Study on fracture propagation characteristics of hydraulic fracturing in steeply inclined coal reservoirs

Abstract

Due to the rapid changes in burial depth in steeply inclined reservoirs (SICR), there is a significant change in stress in the up-dip and down-dip directions within the drainage range of CBM wells, and the expansion of fracturing fractures is different from that of near horizontal reservoirs. Based on the field fracturing data of steeply inclined reservoirs in Fukang West Block of southern Junggar Basin, this study analyzed the bottom hole pressure and fracture morphology characteristics in the process of hydraulic fracture of steeply inclined reservoirs, studied the fracture propagation characteristics of steeply inclined reservoirs by using the finite element numerical method, and expounded the impact of dip angle (stress gradient) on the fracture propagation of coal reservoirs. The results show that according to the stress and fracture propagation characteristics in the up-dip and down-dip dip directions, steeply dipping reservoirs are divided into shallow-steeply dipping reservoirs, middle-steeply dipping reservoirs, and deep-steeply dipping reservoirs. The hydraulic fracture propagation of shallow-steeply inclined reservoirs (with vertical stress less than horizontal stress) can be divided into three stages: fracture propagation in the up-dip direction, fracture stopping propagation, and fracture propagation in the down-dip direction; the fractures in the middle-steeply inclined reservoirs (with the vertical stress in the up-dip direction is less than the horizontal stress, and the vertical stress in the down-dip direction is greater than the horizontal stress) first expand in the up-dip direction, and then the fractures begin to turn perpendicular to the dip direction along the boundary of the up-dip direction and where the vertical stress is greater than the horizontal stress; deep-steeply inclined reservoirs (with vertical stress greater than horizontal stress) fractures propagate perpendicular to the dip direction. As the dip angle of the reservoir increases, the propagation rate of shallow reservoirs increases in the up-dip direction, while the rate decreases in the down-dip direction; the fractures in the middle-steeply inclined reservoirs gradually propagated from along the dip direction to perpendicular to the direction. The difference in fracture propagation in up-dip and down-dip direction of steeply inclined reservoirs is mainly due to the difference in fracture pressure caused by stress changes at different burial depths, causing the fractures to first propagate along the dominant channel with low stress.

Keywords

• coalbed methane
• hydraulic fracturing
• fracture propagation
• abruptly change stress
• steeply inclined reservoir