Pressure relief mechanism and effect evaluation of fracture zones at different layers in surface "L" -shaped horizontal wells

• A mechanical model of the load distribution law in the fractured area of different layers of surface horizontal wells before and after fracturing was established, revealing the unloading mechanism of the fractured area in different layers of surface horizontal wells. • A fracturing effect evaluation method was proposed, which comprehensively considers multiple source parameters such as the construction scale of surface horizontal well fracturing, the fracturing process, the unloading effect, the fracture characteristics, and the fracture mechanism. • From the perspective of different layers of surface "L"-shaped horizontal wells, combined with real-time fracturing parameters, the construction effect of fracturing was comprehensively evaluated from point to surface, from surface to volume, and from macro to micro. This study aims to reveal the pressure relief mechanism of fracture zones at different layers in an “L”-shaped surface horizontal well and evaluate the engineering construction effects during fracturing. A mechanical model was developed to describe the load distribution before and after fracturing, shedding light on the pressure relief mechanisms across various fracture zones. The fracturing effects at different sections and layers of the two wells were analyzed based on real-time monitoring data, microseismic observations, three-dimensional fracture network parameters, and fracture morphology. The analysis, which spans from points to surfaces, surfaces to volumes, and macroscopic to microscopic scales, provides a comprehensive evaluation of the fracturing outcomes. Conclusion: The pressure relief mechanism of the surface horizontal well fracturing zone was revealed by examining the weakening of coal seam strength due to water immersion and the formation of hydraulic fracture networks. An evaluation system for the engineering construction effects of surface horizontal well fracturing was proposed, incorporating real-time fracturing and microseismic monitoring parameters. Real-time fracturing monitoring data shows that the overall fracturing effects of wells ZJ1-1 and ZJ1-2 are similar. In terms of fracturing layer positions, both wells show lower injected fluid and higher sand volumes in the coal seam, while the opposite is true for top and floor fracturing. The coal seam exhibits better fracture creation, with smaller pressure drop data compared to top and floor fracturing. Microseismic monitoring plane data indicates that the overall fracture network effect in well ZJ1-2 is better than in ZJ1-1. The fracture extension range from top and floor fracturing in both wells is larger than that from coal seam fracturing. Overall, the fracture network flow capacity of both wells is comparable and good, with top and floor flow capacities superior to those of the coal seam. Three-dimensional parameter data shows that the average fracture height and volume for top and floor fracturing in both wells exceed those for coal seam fracturing, indicating that the three-dimensional fracture network formed by top (or bottom) board fracturing extends farther than that from coal seam fracturing. The proportion of tensile fractures in wells ZJ1-1 and ZJ1-2 is much greater than that of mixed tensile-shear fractures and shear fractures. The number of tensile fractures in well ZJ1-2 exceeds that in well ZJ1-1, suggesting better fracturing and fracture creation effects in ZJ1-2. The average tensile fracture values in both wells show that the roof fractures exceed those in the coal seam and floor. The mean values for mixed tensile-shear and shear fractures are less distinct, indicating that roof fracturing has a better fracture creation effect than coal seam and floor fracturing. The research aims to offer constructive suggestions for the fine evaluation and efficient development of coalbed methane in similar engineering contexts.