Study On The Law Of Pressure Decline After Fractured Reservoir Lamination

Currently, most of onshore oilfield in China has gradually at middle or past development period, a variety of oilfield development issues become increasingly prominent. Nowadays, horizontal drilling and hydraulic fracturing have become two key techniques to exploit unconventional reservoirs, such as tight reservoirs, thin reservoirs, fractured low-permeability reservoirs, and so on. However, the existing models for fracture pressure decline analysis are mainly for a single hydraulic fracture in vertical well, there is not a theoretical model be aimed at horizontal wells which have several hydraulic fractures due to the difference in characteristic of fracturing fluid flow between them. Thus, the previous related technologies cannot be used directly in horizontal well on fractured low-permeability reservoirs. Therefore, based on sufficient investigation literature relevant literature at home and abroad, the paper mainly researches on the rule of pressure decline and principal influence factors during shut in period and fracturing fluid flowback period. The main contents are as follows:(1) According to the geologic characteristic of fractured low-permeability reservoir, Warren-Root dual-porosity model is applied to establish fracturing fluid leak-off model during shut in period, meanwhile, the stress sensitive effect of fractured low-permeability is taken into account as well. As a result, the main factors influence the fracturing fluid leak-off are analyzed. Meanwhile, the leak-off velocity is calculated under the condition of different stress sensitive coefficient separately, the stress sensitive coefficient is 0,0.02,0.04 and 0.06.(2) The calculation model of change of cracks volume during shut in period is established in this paper, then, based on the material balance equation of fluid in the several hydraulic fractures after pumping, the main factors influenced pressure decline are obtained.(3) A single phase fracturing fluid trilinear flow model is developed. In this model, considering fracturing fluid in the matrix can feed hydraulic fractures directly, rather than fracturing fluid merely depletes from matrix to micro-fractures, and from micro-fracture to hydraulic fracture. The equations are solved using Laplace transforms, Type curves are generated by numerically inverting the resulting Laplace-space to time-space using Gaver-Stehfest algorithm.(4) In fact during flowback operations in fractured low-permeability reservoir, hydraulic fractures are filled with fracturing fluid and hydrocarbon, thus a transient two-phase depletion flow model is established. In the model, on the one hand, equivalent continuous medium model is applied to considering influences of micro-fractures, on the other hand, an explicit dynamic-relative-permeability function of time for the hydrocarbon-phase to account for fracturing fluid drop in hydraulic fractures is adopted. Obviously, connected coefficient is closer to 1, the stronger anisotropy of the reservoir. Meanwhile, natural micro fractures aperture exceed 25 micrometer, the connected coefficient becomes the main factor influence the anisotropy. Consequently, the decline rule of bottom hole pressure can be obtained.In this paper, a thorough research of pressure decline after stimulation in fractured low-permeability reservoir is carried out. Not only considering the influence on fracturing fluid filtration of stress sensitive effect, but also at the aspect of research on pressure decline during a transient two-phase depletion process has certain innovation. Therefore, the research of this dissertation has important significance and reference value to the development of horizontal well fracturing theory and guide the field construction work.