Experiment simulation of hydraulic fracture in colliery hard roof control

Abstract

Hydraulic fracture geometry is of paramount importance to enhance fracture effect in colliery hard roof control. Near wellbore complexity often resulted in low lateral dimension of induced fracture hence the fracking effect of the treatment. In the past, near wellbore fracture geometry is mainly believed to be controlled by the interaction between hydraulic fracture initiation and natural fracture infiltration/opening. Recently, lab experiments have proved that transverse notch plays a role in lowering breakdown pressure and also in reducing near wellbore complexity. Besides, colliery hard roofs are often over-pressured, therefore, with the increase of pore pressure, the influence caused by stress difference is weakened, and the role of notch becomes significant. In this paper, a series of lab experiments are conducted to gain in-depth understanding of the role transverse notch plays on near wellbore geometry and fracture reorientation. Hydraulic fracture process is physically simulated by injecting water (green dyed) at some certain rate into a pre-installed tube in a block (300×300×300 mm) under tectonic stress condition. Different notch parameters and fracturing regimes are used to make comparison. The notch changes the local stress and strain field in the blocks and affects the way blocks deform and fail. Post-mortem analysis and photo image of the block clearly show: (i) the length and angle of initial notch play a role in determining: near wellbore geometry and fracture reorientation; (ii) near wellbore fracture complexity may be reduced by making a longer initial notch and picking an appropriate notch angle; (iii) better understanding can be achieved by conducting further researches under normal stress condition and taking more factors into consideration.