Study On The Mechanism Of Increasing Shale Gas Seepage By Hydraulic Fracturing

Shale gas is a new energy, highly valued in China. In the12th Five Year Planperiod, there are a number of policies specially introduced to encourage thedevelopment of shale gas, and series of key technology research projects launched.Development and utilization of shale gas is steadily advancing. The paper used SichuanBasin Longmaxi Formation shale as research object, which was studied the micro pore-crack structure, adsorption, desorption of methane, seepage characteristics, andmechanical properties of shale, and rupture characteristics simulation of hydraulicfracturing of sandstone. The main results obtained are as followed:There was a lot of pore in shale, and aperture includes four types, ranging ofmacropore, mesopore, ostiole, and micropore. The macropore and mesopore was mainseepage channel, also the ostiole and micropore was storage space in shale; themacroporous accounted for most of pore volume, but specific surface area of themicropore was the largest.The adsorption characteristics of methane on shale could be described byLangumuir equation and D-R equation. The adsorption capacity of methane on shalereduced with temperature increasing, which means parameters a, b of Langmuir modeland Qn of D-R model would decreases gradually, and the temperature sensitivitycoefficient CTdecreased at the same time.The experimental study of methane in shale’s desorption characteistics showed that,as the temperature increased, shale gas eventual desorption amount increased, so it wasbetter for shale gas desorbed from shale. Desorption characteristics can be described bythe empirical formula, diffusion model and seepage model. With the temperatureincreasing, parameter α of the empirical formula and parameter Qd∞of diffusion andseepage model increased.The seepage characteristics of methane in shale were closely related to the stressfield, temperature field, and pore pressure field. The experimental results showed that:the permeability decreased with axial effective stress increasing, when shale stress wasin the initial strain compression phase and elastic stage; there was a small amplitudeincreasing trend near the yield strength; after slit and crack growing and developing, thespecimen got failured, the permeability increased rapidly. The shale’s permeability wasnegative exponential relation with effective ceonfining pressure. Experimental study on mechanical property of shale showed that: with theincreasing of confining pressure, shale tri-axial compressive strength increased, and thedeformation characteristics transformed from fragility to strain hardening, and shaledamaged in the form of shear failure. The shale strongly influenced by water softeningin hydraulic fracturing: higher the water pressure, lowwer the shale’s strength andelastic modulus, but poisson ratio remianing.The acoustic emission monitoring experiment of sandstone fracture character-isticsin hydraulic fracturing showed that primary slits of sandstone was opened first, thennew fractures produced in the whole process of hydraulic fracturing. The crack extenedwith the increasing of water pressure, and ultimately formed the macro fracture, fromwhrere water could flow out, and the crack propagation direction and the minimumprincipal stress σ3were set perpendicular to one another. The rock acoustic emissionsignal characteristics of crack propagation was obvious, because the hit number,amplitude, count, and energy increased in short time, and the damage of rock wasaround the specimen fracture section, or the bottom.