Research On The Mechanism Of Carbon Dioxide Fracturing In Coalbeds And Displacing Coalbed Methane

The low permeability of coalbeds seriously restrains the result of coalbed methane extraction in China.Enchancing the permeability of low permeability coalbeds is the key factor to enchance the efficiency of coalbed methane extraction.Adopting carbon dioxide?CO₂?as fracturing fluid for coalbeds not only can enhance the permeability of coalbeds,but also can displace coalbed methane and enhance methane extraction.In this thesis,theoretical analysis,laboratory experiment,numerical simulation and field application were combined to study the mechanism of CO₂ fracturing and the law of CO₂ displacing methane in coalbeds systematically.The effect of CO₂ property on breakdown pressure was studied by theoretical analysis.The self-designed multi-functional true triaxial geophysical?TTG?apparatus was employed to perform hydrofracturing treatment experiments on coal,shale and sandstone specimens.The law of fracture propagation and the characteristic of hydraulic fractures were investigated.An method was proposed to evaluate the enchancment result of rock permeability stimulated by hydrofracturing treatment under the condition of unloading.This method was adopted to study the result of CO₂ fracturing in coalbeds.Considering the seepage law of fractured coal,the suitable hydrofracturing treatment for coalbeds was discussed.A fluid-soild coupling multi-component gas and multi-porous model was established.The model was applied to simulate CO₂ displacing coalbed methane.At last,the gas-liquid two-phase combined fracturing technology was used to fracture coalbeds in field coal mine.The research results are shown as follows:?1?In hydrofracturing treatment,the initiation pressure of rocks was affected by initial pore pressure and the permeability of the rocks.The extension pressure of fractures was affected by natural fractures and fluid lag zone.Because of the viscosity and permeability difference between CO₂ and water,the effect of rock permeability,natural fractures and fluid lag zone on CO₂ fracturing and hydraulic fracturing were different.It resulted in that the breakdown pressure of CO₂ fracturing is lower than that of hydraulic fracturing.?2?Hydrofracturing treatment experiments were performed under the true triaxial condition.It was found that CO₂ fracturing can induce more secondary fractures,reactivate more natural fractures and forme more complexed fracture network than hydraulic fracturing.In CO₂ fracturing,The effect of horizontal stress on fracture propagation would decline.The effect of natural fractures on fracture propagation would enhance.?3?Through micscopic scanning,it was observed that hydraulic fracturing tended to induce tensile fractures perpendicularly to minimum horizontal stress.However,CO₂fracturing tended to induce slant fractures including wing cracks.The low viscosity property of CO₂ was beneficial for hydrofracturing treatment inducing shear fractures.?4?An method was proposed to evaluate the enchancment result of rock permeability which was stimulated by hydrofracturing treatment.The results of permeability enchanment in different reservoirs were studied through this method.The effect of reservoir property,fracturing fluid and geo-stress on the permeability enchanment were analyzed.The study results indicated that the permeability enchanment of hydrofracturing treatment on coal specimens were ineffective;the permeability enchanment of CO₂ fracturing was effective;the permeability enchanment of hydrofracturing treatment under a condition of high geo-stress was ineffective.?5?The seapage experiments were performed on fractured rocks under the true triaxial condition.It was found that the hydraulic fractures in coal specimens were sensitive to stress.It indicated that the self-supporting property of hydraulic fractures in coal specimens were ineffective.When fluid pressure was unloaded,the increase of effective stress might lead to the sharp decline of fractured coal permeability that made against hydrofracturing treatment.Therefore,the shear stimulation was proposed to perform hydrofracturing treatment that induced well self-supporting shear fractures in coalbeds.The experiments indicated that the shear stimulation could make significant increase of coal permeability.?6?A fluid-soild coupling multi-component gas and multi-porous model was established.The model was applied to simulate CO₂ displacing methane during the process of CO₂ fracturing in coalbeds.The simulation results indicated that CO₂displacing coalbed methane would form methane concentration decline area,methane concentration enhancement area and initial methane concentration area.The methane enhancement circle was affected by displacement pressure,fracture permeability and other parameters.?7?Combining the advantage of hydraulic fracturing and liquid CO₂ phase transitions fracturing,the gas-liquid two-phase combined fracturing technology was proposed.This technology was adopted to fracture coalbeds and enhance permeability in field coal mine.The field investigation indicated that the gas-liquid two-phase combined fracturing technology could improve coalbed permeability and enhance methane extraction.