoirs. (6) CO2 is easy to dissolve in crude oil and greatly reduces its viscosity, which is suitable to stimulate heavy hydrocarbon reservoir.

Adopting above well and layer selection principles, CO2 waterless fracturing were implemented in 6 wells in 2017, and the key parameters, such as success ratio, sand adding amount, production capacity post-fracturing were comprehensively promoted, which effectively supported CO2 waterless fracturing development practices of unconventional reservoirs.

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