Experimental investigation of fracture propagation in N₂ foam integrated layer-penetration fracturing of coal-rock combination body

Integrated layer-penetration fracturing method is a key technology to realize multi-gas co-production in close-layered coal measure strata. Low-viscosity active water is prone to activate weak structural planes, restricting fracture height containment. In view of this, based on the characteristics of high viscosity, low filtration and high flow-back efficiency of foam fluid, a method of N₂ foam low-damage layer-penetration fracturing coal measure strata is proposed. Using the self-developed large-scale true triaxial hydraulic fracturing seepage simulation device, the N₂ foam and active water fracturing are carried out with layered samples, which is combined with limestone, shale and real coal samples, respectively. The injection pressure curve, fracture morphology, fracture network conductivity and main fracture surface roughness characteristics of the two fracturing media are compared and analyzed, and the fracture initiation characteristics and vertical propagation behavior of N₂ foam fracturing are studied. The test results show that: ① The lay-penetration ability of high-viscosity and low-filtration N₂ foam is stronger than that of active water, under the same in-situ stress condition, the number of penetration layers of foam fracturing is more but the peak pressure during fracturing is lower, in favor of layer-penetration fracturing operation. ② Compared to the active water, the pressurization time of high-compressible foam fracturing is longer, and breakdown pressure in the shale layer is 8.7 MPa higher than that of active water. ③ After fracturing, the re-injection pressure of fracture network after N₂ foam fracturing is 3.6 MPa lower than that of active water under in-situ stress condition, indicating that the flow conductivity of fracture network by N₂ foam fracturing is higher. ④ The roughness of the main fracture surface induced by N₂ foam and active water fracturing is 20.53 and 13.56, respectively, the former is more conducive to maintain the flow conductivity in the production process. It is concluded that foam with high viscosity and low filtration characteristics can achieve a better layer-penetration effect, but the complexity of fracture network is relatively low. It is suggested to adopt the hybrid fracturing process of “foam fracturing for main fracture and gas/active water fracturing for branch fracture” to realize the integrated layer-penetration fracturing and fracture network reconstruction in the coal measure strata.