Experimental Investigation Of Breakthrough Pressure Of Carboniferous Shale With Fracture In The Eastern Qaidam Basin

Breakthrough pressure is an important parameter to evaluate rock storage capacity.Shale is a kind of fine-grained sedimentary rocks because of its low porosity and low permeability characteristics,therefore,it has a good gas-sealing efficiency.During the research process,this higher gas-sealing efficiency can be characterized by its high breakthrough pressure.Thus shale formations can be widely used not only in nuclear waste storage,greenhouse gas storage,but also shale gas generated favorable level.In general,under fully water-saturated circumstance,the breakthrough pressure value can reach as high as 40 MPa.However,the shale formation is usually a large number of natural cracks in practice,these cracks may lead to breakthrough pressure relative to the integrity of the core breakthrough pressure and results in of its reduction of more than fifty times.And this will affects the low permeability characteristics of the shale.Under this situatuin,the effects of fracture on the breakthrough pressure of the Carboniferous shale are simulated by making the artificial fissures in the core.In the experiment,three shale samples from the Carboniferous Upper Clark Formation in the zk1-1 well were selected.First,the three cores were cut longitudinally and then bonded again with an adhesive in order to obtain a single pass through the gap.Using the HA-I multiphase flow permeation instrument,all the experiment conditions are invariable except for the confining pressure,to obtain a series breakthrough pressure under different confing pressures.At the same time,the pressures of the upstream and downstream,flow and other data were recroded by the multiphase flow permeation instrument.And the fracture width of the core was calculated by Darcy's law and cubic law.Then,the mineral compositions were obtained by using the X-ray diffraction method.The poisson's ratio and elastic modulus of the samples were measured by the method of measuring the velocity of ultrasonic wave.Based on these analyses,the effects of fracture on the breakthrough pressure were analyzed.Through the experiment we measured the width of artificial are 16?m-50?m and width of natural development fractures are 0.13?m-1?m about 100,they're hundreds of times different.Under the influence of these artificial fissures,the breakthrough pressure of these artificial fractures shows a sharp drop compared with the normal fractures in shale.Combining the analyses of mechanical modulus of the sample,the mineral composition,breakthrough pressure was affected by a combination of both the clay mineral content and elastic modulus,ie,the double effect of stiffness.Under saturated condition,swelling of the clay minerals can make the variation of the width of the fracture becoming relatively small.However,the core with larger elastic modulus under varying confing pressure can keep relatively steady change instead of leading to shape change under higher confining pressure.In this study,the influence of the existence of fractures on the breakthrough pressure was studied,which is of great practical significance in the evaluation of harmful gas storage performance of greenhouse gases and the process of hydraulic fracturing during shale gas extraction.