Study On The Initiation And Extension Mechanism Of Hydraulic Fracturing In The Perforated Oil Shale Horizontal Well

Energy is of great significance to the national economy,and national security is directly linked to energy security to a large extent,which can not be ignored.In 2020,COVID-19 broke out worldwide and spread rapidly,bringing more drastic changes and uncertainties to the previously complex and volatile international energy market,and further aggravating the challenges facing energy security of China.Therefore,on the basis of maintaining the stable production of traditional energy and ensuring the security of imported oil and gas supply,actively seeking unconventional energy that can replace crude oil and forming a new pattern of common development of conventional and unconventional oil and gas resources are important measures to ensure the energy security and sustainable economic development of China.The current unconventional oil and gas resources that can replace crude oil mainly include shale gas,coalbed methane,oil sands and oil shale.At present,China has 719.937 billion tons of proven oil shale resources.Behind this huge resource reserves lies 47.644 billion tons of oil shale oil.Therefore,oil shale is considered as one of the most reliable alternative energy sources of crude oil resources,and is an important strategic material to ensure the energy security of our country.Different from conventional oil and gas resources,oil shale needs to be pyrolyzed by low-temperature retorting to produce the oil of oil shale,and the main development method are ground retorting technology and in-situ exploitation technology,the later one promotes kerogen pyrolyzing underground by in-situ heating of oil shale reservoirs.Compared with the serious environmental problems caused by the ground retorting technology,the underground in-situ conversion technology has the advantages of green environmental protection,less land occupation and the ability to develop deep oil shale resources.However,oil shale reservoirs have very low porosity,permeability and thermal conductivity,which require fracturing technology to provide the necessary channels for heating media and the migration of pyrolysis products.Based on the research of oil shale in-situ exploitation technology,combined with the characteristics of horizontal well perforation completion hydraulic fracturing,such as large discharge area and high single well production,this paper studies the initiation and expansion mechanism of hydraulic fracturing in horizontal well perforation completion in oil shale formation by means of theoretical analysis,numerical simulation and fracturing test.Starting from the commonly used rock failure criteria,this paper analyzes the stress state of the perforation wall,combines the physical and mechanical parameters of the oil shale and the perforation parameters,and calculates the critical fluid pressure corresponding to different failure criteria at each perforation section.The"Buckets Effect"was used to determine the initiation pressure and location of hydraulic fractures.The comparative analysis of theoretical calculations and fracturing test results proves that the initiation position of hydraulic fracturing fractures in oil shale horizontal wells is located at the intersection of the horizontal well bore and perforations;When horizontal wells are completed with open holes or perforated along the direction of maximum horizontal in-situ stress,the fracture pressure calculated by the tensile failure criterion should be used.When the perforation direction is the maximum in-situ stress direction,the rupture pressure calculated by the Mohr-Coulomb criterion is basically consistent with the experimental value.The Drucker-Prager criterion is close to the experimental value when the hydraulic fracturing is performed with high-viscosity fracturing fluid.Based on the CZM and the self-developed grid node separation program,a hydraulic fracturing model of oil shale formation coupled stress-seepage-damage field was established.The effects of perforation parameters and fracturing fluid parameters on the initiation and propagation behavior of hydraulic fractures were discussed.The simulation results show that high-phase or low density perforation should adopt high flow rate and high-viscosity fracturing fluid pumping method,which can significantly improve the hydraulic fracturing effect.It is advisable to use high flow rate and low viscosity for hydraulic fracturing in low phase perforation,and low flow rate and high viscosity in high density perforation.When the initial perforation direction is the maximum in-situ stress direction,it can effectively reduce the fracture pressure and curvature of the fracture,and promote the increase of the cumulative fracture length(C_l)and maximum fracture width(M_w).As the perforation length increases,the M_wincreases,and the fracture pressure and C_l of the reservoir decrease.In order to study the influence of structural fractures on the initiation and propagation of hydraulic fractures in oil shale formation in fault structural region,a hydraulic fracturing model of dual media coupled with stress-seepage-damage field in oil shale formation with structural fractures was established by using a self-developed random structural plane generation program.At the same time,the effects of flow rate,initial perforation direction and perforation length were studied by orthogonal test.The orthogonal test results show that the flow rate of fracturing fluid has a significant impact on the C_l and M_w;the initial perforation direction has a certain impact on the C_l,but has no significant impact on the M_w;the perforation length has no significant impact on the C_l and M_w.The single factor analysis of fracturing fluid flow rate shows that the filtration effect of rock matrix and structural plane limits the expansion of hydraulic fractures when the flow rate of fracturing fluid is 0.0005m~3/s;With the increase of the flow rate of fracturing fluid,the effect of hydraulic fracturing was significantly improved.When the fracturing fluid flow rate is 0.01m~3/s,it promotes the generation and expansion of hydraulic fractures,bedding fractures and branch fractures,and forms a complex fracture network.As the flow rate increased to 1 m~3/s,high density radial fractures are generated near the perforation,which seriously limits the expansion of hydraulic fractures.In addition,by analyzing the C_l and M_w of hydraulic fractures at different flow rate rates,we found that when the fracturing fluid flow rate exceeds0.008m~3/s,the C_l and M_w almost no longer increase with the increase of fracturing fluid flow rate.When there is an angle between the initial perforation direction and the maximum in-situ stress direction,the hydraulic fracture first expands along the perforation direction,and then turns to the maximum in-situ stress direction,resulting in the increase of the curvature and the flow resistance in the fractures.For the simulated reservoir,a flow rate of 0.008 m3/s and perforation direction consistent with the maximum principal stress are the optimal fracturing parameters.Finally,the real triaxial hydraulic fracturing simulation system was used to carry out the hydraulic fracturing test of perforated completion in oil shale horizontal wells,and the effects of different perforation parameters and fracturing fluid parameters are studied in detail.The results show that the existence of perforation can effectively reduce the fracture pressure of oil shale sample,promote the opening and activation of bedding plane,and enhance the connectivity of fracture network in the sample.The increase of the angle between the initial perforation direction and the maximum in-situ stress direction forces the hydraulic fracture tip to turn under the action of in-situ stress,resulting in distorted hydraulic fracture with rough surface,and the fracture pressure of oil shale sample increases significantly.The decrease of perforation length will lead to the increase of fracture pressure of oil shale samples.The change of perforation diameter has little effect on the fracture pressure and fracture morphology of oil shale samples.In addition,the increase of flow rate promotes the generation and expansion of hydraulic fractures,branching fractures and bedding fractures,and the intersecting of these fractures improves the complexity of fracture network.With the increase of fracturing fluid viscosity,the fracture pressure increases sharply,and the hydraulic fracture passes through without causing the activation of the bedding plane,which greatly reduces the complexity of the fracture.This paper uses a combination of numerical simulation,hydraulic fracturing test and theoretical analysis to study the mechanism of hydraulic fracture initiation and propagation of oil shale horizontal well perforation completion under different fracturing conditions,and establish the corresponding theoretical calculation model of crack initiation pressure and position to provide theoretical and technical support for the efficient development of oil shale resources for our country.