Fracturing fracture propagation law of coal seam and its influencing factors in the Shizhuang North Block

Abstract

It is of great significance to reveal the law of fracturing fracture propagation and influencing factors in coalbed methane (CBM) reservoirs for the formulation of fracturing technology and the efficient development of CBM resources. In this paper, the data of core drilling, well logging, downhole observation, fracturing construction and microseismic monitoring in Shizhuangbei CBM block, Qinshui Basin, China, are used to systematically analyze the hydraulic fracturing fracture propagation law and influencing factors. The research results show that clean water fracturing fluid is mainly used in the study area. According to the change in construction pressure, the fracture curves can be divided into four types: descending, stable, ascending, and fluctuating. Hydraulic fracture strikes are mainly N37°–55°W and N42°–70°E, with lengths varying between 69.5 m and 157.5 m and heights of 35 m to 68.5 m. In the type I stress state(σv>σH>σh), the fracturing fractures tend to expand vertically, and most fracturing curves are descending or stable. In the type II in-situ stress state(σH>σh>σv), the propagation direction of fracturing fractures is relatively single, and most of them are vertical fractures of a single shape. The width of the fracture network is small. Under the type III in-situ stress state(σH>σv>σh), the fracturing fractures expand along the horizontal direction, and the fracturing fractures have more complex shapes. The principal stress difference coefficient (λ)is positively correlated with the length of the fracture network and negatively correlated with the width. For fracturing fracture lengths >90 m, the λ is higher than 0.3. The hydraulic fractures can be communicated, penetrated, or captured by natural fractures. The effect of communication fracturing is better, and most coalbed methane wells have the characteristics of high and stable production. The extension length of the penetrated fracturing fractures is limited, and most of the captured fracturing fractures overlap with joints. The gas production effect of penetrated and captured fracturing wells is poor. With the increase of the proportion of primary and cataclastic coal thickness, the length of the fracture network gradually increases and the width gradually decreases. For fracturing fracture lengths >110 m, the proportion of primary and cataclastic coal thickness is higher than 40%. The research results provide a theoretical basis and geological guarantee for coal seam fracturing reconstruction.

Keywords

• qinshui basin
• coalbed methane
• hydraulic fracturing
• fractures morphology
• micro-seismic detection