Study On The Laws Of Hydrofractured Cracks Propagation Of Tight Sanstone Gas Reservoirs In Jiannan Area

Under the background that the natural gas supply of China becomes increasingly serious day by day, the tight sandstone gas gradually becomes the highlight in the regions of unconventional gas exploitations due to its widely regional distribution, enormous resources reserve and ripe technologies of exploitation and prouction. Therefore, to achieve the fractures propagation principles of the tight sandstone reservoirs and to indicate the initiation mechanism of the hydrofractures, are very significant to improve the exploration of the tight sandstone gas reservoirs as well as to realize the huge quantitative of commercial exploitation. By adopting systematical studies of mechanical tests, laboratorial physical simulation testing of hydrofracture, theoretical analysis and numerical simulation, the cracks propation principles in tight sandstone reservoirs and the initiation mechanism of hydrofractures are to be carried out in this thesis. The main contents and conclusions of the thesis are stated as follows:(1) The mineral component, pore structure characteristics and permeability characteristics of tight sandstone were analyzed based on X-ray diffraction, scanning electron microscope and gas-testing method. The results show that the main components of the tight sandstone are granite, solium feldspar, potash feldspar and clay minerals, and the content of quartz and clay mineral vary widely. The tight sandstone is homogeneous, integrated and micro-fractures are not well developed. The average porosity of the tight sandstone is about 2.59%, and the average permeability is about 0.07 mD, mainly belonging to low porosity and low extremely permeability reservoirs, and a few extremely low porosity and low permeability reservoirs.(2) The fracture characteristics and mechanical properties of tight sandstone were analyzed based on uniaxial and triaxial compression tests, Brazilian tests and three-point bending tests of specimens with different formations. The effect of water on the mechanical properties of tight sandstone was also preliminary studied. The results demonstrate that burial depth has a significant effect on strength and modulus of elasticity of vertical tight sandstone core samples obtained from same layer, but a smaller effect on the horizontal coring specimen. Along with the increasing of water saturation content, both the tensile strength and the fracture toughness tend to reduce very fast firstly and then reduce slowly. Under uniaxial compression testing, the tight sandstone samples are mainly splitting fracture, single bevel shear or conjugate shear rupture and mixed fracture. Under triaxial compression testing, the tight sandstone samples behave shear fracture.(3) The hydraulic fracture initiation and propagation in tight sandstone were analyzed based on indoor large-scale hydraulic fracturing simulation experiments. The forming mechanism of fracture was revealed, and a preliminary study on the effect of multiple factors on fracture morphology was conducted. The results show that the initiation mode of the cracks in tight sandstone consists mainly of going along intact rock mass, along weak structural planes, along the micro-cracks of joints and along natural cracks. Hydrofracture mainly induced single and one-wing crack, and a few crossed cracks. When cracks initiate along intact rock mass, it is prone to appear single and one-wing curve-shape crack; when initiating along weak planes or micro-cracks of joints, the cracks tend to be singe and one-winged plannar crack. When the hydrofractured cracks communicate with weak planes, ‘L' or ‘T' shape crossed cracks are prone to be induced. The petrophysical of the rock mass are the main factors to influence the crack initiation and propagation, the output volume only influence the initiation pressure, and the fracturing fluid only influence the amount of leakage loss.(4) The calculation model of induced stress field in subsection fracturing of horizontal wells is established, then obtain the influence law of fracture length and spacing segments on the induced stress field, The fracture length and spacing segments in horizontal well are optimized. The results show that the induced stress of the minimum in situ stress decreases approximate linearly with the increment of the distance to the well wall and the induced stress of the minimum in situ stress decreases approximate nonlinear with the increment of the distance to the hydrofractured crack. With different crack length, along with the increasing of hydrofractured cracks, the influence region of the crack in enlarged, the difference of induced stress gradually decreases, and the difference of induced stress away one length of the crack from the crack can be ignored. With different segmental intervals, along with the increasing of segmental intervals, the difference of induced stress rapidly decreases. When the segmental interval is between 50 and 100 m, complex crack-net is applicable to be formed.