| Hydraulic fracturing technique is widely used in oil exploitation to make cracks arise and propagate in reservoir in order to enhance formation permeability, increase production of oil and gas, and extend the life of the cased boreholes. It is always used in the middle and later production of oil and gas fields, and it is an essential engineering procedure for the exploitation of unconventional oil and gas fields, such as shale gas or tight oil fields. Before hydraulic fracturing operations, a cased borehole should be perforated to make a fluid passage, which penetrates the steel casing and the cement sheath, to connect the borehole and reservoir. However, perforating and hydraulic fracturing may cause the damage of the cement sheath and lead to the channeling phenomenon. This phenomenon should be avoided in oil exploration, because the channeling of oil and water will communicate hydrocarbon reservoir, reduce the efficiency of production, and even cause the disuse of the well. Previous efforts to detect channeling were done before perforating and hydraulic fracturing, by using the method of acoustic logging. Although engineers have noticed the influence of hydraulic fracturing on the packer capacity of cased boreholes, detailed researches are very lack for the influence of the shape and size of perforation on the priority of fracturing in formation and damage of the cased well. In this paper, the stress distribution and damage priority are investigated for cased borehole containing perforation and sustaining hydraulic pressure. Different factors are examined for the influence on channeling, including the shape and size of perforation and the parameters of cement and formation. Some tactics to avoid channeling during hydraulic fracturing are given. The main results are as follows:For the case that cement is well bonded, the 3D model of a cased borehole is built using the finite element software ABAQUS. The stress distribution in the vicinity of perforation is calculated and discussed for the cased borehole sustaining hydraulic pressure.Based on the maximum tensile stress strength failure criterion, the critical injection pressure which causes the damage of the formation and cement sheath is calculated. The priority of the damage of formation and cement sheath under water pressure is investigated, and the dependence of the priority on the parameters of cement and formation is discussed. It is found by simulation that the hydraulic packer capacity of cement sheath can be improved by using the cements with low Young’s modulus and high Poisson’s ratio. Besides, channeling is less likely to arise if hydraulic fracturing is carried out in the formation with high Young’s modulus and low Poisson’s ratio. Channeling is affected by the hydrostatic pressure of crustal stress and nearly unaffected by the partial stress.The effect of perforation conditions on the hydraulic packer capacity of cement sheath is investigated. It is found that perforation conditions have great influences on the stress distribution and priority of damage around a cased well sustaining water pressure. Choosing proper shapes for the top of the perforation is important to guarantee the simulation can reflect the real situation. Conical surface is chosen for the top of the perforations in our simulation, and some key parameters of the perforations are investigated, including perforation density, perforation angle, perforation radius, perforation length. It is found that these parameters, except perforation radius, affect only the fracturing pressure and have little effect on the hydraulic packer capacity. Channeling is likely to take place if perforation radius is too small or too big. Therefore, choosing proper perforation radius is important for hydraulic fracturing technique.A strategy to avoid channeling is proposed: during the cementing stage, it is better to use the cement with low Young’s module and high Poisson’s ratio; during hydraulic fracturing stage, the perforation spots should be located in the formation with high Young’s module, low Poisson’s ratio, high horizontal stress, and low vertical stress; proper perforation radius should be chosen to avoid channeling; it is necessary to detect perforation direction, shape, and size before hydraulic fracturing by the method of acoustic logging. |
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