Lithology, Lithofacies, and Permeability Estimation in Ghawar Arab-D Reservoir

Abstract

Abstract A new methodology was developed to better characterize the lithology, lithofacies and permeability of a pilot area in the Arab-D reservoir of the Ghawar Field. The study area has one hundred and seven wells. All wells have a conventional suite of openhole logs. There are only nine cored wells with detailed petrographic descriptions of the Arab-D. Four of these cored wells only partially sample the Arab-D reservoir. The methodology targets wells lacking core data and where all subsurface interpretation has to be based on wireline log data. The estimation of lithology from well logs was done using explicit stochastic techniques to calculate a multi-mineral volumetric model. A methodology was developed to incorporate various open hole log suites. Rigorous cross validation assured results consistent with core data. Lithofacies were defined in the cored wells combining depositional textural, and porosity-permeability relationships. Permeability and lithofacies were then estimated from the open hole well logs using multivariate statistical techniques. The cored wells were used to create a multi-dimensional ‘Learn’ histogram/database with core permeability, lithofacies porosity, grain density, and well log derived textural parameters as variables. The validation process involved using the database of each cored well to estimate the permeability and lithofacies in every other cored well. The combined histogram/database from the cored wells was then used to estimate the permeability and lithofacies for the uncored wells. As a result, a more accurate quantification of dolomite is developed, major lithofacies units are identified, and permeabilities are estimated from open hole log data. The methodology developed in this study provides reservoir characterization control at uncored well locations. The accurate estimation of these parameters and their spatial distribution is essential for building a technically sound geological reservoir model for managing this Arab-D reservoir. Introduction One of the most difficult problem faced when building a geological reservoir model is not having enough wells with core data to estimate the distribution of the parameters controlling fluid flow within the reservoir (Figure 1). Detailed evaluation of the core data from the Arab-D reservoir in four wells in a pilot area of the Ghawar Field demonstrated that high values of core plug permeability were often associated with the better flow intervals as indicated by flow meter data. It was also established that the core plug permeability could be related to the lithology and lithofacies from the core descriptions (Figure 2). However, the data from these detailed core descriptions coupled with less detailed information from five other cored wells in the area were deemed insufficient to predict the distribution of permeability throughout the reservoir. Additional information is available from the open hole log suites of 107 wells in the study area. Estimation of the core plug equivalent permeability, lithology, and lithofacies at the uncored wells would provide the control to better distribute these parameters and model the fluid flow in the reservoir. Project Objective The objective of this project was to develop a methodology to estimate the lithology, lithofacies, and core plug permeability in the uncored wells for a pilot area in the Arab-D reservoir of the Ghawar Field (Figure 3). The accurate estimation of these parameters and their spatial distribution is essential for building a technically sound reservoir geological reservoir model for managing this Arab-D reservoir. P. 245^