Prediction of Flow Units of the Khuff Formation
Authors: Khalaf A.S. 
Published: December 2007

Bahrain field is located in the Gulf of Sulwa between Saudi Arabia and Qatar (Fig. 1). The Permian Khuff in the Bahrain field has been the subject of many studies aimed at optimizing the development of its gas reserves.

The need to provide continuous profiles of permeability for all present and future Khuff wells prompted this study. As the Khuff wells are not extensively cored, it was necessary to use the openhole logs as the basis for this effort.

Permeability estimation from logs must be done in two steps. The first step is to establish a methodology for identifying the various rock fabric textures from the openhole logs. The second step is simply to generate a porosity to permeability transform for every fabric texture. The method to generate these transforms is straight forward by plotting log permeability versus porosity (log-linear regression).

Introduction

Permeability is a key parameter in reservoir characterization that governs, to a large extent, reservoir development and management. This parameter is normally measured in the laboratory on reservoir cores. Unfortunately the cored intervals represent a small fraction of the total thickness of the producing reservoirs. Thus permeability data are sparse and scattered compared to other data such as porosity and oil saturation.

The problem is compounded in carbonates as against clastic reservoirs. Carbonate reservoirs are much more complex. Porosity and permeability variations in carbonates are far more drastic than in clastics. Moreover, most of the available core analyses were performed on core plugs, which renders the results of limited value in characterizing these carbonate reservoirs.

Various methods have been proposed for deriving permeability from openhole wireline logs. Some had limited success in clastic reservoirs but none proved to be successful in carbonates. The objective of this paper is to identity the potential best flow units in the Khuff carbonates based on the interpretation of the dual laterolog, the density/neutron log and the visual examination of the culling samples.

Culling samples and basic openhole logs are available in almost all wells. This makes the suggested method cost effective as it requires no additional data acquisition.

Khuff Stratigraphy

The gas bearing Khuff Formation is of Permian age and consists of about 2000 feet mix of carbonate lithofacies. Khuff formation is conformably over lain by the Triassic Sudair Shale and un conformably underlain by the Devonian Sandstone of the Jauf formation (Fig. 2). The Khuff formation is described as a fine to coarse crystalline dolomite with a few interbeds of limestone and anhydrite. Percentage of limestone does not exceed two to three percent of the total reservoir thickness.

The formation is subdivided into four main zones, from the top, K-0, K-I, K-II and K-III. K-0 is about 300 feet thick. This interval encompasses the oomoldic porosity zone, about 20 feet thick. Generally it has high porosity and low permeability, except in areas where leaching has broken the barriers between the oomolds.

Khuff-I gross thickness ranges from 456 to 522 feet. The main porosity interval varies in thickness between 40 feet to over 150 feet. K-I porosity is mostly intercrystalline to intergranular with traces of moldic and vugular porosity developed in fine to coarse crystalline brown dolomite. There is extreme vertical variability in both porosity and permeability. For example, the porosity can change from 4 to 20% and the permeability from 3 milidarcys to 1800 milidarcys in an interval of one foot.

Khuff-II gross thickness ranges from 293 to 366 feet. The gross porosity interval varies in thickness from about 45 feet to 180 feet. Like K-I, it is predominantly intercrystalline to intergranular with some moldic, vuggy and fracture porosity in a light brown, fine to coarsely crystalline dolomite.

K-III is the lowest zone with a gross thickness of around 930 feet. K-III lacks any well developed porosity. Fractures constitute the only interconnected porosity in this zone.

Statement of the Problem

Khuff Formation has two thick, mainly interparticle porosity zones developed within it, namely K-I and K-II. Length of these porous intervals range from 40' to 180'. (Fig. 3) is a porosity log through a typical Khuff well.

It has been noted that, in almost all wells, the bulk of the production comes from very discrete thin zones. The problem is how to define these zones to optimize the completion of the Khuff wells. Determination of the best flow units ahead of perforation will result into the following benefits: P. 521