Experiment and simulation for controlling propagation direction of hydrofracture by multi-boreholes hydraulic fracturing

Abstract

Hydraulic fracturing has been applied to enhance CBM production and prevent gas dynamical hazard in underground coal mines in China. However, affected by in situ stress orientation, hydrofracture can hardly continuously propagate within coal seam but may easily extend to the adjacent rooffloor strata, causing ineffective permeability enhancement in coal seam and increasing the risk of gas transfinite during mining coal. Thus, it is very necessary to artificially control the propagation direction of hydrofracture and make it wellaligned in large scale in coal seam. In this study, a method for controlling propagation direction of hydrofracture by multiboreholes is investigated by theoretical analysis, laboratory experiment and numerical simulation. And this is followed by an onsite test in an underground coal mine to verify this method. Firstly, stress intensity factor at the hydrofracture tip is analyzed where pore pressure is taken into consideration. Results show that the pore pressure is able to increase the stress intensity factor and reduce hydrofracture propagation pressure. Based on this, a method of hydraulic fracturing using multiboreholes to control hydrofracture direction is proposed. Afterwards, laboratory experiments are conducted to explore the impact of pore pressure on hydrofracture propagation. The experimental results agree with the theoretical analysis very well. Later on, a series of numerical simulations are performed to examine the influence of principal stress difference, the angle between assistance drillholes and the maximum principal stress, and the fluid pressure of the assistance drillholes on hydrofracture propagation. Finally, an onsite test in an underground coalmine is practiced where this proposed method is used to enhance the CBM production. Results show the scope of the hydrofracture resulting from the multiboreholes hydraulic fracturing method increases 2.7 times compared with that of conventional hydraulic fracturing. And gas production rate also increases 4.1 times compared with that of conventional hydraulic fracturing and 12.3 times compared with direct borehole extraction without fracturing.