| Shale gas reservoirs are most important components of unconventional natural gas resources. According to the advantages of worldwide distribution and long-term available and steady gas production, the study on shale gas is a current spirited topic in the field of new energy. Since the super-low permeability challenges for source rock, the economic viability of shale gas almost totally hinges in effective large volume and high rate hydraulic fracture treatments, therefore, the fracturing design is necessary because of the complex shale formation environments and high fracturing investment.Considering the shale gas formation special characteristics and multi-stage horizontal fracturing features, the fracturing design need to tread the single fracture system created by per stage as a whole system, which is quite different with classical elliptical shape assumption in conventional models, therefore, there’re no available theory can solve this problem at this moment. This paper applied formation geology, reservoir geomechanics, theory of elasticity, fluid mechanics and computational mathematics to study the fracture propagation in per stage during horizontal fracturing. Two types of fracturing model were established due to fracture complexity levels.Borrowing the classical fracturing theory, two fracturing models were presented to study multiple transverse fractures cluster and complex fracture network simulation, which can resolve the fracturing design for each stage during horizontal fracturing and enhance the theory level and application value. Furthermore, the result can help the fracturing design becomes more effective and provide technology support to increase the final production.Based on previous achievements in this research, this paper studied five problems in the field of horizontal multi-stage fracturing.(1) The application of QEMSACN,XRD,SEM core lab technologies, and triaxial tests were conducted on the selected Yanchang lacustrine shale samples and Yangtze marine shale samples, and the formation and stimulation differences were also studied. The results illustrate the marine shale sample is with high TOC and maturity, and the mineral composition is rich in silica materials, which implies good fracability. As for the lab study for lacustrine samples, the TOC value is high for lacustrine shale but maturity is relatively low, so the coexist of oil and gas potential is possible. Owing to the high clay content for most shale formation, the lacustrine shale gas resource only express good brittleness in certain formations. The uniaxial result shows the brittleness of shale sample based on stress vs. strain curve. The results of multi circle triaxial test show that both the value of Young’s modulus and Poisson’s ratio increase with the increase of confining pressure. But when the confining pressure is constant, the Poisson’s ratio in the unloading condition is greater than that in its loading condition and Young’s modulus in the loading condition is greater than that in its unloading condition. Some shale samples with natural fractures behave different stress responses. The shale sample with high brittleness index tends to crack as fracture network according to the multi-stage triaxial compression test.(2) After the investigation of the unique formation geomechanical characteristics, and fracturing treatments, the critical factors controlling the creation of complex fracture network were concluded. An integrated brittleness index was introduced to evaluate the formation fracbility due to uncertainties by traditional formation brittleness index calculation. A fracability model was established to evaluate fracability of shale formation based on the integrated brittleness index, Young’s modulus and I and II fracture toughness through analytical hierarchy process.The combination of lab study and logging data, a screening criterion of perforation points was presented based on calculated integrated fracbility index,closure stress and TOC value,and the application of this method was used on certain shale gas well in Yanchang formation. This screening criterion can effectively evaluate the formation fracbility and provide more exact perforation point, thereby selecting reasonable fracturing techniques and optimizing the fracturing design.(3) This paper mainly studied the single fracture system created per stage in horizontal multi-stage fracturing in shale formation. As for ductile shale, the fracture network is hardly achieved so the horizontal wellbore direction is typically perpendicular to the maximum horizontal stress, and each perforation can create one transverse fracture, so the transverse fractures cluster geometry is established. Borrowing the PKN model theory and rate distribution methodology in multilayer fracturing, the mathematic model for transverse fractures cluster propagation simulation were studied while perforation friction pressure, pipe friction pressure and stress interaction were considered in this model. As for brittle shale, owing to the creation of complex fracture network in per stage, the elliptical shape of stimulated reservoir volume is hypothesized. There assumed one primary fracture parallel to maximum horizontal stress and two sets of secondary fractures at the same plane and the orthogonal plane. Based on semi-like method, the width ratio and aspect ratio can be used to define the relationship of primary fracture and secondary fractures system. Moreover, the P3 D fracturing model and relating elliptical formula were used to simulate the propagation of complex fracture network.(4) A fracturing software was programmed by VB 6.0 and this software manipulate friendly and compute rapidly, especially for multi-stage horizontal fracturing treatments in shale gas formation with different pump rates. The parameters influencing the transverse fracture cluster and complex fracture network propagation were studied through programmed modules. The results can be concluded as: firstly, duiring the transverse fracture cluster propagation, the stress interference between multiple fractures can be observed universally and the increase the perforation numbers in one stage can result in the decrease of fracture width for all the fractures; secondly, the influence of stress interaction on the exterior fractures in this model is more evident than interior fractures. The other controlling factors, like pump rate, fracturing fluid viscosity and formation geomechanical properties, have similar influence degree for exterior and interior fractures but only the influences on the created fracture cluster. As for complex fracture network, the increase of Young’s modulus, the decrease of horizontal stress difference and fluid viscosity can result in larger stimulated reservoir volume,and the stress difference is the most sensitive factor of controlling the reservoir stimulated volume. Furthermore, the sparse distribution of natural fractures means larger stimulated reservoir volume, but the effectiveness of stimulation is doubtful because the complex degree of fracture network falls off steeply, so the effectiveness of created fracture network should be estimated by production analysis.(5) Finally, the software was applied to the stimulation in two shale gas wells at Yanchang formation at Ordos Basin and certain marine shale gas reservoir in U.S.A, respectively; Furthermore, the design of transverse fracture cluster and complex fracture network was carried out. The results show that this software can be not only for fracturing design but also useful in fracturing real-time analysis and evaluation with micro seismic monitoring technology. The software choose fracture system rather than one elliptical fracture as fracturing design target in conventional commercial software so this software especially have a good agreement with multi stage and multi cluster horizontal fracturing shale gas well. The achievement can provide powerful support for the optimization of fracturing methods while provide a useful tool to relate fracturing treatments and formation properties to the fracture propagation. |
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