Fracture Characterization and Modeling of a Tight-Carbonate Reservoir

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 93557, "The Fracture Characterization and Fracture Modeling of a Tight-Carbonate Reservoir: the Najmah/Sargelu of West Kuwait," by O. Fonta, Beicip-Franlab; H. Al-Ajmi, N.K. Verma, and S. Matar, Kuwait Oil Co.; V. Divry, Beicip-Franlab; and H. Al-Qallaf, Kuwait Oil Co., prepared for the 2005 SPE Middle East Oil and Gas Show and Conference, Bahrain, 12-15 March. The Najmah/Sargelu reservoir of west Kuwait is a tight-carbonate oil reservoir in which porosity and permeability result from the fracture network. A multidisciplinary integrated approach combined geology [borehole-imaging (BHI) logs, cores, and wireline logs), geophysics [seismic facies analysis (SFA)], and reservoir-engineering data [production, production-logging-test (PLT), and well-test] to identify the main types of fractures, to predict their occurrence in the reservoir, and to determine the hydraulic properties of the different fracture types. Introduction The objective of a detailed geological and hydraulic characterization of the fracture network within the Upper Jurassic Najmah/Sargelu reservoir was to identify the main geological drivers of natural-fracture occurrence, measure fracture hydraulic properties, and, eventually, use discrete fracture modeling to compute the equivalent fracture properties (porosity, permeability, and block sizes) required for reservoir simulation. This objective was achieved through integration of geological, geophysical, petrophysical, and dynamic data by use of workflows and methods implemented in fracture analysis and modeling software. The main tasks during the project were the following. - Fracture analysis from cores. - Fracture analysis from BHI logs. - Integration of the 3D-seismic data set. - 3D fracture modeling. - Hydraulic characterization of the fracture network. - Computation of the fracture properties in the reservoir grids. Background The study area was approximately 2000 km2 and covered four fields. The reservoir structures are gentle, elongated anticlines. The top-of-reservoir depth ranges from 3350 to 3650 m. The Najmah/Sargelu reservoir has a generally high degree of lateral continuity and gradual thickness changes. The formation has alternating layers defined by volume of shale, Vsh, from well logs of clean limestone (Vsh <30%), shaly limestone (30%< Vsh <60%), and calcareous shale (Vsh >60%). Fifty wells were logged in the reservoir. Cores were available from 21 vertical wells, and BHI acoustic images were available on 18 wells; 10 wells had both cores and BHI logs available for the study. Matrix quality is generally very poor, dominated by micro-porosity (<8%) with low permeabilities (<0.01 md). Locally, higher matrix porosities and permeabilities may be found in Units 5 and 6 of the Sargelu limestones. Production is mostly from the fracture net-work in reservoir Units 2, 3B, 5, and 6.