Quantification of Complex Pore System in Heterogeneous Carbonate Reservoirs Using Borehole Resistivity Image Log from the West Coast of India
Authors:   Oil and Natural Gas Corporation Ltd India, Panda Soumya Chandan , Kumar Suraj ,   Oil and Natural Gas Corporation Ltd India, Prasad Surendra Kumar ,   Oil and Natural Gas Corporation Ltd India, Kumar Vinod ,   Oil and Natural Gas Corporation Ltd India, Bahukhandi Yogesh ,   Oil and Natural Gas Corporation Ltd India
Published: September 2024

The goal of this work is to evaluate and quantify the total porosity contributions (matrix, vugs, connected vugs, resistive, etc.) from high-resolution resistivity image data only in a highly heterogeneous carbonate reservoir from the west coast of India. The study well was drilled as a horizontal well with a 6-in. drainhole to exploit hydrocarbon. Conventional and FMI logging was carried out in the reservoir section of the well. Image textural analysis is carried out in the well to study the heterogeneity in the complex carbonate reservoir. A static and dynamic image is created using histogram equalization. FILTERSIM, a recent multipoint statistics (MPS) (Zhang, 2006; Hurley and Zhang, 2011), is used to fill gaps in the image and create a full image, which is essential for heterogeneity delineation. The mosaic image is created, which represents the heterogeneities against the matrix (background) image using the grayscale reconstruction transform (Luc, 1993), which removes the features not traversing the image. After creating the mosaic image, heterogeneity delineation is carried out in which step size, value, contrast, and surface proportion of each heterogeneity are computed and represented as curves. Heterogeneity delineation comprises three steps that are heterogeneity analysis, heterogeneity cutoff values, and heterogeneity classification. Heterogeneities are classified manually wherever they are not captured by the algorithm. Finally, the porosity map is generated through the below equation (Newberry et al., 1996), where φGVR is the computed porosity, φlog is the porosity, RLLS is the shallow resistivity, Ci is the value of the conductivity image, and m is the cementation factor. φGVR = φlog (RLLS* Ci) 1/m The process produces a set of curves corresponding to the different contributions to the total porosity. Carbonate reservoirs are highly heterogeneous in terms of porosity, permeability, and facies. Porosity and permeability exhibit a nonlinear relationship in carbonates due to porosity creation and destruction during diagenesis, unlike clastic formations. The results from textural analysis indicate various types of porosity systems where hydrocarbon is deposited in a secondary porosity system (vugs and connected vugs). Porosity partitioning in Well-X against the reservoir interval (XX00 to XX13 m) indicates total porosity of 20 p.u., contributed by isolated vugs (green), connected pores (red), resistive (blue), and matrix (beige) porosity. The pie chart prepared shows the contribution from isolated vugs at 15%, connected pores at 15%, matrix porosity at 50%, and resistive porosity at 20%. The above results are in agreement with the results from the well testing carried out in Well-X, which flowed economical quantities of hydrocarbon. The valuable insight into the porosity distribution in this reservoir will also enhance production during the enhanced oil recovery stage. Qualitative and quantitative characterization of heterogeneous carbonate reservoirs from resistivity image logs using a novel approach by carrying out porosity partitioning using image textural analysis and providing quantitative evaluation of carbonate porosity system. In the current study, the contribution of isolated vugs (15%) of the total porosity opens doors for enhanced oil recovery after an acid job in the later stages of reservoir development.