Porosity, Permeability, and MHR Calculations Using SEM and Thin-section Images for Characterizing Complex Mauddud-Burgan Carbonate Reservoir

Abstract

Abstract This paper presents the process, and, results of the analyses of pore/grain morphology of rock fragments, from Mauddud-Burgan reservoir rock in Kuwait, for porosity, permeability and means hydraulic radius (MHR) calculations. The images are captured using SEM and thin-section analyses. In this study, 2-dimensional images are used to characterize the morphology of the grains and pores, using a two step process. In the first step, the image is captured using a backscattered electron detector (BSE) digital electron microscopy imaging, and thin-section photography. All of the grain/ pore features captured in the image are reported in micrometer units. In the second step, the area of such features is scanned using image analysis software that has the ability to accurately measure several morphological parameters of pore and grain spaces. A robust technique of visual estimate is used, which has the advantage of speeding the image analysis process. The visual analysis software tool counts different pores and grains and also measures their shapes and sizes that are crucial for porosity, permeability, and MHR calculations. Several morphological features were been selected for measurement, including: object count, area, perimeter, and roundness. These analyses were conducted twice for each selected image - once for grain morphology and once for pore morphology. Porosity, permeability and MHR features are calculated based on area of the pore/ grain features measured from two-dimensional images. Introduction Mauddud Burgan is a carbonate rock reservoir that is located in southeast Kuwait. It has substantial amounts of crude oil, yet it is described as a complex formation due to the problems experienced in production. This carbonate reservoir has a complex pore system that ranges from touching-vugs caverns to tight pores-matrices(1). Well A is selected to represent the Mauddud-Burgan reservoir. Although the total porosity of Well A is evaluated to be excellent, about 17.4%, the permeability is believed to be humble. The overall reservoir permeability, which is estimated from a well testing study, is found to be about 40 md(2). A core analysis of Well A (Table 1), using a statistical geometric average of air-permeability measurements, found the permeability to be around 0.5 milli-Darcies. Recent studies show that the majority of the permeability is contributed to the large, unconventional cavern pores, which means that the knowledge of the matrix pore network is wholly unknown. As a result, more information is needed at the pore level to evaluate the matrix pore network and its potential for releasing the oil into the large caverns. This knowledge will assist in understanding the nature of this reservoir and how to better drain it. The Mauddud-Burgan reservoir is found to be 40-feet of total thickness(3). Recent studies show that Mauddud-Burgan has two rock types (M1) and (M2), which will be the target for this study. M1 is described as a high-energy limestone formation that is located in the upper part of the Mauddud-Burgan reservoir and is broken into two rock types, an upper dolomitized-limestone and a lower clean-limestone. Since the upper dolomitized portion is relatively small and estimated to be only two-feet thick, this study will consider M1 as one rock-type system. M2, which is located in the bottom of the reservoir, is classified as a low-energy glauconitic-limestone. It is located towards the bottom of M1. The measured data for M1, roughly, is spanned over 26-foot layer and M2 is spanned, again roughly, over a 12-foot-layer. M1 has an average porosity of 20.1% and a geometric-average permeability of 5.2 md. M2 has an average porosity of 13.9% and a geometric-average permeability of 0.51 md. Evidence from a core study generally suggests that the Mauddud-Burgan reservoir is a tight limestone formation with large storage porosity. In order to stretch the understanding of these petrophysical descriptions, the scope of this study is set to examine the rock pore spaces and permeability at a close range.