When cold fluid is injected into low-temperature, low-pressure, low-permeability reservoirs containing wax-bearing heavy oil, cryogenic paraffin deposition and heavy oil condensation will occur, thus damaging the formation. Moreover, the formation pressure coefficient is low and the working fluid flowback efficiency is low, which affects the fracturing stimulation effect. Therefore, an in situ heat/gas clean foam fracturing fluid system is proposed. This system can ensure that conventional fracturing fluid can create fractures and carry proppant in the reservoir, generate heat in situ to avoid cold damage, reduce the viscosity, and improve the fluidity of crude oil. The in situ heat fracturing fluid generates a large amount of inert gas while generating heat, thus forming foam-like fracturing fluid, reducing fluid loss, improving proppant-carrying performance, improving gel-breaking performance, effectively improving crack conductivity, and is clean and environmentally friendly. Based on the improved existing fracturing fluid system, in this paper, a new type of in situ heat fracturing fluid system is proposed, and a system optimization evaluation is conducted through laboratory experiments according to the performance evaluation standard of water-based fracturing fluid. Compared with the traditional in situ heat fracturing fluid system, the fracturing fluid system proposed in this study generates a large amount of inert gas and form foam-like fracturing fluid, reduces fluid loss, enhances the proppant-carrying capacity and gel-breaking performance, improves crack conductivity, the gel without residue and that the gel-breaking liquid is clean and harmless.
Based on the analysis of the typical Ordos well groups, this study began with the accurate characterization of the fracture geometry by adopting advanced laboratory experiment methods and monitoring techniques. Then, with the integration of fracture geometry characterization and in situ stress distributions, fracture optimizations of the target wells were performed through numerical simulations methods. Finally, this study established a sweet spot prediction and identification method for long horizontal shale oil wells and constructed a set of optimization design methods for multistage hydraulic fracturing. This investigation revealed that the hydraulic fractures in Chang-7 terrestrial shale oil reservoirs exhibited the belt pattern, and the primary fractures generated the secondary fractures, which activated the natural fractures and induced shear failure. Macroscopic fractures were found to be perpendicular to the direction of the minimum principal stress. Secondary fractures and activated natural fractures were distributed around the primary fracture in the form of fracture types I and II. Multicluster perforation optimization techniques, which were based on shale reservoir classification and evaluation, and aimed at activating multiclusters and determining fracture sweet spots, were developed. These were successfully applied to the field operation and achieved production enhancement performance.
For tight oil horizontal completion, optimization of reservoir modeling and fracture design relies highly on correct understanding of the relationship between production and reservoir characteristics. Due to reservoir heterogeneous properties, it is also challenging to understand the relevant correspondence between the production and petrophysical and petromechanical stage characteristics. Understanding stage contributions during initial and short to medium term production is the first and important step to reveal such a relationship.
This article describes an effort in using chemical tracers to evaluate initial oil and water productions in a horizontal multi-stage fracture well. Determining an exact contribution per stage of oil and water in initial production is achieved here for the first time. This quantitative understanding of how each stage has contributed to the comingle production in initial production provides increased understanding of the correlation between stage production and reservoir characteristics.
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