A Multiscale Approach to Investigate Hydraulic Attributes of Natural Fracture Networks in Two Tight Sandstone Fields, Ahnet, Algeria
Authors: Irofti Doina , Ifrene Ghoulem E. H. , Pu Hui , Djemai Safouane 
Published: September 2022

Regardless of the porosity of the matrix, fractures have a crucial role in increasing permeability. In this article, the critical role that natural fractures may play in enhancing fluid flow characteristics is considered. An attempt to demonstrate the contribution of fractures on enhancing the permeability was done, assuming the genetic relationship between complex fracture systems at different scales. A multiscale-based statistical approach was adopted to compare hydraulic properties of two tight sandstone fields in a Cambro Ordovician reservoir situated in the Ahnet Basin, Algeria. The first field, situated in the Eastern region, denotes smaller fractures with three different orientations. In comparison, the second field, located in Western domain, is dominated by long faults, oriented N-S. However, the most predominant direction is NW-SE. In the first part, the connectivity of the networks was analyzed, and the results were plotted in a ternary diagram showing that the first field is characterized by a strong connectivity, probably due to the higher intensity of faults. Then, the dilation and shear tendency were displayed based on the current stress of the field. The outputs showed that fractures in the first field have a dilation and slip tendency higher which means that they are probably open and can participate in the creation of a fluid flow path. This is probably due to their orientation, parallel to the maximum horizontal stress. In the second part of this paper, naturally open fractures at the borehole scale were investigated and then compared to the seismic scale. Finally, the matching of the results at the two scales was studied.

1. INTRODUCTION

A fractured reservoir consists of initially continuous deposits, which, as a result of the settlement, diagenesis, and deformation, have fractures. This type of reservoir would contain between 20% and 25% of the world’s available hydrocarbon reserves. Natural fracturing is a very important parameter in the assessment of an oil reservoir (Aoun et al., 2021). It can have several effects on production performance, both in primary recovery and in secondary or tertiary recovery.

Traditional techniques used in the oil industry to characterize natural fracturing and its distribution at the reservoir level are insufficient. The seismic investigation provides a good visualization of the distribution of large-scale fracture zones. Well data can sometimes provide access to a satisfactory characterization of small-scale fracturing as long as the analysis of the images and the cores are done properly. On the other hand, intermediate-scale features (small faults, fractured corridors), undetectable by seismic tools and too much localized to be systematically characterized by the analysis of well data, remain very difficult to apprehend. However, their presence, detected at random by the progress of the drilling, proves to be decisive in controlling the hydrodynamic behavior of reservoirs. In this paper, we propose a qualitative and quantitative characterization of fracture networks.