Pressure transient analysis of a vertical well with multiple etched fractures in carbonate reservoirs

Erhui Luo,Wenqi Zhao,Yongle Hu,Meng Sun,Zifei Fan,Chenggang Wang,Xuanran Li

doi:10.1177/0144598719885066

Abstract

Acid fracturing has been widely used as an industry practice in explored and developed carbonate reservoirs. It is very important to understand responses of reservoirs and improve production performance of a well due to the presence of fracture networks by stimulation treatments. Pressure transient analysis is one of the most effective diagnostic techniques available to enhance our understanding of natural and artificial-etched fracture behavior. This work presented a novel mathematical model for unsteady state flow of naturally fractured porous medium into multiple etched fractures intersecting a vertical well, and three different geometric shapes of matrix blocks containing slabs, cylinders and spheres were considered. The new solution was derived by using the Laplace transformation and the point source function integral approach. The polar coordinate transformation was used to deal with the radial distribution of arbitrary fracture number and angle. Then the model was validated by comparison with three published cases. Finally, type curves were plotted to identify flow regimes: linear flow, transitional flow, pseudoradial flow, and boundary dominant flow if the closed or constant pressure boundary exists. Furthermore, sensitivity analysis was investigated. The results showed that the acid-etched fracture parameters containing fracture number, fracture distribution and conductivity had a significant impact on pressure behavior at early times. However, natural fracture storativity coefficient and interporosity flow parameter mainly affected the transitional flow at intermediate times. Moreover, the shape of matrix blocks had a little influence on transient responses at intermediate times. It is found that multiple etched fractures existing near the wellbore consume less pressure drop and increase the productivity of a well as a whole.

Highlights

Carbonate reservoirs possess about half of the world proven oil reserves and over 60% of world oil production, and have become increasingly important resources (Bagrintseva, 2015)
The dual-porosity model that takes into account the fluid transfer between matrix and fractures includes two different conditions: the pseudosteady state and unsteady state flow
The biggest difference between the pseudosteady state and nonsteady state flow is that the pressure derivative curve of the former has a deeper trough like a bell-shape than the latter one being relatively flat at the transition flow period

Summary

Introduction

Carbonate reservoirs possess about half of the world proven oil reserves and over 60% of world oil production, and have become increasingly important resources (Bagrintseva, 2015). The dual-porosity model that takes into account the fluid transfer between matrix and fractures includes two different conditions: the pseudosteady state and unsteady state flow. For these models, two key parameters, the storativity coefficient and interporosity flow parameter, are sufficient to characterize the behavior of the double medium. Since Warren and Root (1963) first proposed an idealized model that assumes a network of orthogonal, spaced feature of fracture system, with the pseudosteady-state flow between the matrix and fracture, more than a hundred authors have used the dualporosity model under the pseudosteady state flow to investigate and interpret pressure transient responses of naturally fractured reservoirs in the petroleum engineering domain (Samaniego and Cinco-Ley, 2009). The biggest difference between the pseudosteady state and nonsteady state flow is that the pressure derivative curve of the former has a deeper trough like a bell-shape than the latter one being relatively flat at the transition flow period

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Conclusion