Physical Simulation Experiments On Liquid Nitrogen Assited Fracturing Of Shale Formation

The world energy shortage situation is grim,and conventional oil and gas resources have been unable to meet domestic demand.Large-scale development of shale gas can effectively ease the tense situation based on China’s abundant resources.As an effective measure of increasing production,hydraulic fracturing has been widely used in the exploitation of shale gas.However,with the application of hydraulic fracturing in unconventional gas reservoirs,the large scale fracturing operation makes the shortage of water resources and environmental pollution problems,and the effect of hydraulic fracturing is not satisfactory.These problems have become increasingly prominent and the liquid nitrogen fracturing becomes one of the key technologies to alleviate the shortage of water resources.Therefore,it is of great significance for the development of shale gas in China to study the mechanism of brittle fracture of shale under super-low temperature condition of liquid nitrogen,and to explore the feasibility of liquid nitrogen fracturing on the shale reservoirs.This paper aims at the feasibility of shale volume fracturing under the impact of ultra-low temperature liquid nitrogen,Starting with the physical properties of liquid nitrogen and the compressibility of shale,a liquid nitrogen impact simulation experiment system was established.The compressibility of shale under ultra-low temperature is analyzed and the differences of artificial rock and shale in liquid nitrogen before and after the shock fracturing effect in the open hole section of well depth,the displacement,stress difference,bedding development.The fracturing effect of shale before and after liquid nitrogen impact is analyzed on the basis of fractal theory.Research results show that after the impact of liquid nitrogen,the brittleness of shale decreasing,the fracture development degree increasing,the difference coefficient of horizontal stress decreasing,the crack propagation resistance decreasing,the weak surface mechanical property being weakened,and the compressibility of the reservoir being enhanced.Therefore,it is believed that after the liquid nitrogen treats rock sample,there is the potential to increase the compressibility of the reservoir.The results of fracturing simulation before and after the impact of artificial rock sample and shale by liquid nitrogen are shown that the effect of stress difference and displacement on the fracturing effect is remarkable compared with conventional fracturing.The water saturation of core is the most influential factor after impacted by the liquid nitrogen.It is proved that the frost heaving force of water in core is not negligible.Before and after the impact of liquid nitrogen,the effect of stress difference and fracturing fluid displacement is unchanged.The fracture pressure of core decreases with the increase of stress,and increases with the increase of displacement.The impact of liquid nitrogen does increase the brittleness of shale core and is more prone to crack.However,in the later stage,the brittleness of the shale core is decreased and the plastic deformation is strengthened.Under the premise that all the experimental factors are consistent,before and after the impact of liquid nitrogen,the decrease of fracture pressure is 30% to 50% basically,and core permeability is greatly improved.It is proved that the effect of liquid nitrogen impact is obvious.The fractal theory is used to deal with the distribution of reservoir fractures and to establish the evaluation criteria for the capacity of liquid nitrogen.The results of core fracturing were evaluated by using box dimension method.Based on the evaluation criteria of the summation of each end dimension D,The greater the value of D,the better the fracture effect after fracturing.