AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Quality Fracture Network Assessment for IOR Feasibility Case Study in Fractured Carbonate Rock-UAE
(1) Consulting Services, Schlumberger, Ahmadi, Kuwait.
(2) Engineering Geology & Hydrogeology, Aachen University, Aachen, Germany.
The implementation of the Improved Oil Recovery (IOR) methods to increment the recovery factor in naturally fractured reservoirs is often expensive and time-consuming; therefore, a detailed quality reservoir assessment of key attributes of the fracture network in an analogue outcrop carbonate rocks will help unravel its impact on the IOR pilot.
Detailed fracture analysis was conducted on collected data from the Eocene-Miocene outcrop exposure of the fractured carbonate rocks in Jabal Hafit anticline/Abu Dhabi. This paper aims to assess the effect of critical fracture parameters such as geometry, interconnectivity, density, aperture, size, mechanical layering and ambient stress condition on fractures openness and reactivation mechanisms. It also introduces a method to estimate the seal potential ratio based on aperture measurements of representative fractures chosen over areas of different structural settings in Hafit structure.
Results indicate that there are two fracture systems; an older E-W trending and a younger N-S striking system, which have been formed in at least two different tectonic settings from Cretaceous to present. Each of these systems is divided into three vertical to sub-vertical fracture sets; an extensional (joints) and two conjugate shear sets (faults). Some of these extensional fractures and faults are partially or entirely sealed by calcite or clay gouge filling. Fracture density shows a log-normal relationship with bed thickness, which increases in dolomitized limestone facies, in the crestal area and in the vicinity of pre-existing faults and shear zones. Fracture size is inevitably constrained by the outcrops exposure. Fractures aperture varied between 0.5 to more than 30 cm, depending on fracture geometries, positions in the anticline and lithology, and calcite fillings.
We consider that only fractures of the second system is preferred for fluid flow along corridors that are held opened by the current N-S to NE-SW ambient stresses unless locally sealed by clay or calcite mineralization. However, at the vicinity of a fault, the estimation of fractures openness depends on the fault geometry and the associated in-situ stress tensors around the fault. Fractures connectivity is controlled by individual fracture set geometry together with the current in-situ stress. Riedle and diffused fractures connect those opened fractures and faults of this system together with the bedding planes giving rise to a dual permeability reservoir.