Enhanced Reservoir Description of Heterogeneous Carbonate Reservoirs
Authors: Shedid S.A. , Almehaideb R.A. 
Published: March 2010

This study is conducted to test and evaluate the use of current methods of reservoir characterization, namely the permeabilityporosity correlation, the J-function, and the Reservoir Quality Index (RQI) concepts, for reservoir description of heterogeneous carbonate formations. These approaches were compared with a new technique developed in this paper for improved reservoir description of carbonate reservoirs. This technique is called the Characterization Number (CN) technique and it is based upon considering fluid, rock, rock-fluid properties, and flow mechanics of oil reservoirs.

To compare these reservoir characterization techniques, measurements of porosity, absolute permeability, oil and water relative permeability, and irreducible water saturation for 83 actual core samples extracted from eight different wells for a new oil reservoir in the U.A.E. were obtained. These experimental data are used first to develop a permeability-porosity correlation. Then, the J-function and the RQI concepts along with the newly developed CN approach are applied and evaluated for the reservoir description of the UAE carbonate reservoir under investigation. The results show that the Reservoir Quality Index concept is capable of identifying the flow units while the J-function concept is quite poor. Also, a more refined identification of flow units is obtained by using the newly-developed Characterization Number. This improved description for the Characterization Number approach may be attributed to the consideration of rock/fluid properties of flowing fluid(s) and flow dynamic conditions of its containing formation.

Introduction and Review

Reservoir characterization techniques are quite valuable as they provide a better description of the storage and flow properties of a petroleum reservoir and thus provide the basis for developing its simulation model. Also, carbonate reservoirs, in particular, present a tougher challenge to engineers and geologists to characterize because of their tendency to be tight and heterogeneous.

Permeability and porosity of the reservoir rock have always been considered as two of the most important parameters for formation evaluation, reservoir description, and characterization. Beyond evaluating permeability and porosity, one can also use combinations of two or more rock properties to gain insight into the character of flow through porous media. The J-function and the Reservoir Quality Index (RQI) are two of the concepts that the oil industry has used to characterize the reservoir media. They incorporate parameters such as porosity and permeability into a single quantity that describes/characterizes the formation. The application of the J-function and/or the Reservoir Quality Index (RQI) concepts, however, may or may not determine whether a formation can be considered to have a single flow unit or multiple ones(1–4).

It is well recognized that an improved and effective reservoir description is a prerequisite for efficient development of oil reservoirs. The following is a brief review of the most common techniques available for reservoir description.

Permeability-Porosity Correlation Technique

The effective porosity(1) of a rock is defined as the ratio of its interconnected pore volume to its bulk volume. The permeability of the reservoir rock is defined as the ability of that rock to allow fluids to flow through its interconnected pores.