3D Geological Modeling of a Carbonate Reservoir, Utilizing Open-Hole Log Response - Porosity & Permeability - Lithofacies Relationship

Abstract

Abstract A lithofacies based reservoir characterization study was conducted for a Lower Cretaceous carbonate reservoir in order to build a numerical 3-dimensional geological model including permeability prediction and rock typing for reservoir flow simulation. This paper describes a procedure for constructing a lithofacies based geological model for reservoir flow simulation. This is the first time a lithofacies based reservoir characterization has been applied to the target reservoir. We believe this model is a reasonable representation of the reservoir as it gave a good match between predicted permeability and well test permeability. Nine lithofacies were identified from detailed slabbed core observations and lithofacies of non-cored wells were predicted from the cross plots between DT, RHOB, PHIE and MSFL of each lithofacies. The relationship between lithofacies and open-hole log response was also effective in estimating the spatial distribution of each lithofacies. A lithofacies trend map, which shows the lateral variation of lithofacies, was produced by applying this relationship to maps of vertically averaged log response. A lithofacies distribution map was prepared by incorporating a geological interpretation of thickness and porosity variations into the trend map. Horizontal permeability was predicted from simple core porosity-permeability regressions for each lithofacies using log porosity as the input. Using the lithofacies distribution map and the relationship between porosity and permeability of each lithofacies, the lateral variation of horizontal permeability was predicted without direct interpolation or extrapolation of permeability data between wells. The general trend of this modeled permeability was similar to the permeability derived from production tests. However, the absolute value of modeled permeability was lower than the well test permeability across the reservoir. This phenomenon might be due to the existence of high permeability streaks which were not cored. Investigation of the relationship between lithofacies and petrophysical data, such as special core analysis, was carried out based on pore throat size distribution curves derived from capillary pressure data. As a result, four rock types were identified in the reservoir. Introduction The studied field is located offshore Abu Dhabi, UAE (Fig.1) and the target reservoir is composed of Lower Cretaceous limestone succession (Fig.2). The reservoir has been divided into two major units: the upper dense zone including a minor porous part and the lower porous zone. The porous zone has been further divided into five layers based on petrographics and petrophysical characteristics. The importance of a detailed 3-dimensional geological model has been recognized for the past decade in order to construct a reliable flow model. The objective of this study was to construct a 3-dimensional geological model for reservoir flow simulation. The procedure used in the study is illustrated in Fig.3. The first step of the study was the observation and description of cores in 15 key wells in order to identify lithofacies. Thin section observation was used for a more detailed lithological characterization in one deviated well with high core recovery. Suites of open-hole logs with computer processed interpretation logs (CPI logs) were available in 215 wells. Conventional core analysis (CCAL) data and special core analysis (SCAL) data were also available in 22 wells and at 58 points from 10 wells respectively. The reservoir layering scheme is based on a modified version of the above-mentioned five layer model. The reservoir was divided into 13 layers based on lithofacies, CPI profile, open-hole log and CCAL data.