Abstract

A fundamental component in the construction of most reservoir performance models is an empirical relationship between permeability as measured in a limited number of cored wells and other petrophysical properties measured in well logs. This paper presents a permeability model specially designed for carbonates. The model relates permeability to interparticle porosity, makes special accommodation for separate-vug porosity, and includes a rock-fabric classification scheme with an important dual petrophysical-geological significance. Methods to estimate the separate-vug porosity from sonic logs and the rock-fabric from initial saturation are presented.

The dual petrophysical-geological significance of the rock-fabric classification is important for providing a link to geological models for use in distributing permeabilities between wells. Porosity and permeability are highly variable and difficult to predict spatially in most carbonate reservoirs, but rock-fabric changes tend to be systematically organized in a predictable manner within a sequence stratigraphic framework.

Introduction

Reservoir characterization and modeling is primarily a problem of understanding the 3D spatial arrangement of petrophysical properties. Petrophysical measurements must be linked to spatial information when building a reservoir model, and geologic models contain vital spatial information to be applied in interwell areas where direct petrophysical measurements are difficult. The link is best accomplished through the integration of geologic rock-fabric descriptions and petrophysical measurements.

A method for linking basic rock-fabric descriptions and petrophysical properties has been proposed by Lucia.1,2 Carbonate pore space is divided into interparticle, which includes both intergrain and intercrystal, and vuggy pore space (Fig. 1). Vuggy pore space is subdivided into separate and touching vugs on the basis of vug interconnection. Separate vugs are connected through the interparticle pore space (grain molds, for example), and touching vugs form an interconnected pore system independent of the interparticle pore space (caverns and fracture pore space, for example). Interparticle pore space is subdivided into rock-fabric classes on the basis of geologic descriptions of particle size and sorting.

In this paper we present an approach to permeability modeling in carbonates on the basis of this rock-fabric classification. The paper is organized into five main sections:

  1. the carbonate rock-fabric classification is summarized and its relationship to porosity and permeability is presented;

  2. exponential and power-law porosity-permeability models are compared, and a generalized power-law model relating porosity, permeability, and rock fabric is presented;

  3. the generalized permeability model is compared with three others from the literature;

  4. rock-fabric based methods for permeability prediction from well logs are summarized; and finally

  5. an approach to 3D modeling of carbonate permeability taking advantage of the geological link provided by the rock-fabric method is described.

Carbonate Rock-Fabric Petrophysical Classification

Permeability and capillary properties of interparticle pore space can be related to interparticle porosity and geologic descriptions of particle size and sorting called rock fabrics.1,2 These rock fabrics were initially grouped into three categories called rock-fabric petrophysical classes on the basis of porosity, permeability, and capillary properties1 (Fig. 2):

  • Class 1 is composed of grainstones, dolograinstones, and large crystalline dolostones.

  • Class 2 is composed of grain-dominated packstones, fine and medium crystalline, grain-dominated dolopackstones, and medium crystalline, mud-dominated dolostones.

  • Class 3 includes mud-dominated limestones and fine crystalline, mud-dominated dolostones.

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