Abstract
In the past few decades, scholars have made great breakthroughs in the study of well test analysis of carbonate rock. The previous studies are based on horizontal wells, straight wells, fractured wells, and inclined wells. With the development of fracturing technology, acid fracturing technology is considered to be the most effective measure to develop carbonate reservoirs. As the carbonate rock is easily dissolved in carbonic acid, multi-branched fractures will be produced near a vertical well. This article presented a semi-analytical model for multi-branched fractures in naturally fractured-vuggy reservoirs for the first time, which laid a theoretical foundation for solving well test analysis for finite conductivity multi-branched fractures. The model can quantify the wellbore flow pressure and applied to obtain more parameters reflecting comprehensive flow characteristics through using history matching procedure. The results were compared with numerical simulation and the existing analytical solutions of a single fracture model. Then in this paper, flow characteristics are recognized and there are five flow regimes found in the type curves, e.g. bi-linear flow region, linear flow region, inter-porosity flow region between vugs and fractures, inter-porosity region between matrix and fractures, and radial flow region. Finally, the influence factors analysis shows fracture number will mainly affect flow behavior of bi-linear flow and linear flow. The angle analysis showed that as the fractures were closer, their interaction became stronger. The conductivity would seriously affect the flow behavior in the early time. Linear flow cannot be observed when the conductivity is less than 1 and bi-linear flow cannot be observed when the conductivity is more than 20. And the effect of fracture length on flow behavior occurs in the early time. Bi-linear flow and linear flow characteristics cannot be observed when the fracture length is reduced.
Highlights
Fractured-vuggy rock with complex pore structures has been widely studied by many countries[1,2,3,4,5,6,7,8,9]
We focused on a single phase transient flow behavior in fractured-vuggy rock, conceptualized as multiple-continuum medium, consisting permeable natural fractures, low-permeability rock matrix, vugs, and multi-branched fractures
For early time, the results from numerical simulation cannot well agree with the semi-analytical results and the accuracy of numerical simulation is controlled by the number of grids near the wellbore
Summary
We compared the presented semi-analytical solution with Darcy flow and numerical simulation with non-Darcy flow (different Forchheimer number values). (3) The first V-shaped segment on the pressure derivative curve represents inter-porosity flow region from vug system to the fracture system shows. The uniform conductivity for case 3 exhibits the complete flow characteristics, the starting time of the linear flow becomes late and the pressure depletion increases as the each fracture conductivity decreases. This point is coincided with the analysis above under the uniform conductivity. The matching results implies that the presented model can be used to explain the well test data in the early time, which cannot be done in previous study[34]
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