Study On Mechanical Behavior And Fractures Cracking Mechanism In Hydraulic Fracturing Of Coalbed With Cleats

The main technical means for developing coalbed methane is hydraulic fracturing, it canform complex fracture in coalbed, make the pressure drop in a large area around the wellbore,increase the surface area for coalbed methane desorption, and guarantee coalbed methanerelease quickly and relatively permanently. There are lots of weak structure surface in coalbedsuch as cleats and cracks, which have the characters of low strength, low Young's modulus,high Poisson's ratio and frangibility, it make difference in rock mechanics properties betweencoal and rock. Studying fractures cracking mechanism must be basing on mastering coalbedmechanical characteristics.In this paper, firstly the macroscopic and mesoscopic measurement of the coalbed cleatscharacters were conducted, which included cleats density, length and width, and the cleatdevelopment status in the test area was evaluated.Secondly, the indoor experiments for measuring the coalbed mechanics characteristicswere carried out and the measured parameters included tensile and shear strength, uniaxialand triaxial compressive strength and Young's modulus, Poisson's ratio in different directionof coring. It can be concluded from the experiments that the coalbed mechanics parameterswere difference in different coring direction, existing obvious anisotropic characteristics.The equivalent Young's modulus, Poisson's ratio calculating model of the coalbed wasestablished by elastic mechanics theory. The relationship between equivalent Young'smodulus, Poisson's ratio and coalbed matrix, face cleats, butt cleats mechanical parameterswas calculated by using the model. When the coring direction is vertical to the face cleats, theYoung's modulus is depending on coalbed matrix Young's modulus, the normal stiffness offace cleats and the size of the spacing. And the Poisson's ratio is only related to coalbedmatrix Young's modulus, Poisson's ratio, the normal stiffness of face cleats and the size of thespacing. When the coring direction is parallel to the face cleats, the Young's modulus isdepending on coalbed matrix Young's modulus, the normal stiffness of butt cleats and the sizeof the spacing. And the Poisson's ratio is determined by coalbed matrix Young's modulus,Poisson's ratio, the normal stiffness of butt cleats and the size of the spacing.The borehole wall surrounding rock stress distribution calculating model for orthotropiccoalbed was established considering the unique mechanical properties of coalbed. Thefractures cracking pressure calculating formula in fracturing for coalbed open holecompletion was deduced basing on the maximum tensile stress failure criterion. By studying, open hole completion borehole wall circumferential stress was closely related to the coalbedcleats mechanics characteristics. The stress distribution status was affected by the anglebetween the face cleats and horizontal maximum principal stress, face cleats distributiondensity, butt cleats distribution density and the connected rate of butt cleats. When thedirection of face cleats was the same as maximum principal stress direction, the fractureswere easily to crack. With the increasing of face cleats spacing, the fractures crackingpressure needed for open hole completion was increasing.Finally, the fractures cracking pressure calculating model for perforation completion inorthotropic coalbed was simulated by finite element method with different perforationparameters, ground stress status, and coalbed mechanics parameters, and the effects ofperforation azimuth, radius and depth on cracking pressure was obtained. The effects ofprincipal stress variation in different direction on perforation completion cracking pressurewere analyzed. And the effects of coalbed matrix Young's modulus, Poisson's ratio, the anglebetween face cleats and horizontal maximum principal stress and face cleats spacing onfractures cracking pressure were discussed.The coalbed is discontinuous medium, which has the weak surface with cleats, and it isequivalent to continuum when studying in the paper. The mechanics parameters variationregulations of coalbed equivalent continuum were described by theoretical model, and onwhich the fractures cracking pressure calculating model in fracturing was established. Theachievements of the paper provide new thought and direction for studying hydraulicfracturing mechanism under coalbed complicated cleats system.