Research On Stress Disturbance Effect Of Radial Lateral Pulsating Hydraulic Fracturing In Coal Seams

Low production and poor hydraulic fracturing effect for a single well are the main problems on coalbed methane(CBM)development in China.Thus a new stimulation technology is proposed to enhance the CBM single well production,namely the “radial lateral pulsating hydraulic fracturing(RL-DHF)”.In this technology,pulsating hydraulic fracturing is applied on the basis of several radial laterals drilled in the coal seams.This paper is mainly focus on the stress disturbance and damage caused by the RL-DHF in coal seams.Therefore,a static and dynamic stress response numerical model was established,an evaluation methodology on stress disturbance effect was proposed,the influences of technical parameters and geomechanics parameters on stress disturbance effect were studied,an experimental research on RL-DHF fracturing effect was conducted,and finally the optimum selecting principles on technical and geomechanics parameters were achieved.The detailed results are as follows:1.Based on the continuum mechanics,rock pulsating mechanics and fracture linear sliding theory,a static and pulsating stress response numerical model was established.This model applied a high-order,staggered-grid,finite-difference scheme to investigate the stress response of coal seams caused by the RL-DHF.In details,the first order velocity-stress wave equations in fractured coal seams were derived.The implementation of in-situ stress condition made use of preloading method,and the implementation of infinite formation made use of the perfectly matched layer technique.The accuracy of the model was verified for the static case and the pulsating case.The concepts of “effective stress disturbance zones” and “the percentage of effective stress disturbance nodes” were proposed to describe the stress disturbance effect.With this numerical model,the influences of a series of technical and geomechanics parameters on stress disturbance caused by RL-DHF can be studied.2.Based on the numerical model,the influences of technical parameters on stress disturbance were studied.Generally,the length and branch of radial laterals,the frequency and amplitude of pulsating hydraulic fracturing are all positively correlated with stress disturbance effect.But above technical parameters show some unique stress behaviors with RL-DHF: First,there is a critical value of the length of radial laterals(10~15 m).When the length is beyond the critical value,the effective stress disturbance zone will increase rapidly.Second,with the low amplitude(5 MPa),the three branch radial lateral generates the best stress disturbance effect,because it can eliminate the negative influence of the stress shadow.Third,limited by the amplitude,there is a threshold(40 Hz)on frequency.When the frequency is beyond the threshold,the stress disturbance effect will not be increased significantly.The impact strength order of above parameters is: amplitude > branch > length > frequency.Thus during the technical construction design,the amplitude should be firstly enhanced as far as possible,then the branch and length of radial laterals should be increased as much as possible,but the frequency is not required to be too high.3.Based on the numerical model,the influences of geomechanics parameters on stress disturbance were studied.First,low Young's modulus and high Possion's ratio are helpful to enhance the intensity of shear stress around the wellbore toe,which will increase the stress disturbance effect.Through the above characteristics,RL-DHF can produce a large range of permeability-enhanced area in the coal seams of high compressibility.Second,high in-situ horizontal stress can decrease the magnitude of the stress disturbance and thus weaken the stress disturbance effect,in which the minimum in-situ horizontal stress plays a dominant role.Third,increasing crack density will hinder the propagation of stress wave,but its influence can be ignored compared with other geomechanics parameters.The impact strength order of above parameters is: minimum in-situ horizontal stress > Young's modulus > Poisson's ratio > maximum in-situ horizontal stress > crack density.Thus during choosing the coal seams conditions,a lower minimum in-situ horizontal stress should be firstly taken into consideration,then a coal seam with lower Young's modulus and higher Possion's ratio should be chosen,and the maximum in-situ horizontal stress and crack density can be taken into account as the relatively secondary factors.4.Based on the pulse servo fatigue testing machine and acoustic emission detector,experimental researches on RL-DHF using coal briquettes were conducted.The following conclusions were drawn under the condition of this experiment.First,larger length of radial laterals and higher frequency can generate stronger acoustic emission signals and a better fracturing effect.Second,the optimal branch number of radial laterals is three.Third,lower amplitude is helpful to develop micro-fractures,and higher amplitude is helpful to make the main artificial fracture propagate quickly.Finally,compared with radial lateral quasi-static hydraulic fracturing(RL-QHF),RL-DHF can produce more acoustic emission signals with lower peak pressure.Thus under same operating conditions,RL-DHF can produce a larger range of stimulated volume and better fracturing effect than RL-QHF.The models and results presented in this paper can provide theoretical guidelines for the stimulation mechanism study,technical parameters and geomechanicas parameters optimization on RL-DHF.