Application of Numerical Simulation, Flowmeter Analysis and Conditional Simulation for Modeling Dual-Permeability Reservoirs
Authors: Olarewaju Joseph S. 
Published: December 2007

Naturally fractured reservoirs present special challenges with regards to the description of the grid properties required for flow simulation. When the reservoir is considered dual-permeability, both fracture and matrix permeability values are needed for each grid block. Matrix permeability field may be derivable from a transform of well log and core data, but the fracture permeability field cannot be obtained this way and might need to be estimated from well test and any other engineering data. However, the effective permeability derived from well test analysis is associated with a large volume and is a thickness weighted average over a large radius of investigation. This does not yield information that can be used for geocellullar modeling. This paper presents a technique of combining reservoir engineering and geostatistical simulation methods for generating geocellular fracture permeability field. The resulting models will be used for reservoir management studies of Hanifa reservoir.

Introduction

This research came about as a response to the need to develop 3-D high resolution reservoir description of dual-permeability Hanifa reservoir. The desired results from the research include permeability and porosity fields for the fracture network and for the background matrix system. Previous researchers including Grover, and Luthy and Grover discussed the significance of the fracture related production in the Hanifa. They also used borehole imaging logs combined with other petrophysical logs and structural curvature information to quantify fracture orientation and distribution in Hanifa reservoir.

This paper presents the reservoir engineering method that has been developed and applied for generating fracture permeability fields for flow simulation in Hanifa dual-permeability reservoir. Information concerning the vertical distribution of fracture permeability and vertical flow units were derived from flowmeter survey data. Effective permeability data collected from previously analyzed well test data were used to estimate the permeability of each flow unit. A numerical flow simulator was used to match the pressure transient data using the permeability arrays estimated from a combination of flowmeter and well test analysis resulting in a fracture permeability trace along each well. This numerical pressure transient analysis step was also necessary to reduce the volume represented by the effective permeability values to well gridblock volume level compatible with those traces from petrophysical log.

The details of the numerical single-well pressure transient analysis is presented here. The resulting permeability traces from the pressure transient history match from all available wells were used to build a 3D permeability field using Sequential Indicator Simulation (SIS). Sample fracture permeability surface and cross-sectional slices are also presented.

The Hanifa Reservoir System

The Hanifa reservoir was deposited as a carbonate build-up within the Arabian intra-shelf basin. These carbonates are dominated by micropores with porosity up to 30% and matrix permeability of less than 6md. Extremely high flow rates from certain intervals has confirmed the presence of fractures in this reservoir. Some wells in this reservoir with average matrix permeability less than 5md produced more than 28,000b/d during flowmeter survey. It was clear that such high rate could not have been delivered by such low matrix permeability measured from the cores. The presence of fractures has also been confirmed from cores and FMS logs, and the discrepancy between core and well test derived permeabilities. Permeability from well test was found to be 40 times greater than core permeability due to the presence of fractures.

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