Numerical Investigation On Hydraulic Fracture Initiation And Propagation Behavior In Horizontal Wells Of Shale Reservoir

Affected by the property of low permeability of shale reservoir, it is essential to adopt hydraulic fracturing measures before shale gas (oil) production. Due to obvious superiorities like sufficient contact with the reservoir, reduce costs, and long-term and stable yield ability, horizontal wells, especially when coupled with multi-stage fracturing with gliding water, are more prone to substantially increase the gas production and have been widely used in production in shale reservoirs. Since the vast majority of fracturing operations are conducted after the casing perforation and the hydraulic fracture initiation is closely correlated with the characteristics of shale rock mechanics and the heterogeneity of shale reservoir, the actual fracture initiation and propagation are complicated and vary in different forms. In addition, the shale reservoirs always born with large quantities of natural fractures and beddings, which usually lead to large-scale fracture nets when subject to hydraulic fracturing, and in this way the technology of the so called Stimulated Reservoir Volume take on effect.Supported by China Petroleum Science and Technology Innovation Fund (2013D-5006-0211), the National Natural Science Foundation of China (51479024), and National Program on Key Basic Research Project of China (973 program) (2014CB047103), and with the understanding of previous research about the mechanism of hydraulic fracturing in rock mass, this paper is set off on the basis of the coupling flow-stress-damage model in the process of hydraulic fracturing and make it in application in the FEM software RFPA seepage version. Given the difficulty of lab experiments about the shale cores, the paper deals with some numerical experiments based on the limited laboratory results and some related mechanical parameters for subsequent chapters are obtained. Then numerical simulations are applied to carry out the research on fracture initiation and propagation characteristics in hydraulic fracturing process of the horizontal wells in the shale reservoir.First of all, the initiation characteristics of hydraulic fracture in horizontal wells after directional perforation is studied in the numerical simulation, and the influence of two corresponding factors of geostress and perforation azimuth on the fracture initiation is analyzed in detail. Result shows that there exists an initiation mode that hydraulic fracture doesn’t arise from the perforation end, and on this basis further researches are conducted to explore the possibility of double-fracture initiation mode and the related critical conditions. In addition, hydraulic fracture behavior when extending near interfaces between the reservoir and interlayer is also studied by means of numerical simulation. The results may further enrich study and research about perforation parameters optimization of horizontal wells and hydraulic fracturing operation.Moreover, in view of the existence of quantities of natural fractures in the shale reservoir, numerical models are established to study the interaction of hydraulic fractures and natural fractures and the rule to form a fracture net in shale reservoir with natural beddings. With the help of a parameter called "horizontal stress difference coefficient", preliminary analysis of the effect of Stimulated Reservoir Volume fracture nets is achieved, and then influence of geostress, brittleness index, bedding characteristics on the fracture network complexity is analyzed separately, with the hope to provide some reference to the design and study of fracture network characteristic in shale reservoir.Finally, according to the in-situ geological condition of a certain reservoir, numerical model is set up to obtain the stress distribution phenomenon of stress interference in around a single horizontal well and with different fracture spacing. As a result, an optimized value about fracture spacing is obtained. Further, the value of fracture spacing in two synchronous fracturing wells is also numerically selected and two fracturing schemes are analyzed and evaluated from a comprehensive view.