GeoArabia, Vol. 6, No. 4, 2001 Gulf PetroLink, Bahrain Permeability and Rock Fabric from Wireline Logs, Arab-D Reservoir, Ghawar Field, Saudi Arabia F. Jerry Lucia, James W. Jennings, Jr., and Michael Rahnis, The University of Texas at Austin, and Franz O. Meyer, Saudi Aramco ABSTRACT The goal of reservoir characterization is to distribute petrophysical properties in 3-D. Porosity, permeability, and saturation values have no intrinsic spatial information and must be linked to a 3-D geologic model to be distributed in space. This link is provided by relating petrophysical properties to rock fabrics. The vertical succession of rock fabrics was shown to be useful in constructing a geologic framework for distributing porosity, permeability, and saturation in 3-D. Permeability is perhaps the most difficult petrophysical property to obtain and image because its calculation from wireline logs requires the estimation of pore-size distribution. In this study of the Arab-D reservoir, rock fabric and interparticle porosity were used to estimate pore-size distribution. Cross- plots of water saturation and porosity, calibrated with rock-fabric descriptions, formed the basis for determining the distribution of rock fabric and pore size from resistivity and porosity logs. Interparticle porosity was obtained from travel-time/porosity, cross-plot relationships. A global porosity-permeability transform that related rock fabric, interparticle porosity, and permeability was the basis for calculating permeability from wireline logs. Calculated permeability values compared well with core permeability. In uncored wells, permeability was summed vertically and the horizontal permeability profile compared with flow-meter data. The results showed good correlation in most wells. INTRODUCTION A key aspect in constructing a reservoir model is distributing petrophysical properties in 3-D space. Because petrophysical data contain no intrinsic spatial information, this process requires not only measuring the petrophysical properties of core samples and calculating properties from wireline logs, but also of linking them to the stratigraphy. This paper illustrates a method of calculating matrix permeability profiles and rock-fabric successions using some common wireline logs, such as gamma ray, neutron, density, acoustic, deep resistivity, and induction. Core and log data from the Haradh area (Figure 1) of the Ghawar Arab-D (Jurassic) reservoir (Figure 2) were used as follows: (1) To link petrophysics and stratigraphic-related petrophysical measurements to rock fabrics. (2) To calibrate rock-fabric petrophysical class and interparticle porosity to wireline-log responses. (3) To calculate permeability from common wireline logs using a global rock-fabric interparticle porosity permeability transform. Numerous ways of calculating permeability from wireline logs have been tried. The most popular has been the porosity-permeability transform. This approach fails because permeability is a function of interparticle porosity and pore size, not simply of porosity (Pittman, 1992). Another approach is to relate permeability to water saturation, porosity, and capillary pressure (Timur, 1968; Saner et al., 1997). Although this method has been used successfully in siliciclastic reservoirs, it is difficult to apply to carbonate reservoirs because of the complexity of the pore space. Several statistical approaches are in vogue to calibrate core data with log responses, including various statistical regression methods (Neo et al., 1998), cluster analysis, neural networks (Mohaghegh, 2000), and fuzzy logic (Cuddy, 2000). The rock-fabric approach of this study used interparticle porosity and rock-fabric petrophysical class to characterize pore size. A single porosity-permeability transform can be used in those reservoirs 619 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/4/619/4560598/lucia.pdf by guest on 30 September 2021 IRAN Rimthan Wafra Lucia et al. Fuwaris Marjan Lawhah Dibdibah South N Safaniya Hamur Hasbah Kangan Jauf Ribyan Maharah Sadawi Harqus North Pars Suban Sharar Manifa Karan Habari Kurayn Wari’ah Watban Jurayd Arabian Juraybi’at Jana Gulf Abu Hadriya El Haba Bakr Khursaniyah Jaladi Berri Faridah Fadhili Dhib Abu Sa’fah Samin Qatif Al-Rayyan Dammam BAHRAIN Al-Shaheen Abqaiq Awali Jaham Fazran ’Ain Dar SAUDI ARABIA Shedgum Doha ’Uthmaniyah Dukhan Khurais Ghawar QATAR Hawiyah Abu Jifan Qirdi Riyadh Farhah Manjurah Harmaliyah Mazalij Reem Jafura Haradh Mazalij-24 Sahba Ghazal Wudayhi Tinat Dilam Shaden Waqr Raghib Lughfah Abu Shidad Tinat South Shiblah Abu Rakiz Niban Shamah Jawb Hilwah Mulayh Abu Markhah Khuzama Burmah Nisalah Nuayyim Hawtah Hazmiyah Sabha Ghinah Umm Jurf 0 150 Layla Usaylah Faydah km Figure 1: Location map of the Haradh area of Ghawar field, Saudi Arabia. that are characterized by interparticle porosity and a uniform rock fabric. Most carbonate reservoirs, however, are more complex, being composed of varying amounts and types of vugs, as well as variable interparticle porosity and several petrophysically significant rock fabrics. Once permeability values have been calculated from wireline logs, they must be distributed into 3-D space to form a reservoir model. Several methods have been used. The most common approach has been to construct a correlation framework and interpolate permeability between wells constrained by the correlation surfaces. No link between permeability and geologic description is required, and the correlation structure may be based simply on gamma-ray and porosity logs. This method tends to average high- and low-permeability values and results in a reservoir model that is unrealistically uniform. A more realistic permeability distribution can be obtained by using modern spatial statistical methods, such as variography (Jenson et al., 1997). However, if not constrained by a geologic model, the resulting permeability model may also be unrealistic. Geologic models typically consist of facies distributed within a sequence stratigraphic framework (Kerans and Tinker, 1997). Permeability values from core data are linked to facies by using various rock-typing methods, such as regression analysis and cluster analysis. In this paper, permeability is linked to rock-fabric facies, and the rock fabrics are an integral part of the permeability calculations. The principal ways of distributing petrophysical properties in 3-D space are as follows: (1) Calculation of a reasonable vertical permeability profile for each well. (2) Linking the permeability profile to the vertical succession of rock fabrics that can be incorporated within a sequence stratigraphic model to distribute the data in the interwell space. 620 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/4/619/4560598/lucia.pdf by guest on 30 September 2021 Permeability and Rock Fabric ARAB-HITH TERMINOLOGY Fm Member Reservoir Manifa GENERALIZED UPPER JURASSIC HITH STRATIGRAPHIC SEQUENCE FORMATION MEMBER LITHOLOGY ARAB-A Arab-A HITH Arab-A ARAB-B Arab-B Arab-B ARAB Arab-C ARAB-C Arab-D Arab-C ARAB JURASSIC JUBAILA 1 ARAB-D 2A HANIFA 2B 3A Wackestones/ Arab-D Anhydrite Grainstones/ Packstones Mudstones 3B JUBAILA 4 Figure 2: A schematic display of the stratigraphy and reservoir zonation of Ghawar field (from Cantrell et al., 2001). This is not a two-step process, such as defining petrophysical rock-types first and then attempting to find stratigraphic links, or of developing the stratigraphic model first and then characterizing the facies petrophysically. It is an iterative process, with geological and petrophysical interpretations being done in concert. METHOD Approach The approach was in two parts as follows: (1) Develop the methodology for calculating permeability and rock fabric using data from two cored wells. (2) Test the method by using core data from other cored wells and flow-meter data from uncored wells. The first step was to relate core measurements to rock-fabric descriptions using the classification described by Lucia (1995). This resulted in a relationship between rock-fabric petrophysical class, interparticle porosity, and permeability that was the basis for calculating permeability from wireline logs. A similar approach to characterizing Arab-D reservoirs was presented by Wilson (1981), Munn and Jubralla (1987), and Saner and Sahin (1999). The second step was to develop a method for estimating rock fabric and interparticle porosity from common wireline logs. This resulted in a relationship between rock fabric/water saturation/porosity and interparticle porosity/acoustic transit time/ porosity. The method was tested on three cored wells by comparing rock fabric and permeability results with core data, and on eight uncored wells by comparing calculated permeability with flow-meter data. The stratigraphic implications of the calculated rock-fabric successions were examined by correlating the vertical succession of rock fabrics. 621 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/4/619/4560598/lucia.pdf by guest on 30 September 2021 Lucia et al. Database and Analyses Core descriptions, core analyses, and thin sections were provided by Saudi Aramco. Boyles Law porosity and permeability measurements were made on 1.25-inch-diameter horizontal core plugs. No capillary-pressure curves were available. Rock fabrics were identified from thin sections cut from the ends of the core plugs used for the petrophysical measurements. The samples were impregnated with blue dye to simplify pore-space identification.