| Unconventional oil and gas reservoirs are dense and low permeable media with a large number of non-uniform multi-scale pores and a large number of randomly distributed fractures.Micro pores and fractures provide the storage location and flow channel for oil and gas resources.Because the permeability of the unconventional oil and gas reservoirs is very low,and the storage of oil and gas is more dispersed.For a long time,the United States,Canada and other countries are using hydraulic fracturing technology to carry out reservoir stimulation,improve the recovery efficiency and production of unconventional oil and gas.However.there are water shortages,environmental pollution and other technical bottlenecks in large-scale hydraulic fracturing.Current research shows that supercritical CO2 fracturing have the advantages of saving water resources,not polluting the natural environment displacing CH4 and thus burying greenhouse gases etc.Supercritical CO2 fracturing is the key to solve the bottleneck problem of shale gas exploitation in China,where there is no perfect hydraulic fracturing system and abundant water resources.Because of the different sedimentary environments in the long-term geological age,the rock of different reservoirs in different geological environments have different pore structures,and the number and distribution of micro-cracks are also different.On the one hand,the nonuniformity of these random pores and cracks determines the anisotropy of reservoir rock;on the other hand,in the process of fracturing,it also leads to the nonuniformity of seepage field and stress field.This nonuniform seepage effect is superimposed on the anisotropy of the physical and mechanical properties of rock,which will have an important impact on the distribution and evolution of cracks induced by fracturing.Do the mechanism of artificially induced fracture propagation and evolution for supercritical CO2 fracturing influenced by unconventional reservoir pore,fracture distribution,in-situ stress and other characteristics have similarities and differences compared with common hydraulic fracturing?The challenges faced by numerical simulation of supercritical CO2 fracturing comparing with common hydraulic fracturing are mainly how to simulate the special physical and mechanical properties of supercritical CO2(such as ultra-low viscosity,high density,ultra-low surface tension,compressibility,etc.),and how to consider the transformation of its state,thermal stress induced by endothermic and exothermic reactions,and the special chemical effects of supercritical CO2 on the induced cracks.This paper attempts to study the above problems by numerical simulation of particle flow discrete element method.Firstly,the computational mechanics techniques involved in the numerical simulation of supercritical CO2 fracturing are elaborated from the point of view of particle flow discrete element method.The intrinsic mechanism of failure of rock media under stress is the generation and propagation of micro-cracks.BPM(Bonded Particle Model)simulates the response of rock media to stress from the point of view of the initiation and development of micro-cracks.This method of mechanical simulation from a fundamental point enables BPM can explore the intrinsic mechanism of fracturing from a deeper level,which is not available in the continuous medium simulation method.At present,the fracturing simulation based on BPM method mainly focuses on the research of common hydraulic fracturing,and the simulation of supercritical CO2 fracturing is still in its infancy.The main difference between common hydraulic fracturing and supercritical CO2 fracturing lies in the special physical and mechanical properties of supercritical CO2 and its special percolation behavior in ultra-low permeability reservoirs.Therefore,how to making up an appropriate set of algorithms with particle flow discrete element method to simulate the special mechanism of supercritical CO2 fracturing is a research focus of this paper.According to the seepage algorithm of BPM(Bonded Particle Model),the constitutive behavior of cohesive particles,the coupling between fracturing fluid and solid particles,etc.Using FISH language and PFC3D program interface,a large number of loading programs are written and embedded into PFC3D,such as seepage simulation,pore structure fitting,fracture generation and distribution uniformity calculation,algorithm of servo-controlled geostress boundary conditions,fluid-solid coupling calculation in fracturing process,induced fracture propagation,etc.The established BPM numerical model can simulate the complex constitutive behavior which is difficult to be described by the continuous constitutive relation formula.It can also simulate the influence of in-situ stress,rock fracture distribution,pore structure,rock macro-mechanical properties,fracturing fluid characteristics(viscosity,density,surface tension,compressibility,etc.)on fracture propagation and evolution.Especially,based on the three-dimensional Delaunay triangulation,the calculation range of fluid is divided into many "domains" and "flow channels".The discrete seepage algorithm can take capillary effect and the relationship between bulk modulus and pressure into account.Because rock is simulated as bonded particles in BPM,the fracture of parallel bond represents the formation of micro-cracks,so the evolution characteristics of micro-cracks can be simulated more effectively.However,there is no algorithm for directly simulating the pore structure of rocks in the BPM.In this paper,a method of calculating pore size in BPM is proposed.Combining with the seepage calculation principle of BPM,the numerical distribution and spatial distribution of pores in actual rock are simulated.According to the mechanical parameters of shale obtained from laboratory tests,a 3D numerical model is established based on a BPM approach utilizing PFC3D.In order to quantitatively analyze the influence of original natural micro-fractures and different pore structure distribution on fracturing network in reservoir rocks,distribution uniformity index(DUI)was proposed to quantitatively characterize the distribution uniformity of fractures.The matching relationship between DUIs of different size range pores and the spatial distribution of pore structure are characterized by the mean value and quadratic mean square deviation of DUIs of pores.On this basis,the effects of pore distribution characteristics,original natural micro-fracture structure and in-situ stress characteristics of reservoir on the spatial lorphology and distribution of fracture induced by supercritical CO2 fracturing,and conventional hydraulic fracturing were studied.Through the above numerical simulation,the mechanism of fracture propagation and evolution induced by fracturing under the combined action of reservoir characteristics(pore structure,fracture distribution.in-situ stress,etc.)and fracturing fluid characteristics(viscosity,density,surface tension,compressibility,etc.)is revealed.The main conclusions are as follows:1.Seen from visual macroscopic fractures,there is no obvious difference between supercritical CO2 fracturing and common hydraulic fracturing when the in-situ stress is very small.BLit the distribution uniformity index(DUI)of supercritical CO2 fracturing is higher than that of common hydraulic fracturing.When the in-situ stress is large,the bif'urcation of induced fracture of supercritical CO2 fracturing is more,and the fracture geometry of supercritical CO2 fracturing is obviously different from that of common hydraulic fracturing.2.the distribution uniformity index(DUI)of induced microcracks in supercritical CO2 fracturing increases with the increase of in-situ stress.But there is no such change trend in common hydraulic fracturing.This indicates that the advantage of supercritical CO2 fracturing is more obvious when the in-situ stress is large.3.When the other conditions are the same,the DUI of the induced cracks of supercritical CO2 fracturing is considerably larger than that of the common hydraulic fracturing and the increase trend is more notably with increases in the DUI of the initial microcracks.A larger DUI indicates that the distribution of cracks is more uniform and the induced cracks are more complex with multiple branches.This result demonstrates that the oil and gas recovery effect of supercritical CO2 fracturing is better than that of hydraulic fracturing under certain conditions.4.The effect of the initial DUI on the DUI of induced cracks is only obvious when the horizontal stress ratio is relatively small.With a higher horizontal stress ratio,the DUI of induced cracks is not sensitive to the initial microcracks' DUI.This shows that supercritical CO2 fracturing can give full play to its advantages only under good in-situ stress conditions(i.e.,horizontal stress ratio is 1.0 or their numerical values are basically equivalent).The productivity of a shale reservoir after a fracturing stimulation is closely related to the distribution of the induced fracture network.The numerical results show that the characteristics of pore structure and the distribution uniformity of natural micro-cracks have important effects on the evolution of induced fractures.Therefore,in addition to the macroscopic physical and mechanical properties of reservior rock,it is necessary and important to consider the effects of pore structure and natural micro-cracks on the propagation and distribution of induced fractures when predicting the characteristics of supercritical CO2 fracturing.It must be pointed out that the numerical model in this paper only deals with the effects of the special physical and mechanical properties of supercritical CO2(such as ultra-low viscosity,high density,ultra-low surface tension,bulk modulus varying with pressure,etc.)on induced cracks.However,the state transition of supercritical CO2 during fracturing.the thermal stress caused by endothermic and exothermic reactions.and the special chemical effects of supercritical CO2 have not been involved.Supercritical CO2 fracturing is a very complicated problem.This paper only involves a small part of it.In the future work,we will further improve the BPM numerical model,comprehensively consider various effects,and more thoroughly understand the supercritical CO2 fracturing mechanism.The main innovations of this study are as follows:l)By embedding discrete seepage algorithm into BPM method,the fracturing model can simulate the ultra-low viscosity,high density,ultra-low surface tension and the variation of bulk modulus with pressure.2)The distribution uniformity index(DUI)is proposed to quantify the degree of fracture distribution uniformity,and the similarities and differences in fracture evolution characteristics of supercritical CO2 fracturing and conventional hydraulic fracturing under the combined action of in-situ stress and original natural micro-fractures with different distribution uniformity index in reservoir rocks is studied.3)Based on the seepage calculation principle of BPM method,an algorithm which can directly simulate the pore structure of rock is proposed.The matching relationship between the uniformity of pore distribution in different size ranges and the quantitative characterization method of the spatial distribution characteristics of pore in rocks are proposed.The number and spatial distribution of real rock pores are simulated.The similarities and differences of fracture propagation characteristics of dense reservoir rocks influenced by pore structure and in-situ stress under supercritical CO2 fracturing and conventional hydraulic fracturing are studied. |
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