Experimental on N2 foam fracturing characteristics of tight siltstone in coal measures

Foam fracturing with low water consumption, low reservoir damage and high flowback efficiency is one of the potential technologies for the development of low-pressure, low-permeability and strong water-sensitive coal measures reservoirs. However，the related research and application mainly focus on foam stability and rheological property design, and the research on fracturing and permeability enhancement characteristics is rare. In view of this, this experiment carried out a comparative study on the law of crack initiation and propagation of N2 foam with different foam quality (volume percentage of gas in the foam), clean water and N2 gas fracturing tight siltstone in coal measures, and evaluated the characteristics of fracture surface morphology and conductivity. The test results show that: ① The foam is a gas-liquid mixture, and its compressibility is between water and N2 gas. The higher the foam quality, the stronger the compressibility and the longer the fracturing pressurization time. ② The foam has the characteristics of high viscosity and low filtration, rock breakdown pressure is higher than that of water and N2 gas, and the rock breakdown pressure is positively correlated with the foam quality, the higher the foam quality is, the higher the rock breakdown pressure is. ③ The flow rate of fracturing fluid directly affects the rock breakdown pressure, with the flow rates of 40 and 20 mL/min, the breakdown pressures of 90% foam quality are 15.66 and 6.08 MPa higher than those of clear water, respectively. ④ The hydraulic input energy and acoustic emission energy of foam fracturing are higher than those of clear water, and with the increase of foam quality, the maximum and cumulative acoustic emission energy at the time of rupture are greater. ⑤ The hydraulic input energy of pure N2 gas fracturing tight siltstone is the highest, but it is mainly used for gas compression. The acoustic emission energy is the lowest when it breaks, and the reinjection pressure is high after fracturing, then the fracturing capacity to induce fracture is relative weak. With the increase of foam quality, the dimensionless fracture area and roughness increase, and the fracture conductivity after foam fracturing is higher than that of clean water and N2 gas. Foam fluid in strong water-sensitive reservoirs can replace conventional water-based high-viscosity fracturing fluid, and the high conductivity fracture with large roughness and surface area can be formed This study provides theoretical guidance for high-efficiency fracturing and permeability-enhancement of strong water-sensitive coal-bearing reservoirs.