Abstract

The West Sak field located on the North Slope of Alaska is estimated to contain 25 billion barrels of heavy oil in place. This heavy oil of 10.5–22.5 degrees API gravity lies at depths ranging from 2500 to 4500 feet. The reservoir temperature due to the overlying permafrost is between 45 and 100 degrees F and therefore, the recovery of this oil will require the application of some enhanced oil recovery (EOR) process. Deployment of an appropriate EOR process for commercial production will require a detailed reservoir description. Delineation of the reservoir system, both at the microscopic and macroscopic scale, should include the depositional environment, the reservoir rock and fluid properties, and the rock-fluid property interactions.

This study presents the reservoir description of the West Sak sand. Salient reservoir properties such as the porosity, water saturation, and pay zone thickness are porosity, water saturation, and pay zone thickness are determined using well logs from the West Sak field. isopach maps, subsurface correlations, and 3-D surface plots are prepared to determine the spatial variations in plots are prepared to determine the spatial variations in porosity and water saturation in the pay zones. The porosity and water saturation in the pay zones. The correlations of the Spontaneous Potential (SP), Gamma ray and resistivity traces of wells from the West Sak field are used to determine lateral and vertical gradients of the lithofacies of various horizons. The lithofacies distributions are then used to describe the depositional environment of these sands.

The results of the reservoir description stud, were used as EOR screening criteria and a number of EOR processes were found to be technically applicable to the processes were found to be technically applicable to the West Sak field.

Introduction

West Sak field is a megagiant and the largest known heavy oil deposit on the Alaskan North Slope. It contains over 25 billion barrels of heavy ail in place and thus is the largest ail reservoir in North America. The gravity of the oil varies typically between 10.5–22.5 degrees Reservoir rock in the West Sak field lies at depths ranging between 2500 and 4500 ft. Inspite of significant depth, the reservoir temperature varies between 45 and 100 degrees F due to the proximity of overlying permafrost. Degradation of crude proximity of overlying permafrost. Degradation of crude at shallow depths and the low temperature due to overlying permafrost makes the West Sak crude viscous. Commercial production of this heavy oil will therefore, require use of some enhanced oil recovery (EOR) process. Understanding of the reservoir framework, architecture, and blind properties therein is a prerequisite for the design and application of any EOR process. Such understanding will require knowledge of the basinal analysis, the source and origin of the hydrocarbons, the migrational attributes, the migrational path, and eventual accumulation of the hydrocarbons. These attributes provide basis for the selection of most suitable and efficient EOR process.

In this paper, we present a reservoir description of the West Sak field. Important reservoir properties such as porosity, water saturation, and pay zone thickness are porosity, water saturation, and pay zone thickness are determined using well logs from the West Sak field. Contour maps. cross-sectional correlations and 3-D surface plots are prepared to determine the spatial distribution of plots are prepared to determine the spatial distribution of these parameters within the pay zones. Correlations of the Spontaneous Potential (SP), Gamma Ray and resistivity traces of several wells from the West Sak field are used to determine the lateral and vertical gradients of the dominant lithofacies of the West Sak. The spatial variation within each lithofacies distributions are then used to interpret the depositional environment for these sands.

The reservoir properties were used as EOR screening criteria and a number of EOR processes were found to be technically applicable to the West Sak field.

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