Experimental investigation of shale breakdown pressure under liquid nitrogen pre-conditioning before nitrogen fracturing

Cryogenic fracturing with liquid nitrogen(LN_(2))offers the benefits of reducing the water consumption and adverse environmental impacts induced by water-based fracturing,as well as potentially enhancing the fracture complexity.We performed a series of laboratory experiments to explore the key mechanisms governing the breakdown pressures of shale during cryogenic fracturing.In this study,cylindrical shale samples were pre-conditioned by exposing a borehole to low-temperature LN_(2) for a certain time period,and then,the samples were fractured using gaseous N_(2) under triaxial stress and a high reservoir temperature.The effects of various key parameters on the breakdown pressure were investigated,including the duration of the low-temperature LN_(2) treatment,the confining pressure,the reservoir temperature,and the direction of the shale bedding relative to the borehole axis.The results demonstrate that the injection of low-temperature LN_(2) as a pre-fracturing fluid into a borehole can significantly reduce the breakdown pressure of the shale during subsequent nitrogen fracturing.This reduction in breakdown pressure can be further intensified by increasing the duration of the LN_(2) pre-conditioning.Without LN_(2) pre-conditioning,the breakdown pressure initially increases and then decreases with increasing reservoir temperature.When LN_(2) pre-conditioning is applied,the breakdown pressure keeps decreasing with increasing reservoir temperature.As the confining pressure increased,the breakdown pressure increased linearly in the tests with and without LN_(2) pre-conditioning.The experimental results demonstrate that LN_(2) preconditioning before N_(2) fracturing is a promising waterless fracturing technique that reduces the breakdown pressure and enhances the fracture complexity.

Experiment and Simulation for Controlling Propagation Direction of Hydrofracture By Multi-Boreholes Hydraulic Fracturing

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 roof-floor 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 well-aligned in large scale in coal seam.In this study,a method for controlling propagation direction of hydrofracture by multi-boreholes is investigated by theoretical analysis,laboratory experiment and numerical simulation.And this is followed by an on-site 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 multi-boreholes 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 on-site test in an underground coalmine is practiced where this proposed method is used to enhance the CBM production.Results show the scope of the hydro-fracture resulting from the multi-boreholes 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.