NMR Supported Near Wellbore Data Analysis to Improve the Petrophysical Evaluation of a Sandstone Reservoir Drilled with Barite Enriched Water Base Mud – A Case Study from Kuwait Onshore

Abstract

The effect of mud filtrate and eventually fine migrations in high quality rocks considerably influence the reading of NMR and conventional logs. It poses a challenge in extracting useful information that can contribute to the petrophysical evaluation of the virgin zone, e.g. the rock quality index. This is critical for determining the permeability index and the saturation exponent n, required for accurate determination of the water saturation.

The rock quality index is understood as the ratio between free fluid volume (FFV) and bound fluid volume (BFV). This is equivalent to the pore-throat radius, which dominates fluid flow, that is negatively affected by the fine migration. This is reflected in a reduction of the porosity filled by free fluid. The free fluid porosity undercall is corrected with the help of a deeper reading Neutron/Density data. After this correction, the NMR derived permeability in the flushed zone, probably damaged by migrated fines, is used to calculate the permeability in the virgin zone.

The determination of the oil saturation in the flushed zone from NMR logs was done in two ways. First by applying statistically based Machine Learning Tools. Secondly, applying the widely used 2D-NMR methods, Diffusion vs. T2 and T1/T2 vs. T2 maps. Comparative analysis of the oil saturation in the flushed zone against the oil saturation in the virgin zone indicated a high degree of invasion.

The Great Burgan field is recognized as the world’s largest sandstone reservoir. Located in the south-eastern part of Kuwait near the crest of the north-south trending anticline with an aerial extent of 450 square miles (Fox, 1961; Adasani, 1965; Brennan, 1990b; Carman, 1996), illustrated in Figure 1.

The Great Burgan field is made of a group of three main subsurface structures: Ahmadi, Magwa and Burgan which are characterized by their domal structure. There are five main oil-producing Cretaceous-age reservoirs: Wara Sand, Mauddud Limestone, Burgan Sand- Upper, Middle and Lower layers, shown in Figure 2. The Burgan and Mauddud formations are the parts of the Wasia Group. This group overlies the Lower Cretaceous Thamama Group of the Arabian plate (Alsharhan and Nairn, 1993). The Burgan field was discovered in 1938 and started producing in 1946.

The current study focuses on the Wara and the Burgan formations, the major producers of Burgan field. Both formations were deposited in a fluvial deltaic environment on the continental shelf margin of the ancient Tethys Ocean. The two formations separated by a carbonate succession of the Mauddud formation, deposited in a shallow marine environment. (Al-Jallad et al.)