Abstract

Reservoir engineering studies of the Kuparuk River Field require reservoir description models at multiple scales. Kuparuk reservoir types range from relatively low permeability, vertically stratified and imbricated lower shoreface sandstones (A Sands) to "thiefy", glauconitic and sideritic shallow marine sandstones with complex permeability structure (C Sands). The field is also heavily faulted, having over 5200 mapped faults with throws typically between ten and several hundred feet.

Geocellular models built for type curve modeling of waterflood and enhanced oil recovery (EOR) processes capture a range of A and C Sand reservoir types. These models are 2600×2600 feet with cell dimensions of 50×50×0.5 feet. Cored wells provide a robust facies and petrophysical model. Both object- and pixel-based techniques are used together with a flow unit-scale zonation to capture key heterogeneities. In the A Sands, this requires capturing the interstratification of sandy and shaly facies using object-based methods. In the C Sands, this requires capturing permeability heterogeneities caused by facies changes and siderite diagenesis using pixel-based methods.

A geocellular model was built to evaluate options to improve throughput rate in the A Sands. This model is 24,000×20,000 feet with cell dimensions of 150×150×0.5 ft. It incorporates 237 faults and 11 flow units. Both object- and pixel-based techniques were used to distribute facies and petrophysical properties. A challenge in this model was in representing reservoir compartmentalization due to faulting of thinly bedded sandstones.

These examples demonstrate the need for "fit-for-purpose" models that best represent key heterogeneities for a given reservoir-modeling/reservoir engineering objective.

Introduction

The Kuparuk River Field, located on the North Slope of Alaska (Figure 1), is a mature giant field with substantial opportunities for in-field and peripheral development and enhanced oil recovery. Of the 43 drill sites already existing or under development, all but 10 are in various stages of miscible water-alternating-gas injection (MWAG) recovery. Eventually, all but one drill site will be under MWAG. In addition, an aggressive program for added recovery and rate acceleration through in-field development of poorly swept or isolated fault blocks is also planned using coiled tubing drilling technology.

The diverse reservoir engineering issues associated with managing a giant field in this state of maturity requires reservoir descriptions at a variety of scales.This paper describes the development of two types of 3D geocellular models for use in reservoir management of the Kuparuk River Field. The first involves the creation of pattern-scale, highly detailed descriptions for use in the development of dimensionless type curves for EOR performance prediction. The second model was created to evaluate options for improving throughput rates in the lower reservoir interval.

These description efforts have underscored the importance of matching the detail of the geological description to the intended purpose of the model. They also highlight the importance of developing a workflow for iterating between the geological and reservoir simulation models to rapidly screen descriptions and modify them during history matching.

Reservoir Geology of the Kuparuk River Field

The Kuparuk River Field is productive from two major sandstone reservoir intervals of the Kuparuk River Formation Lower Cretaceous), the underlying A Sands and overlying C Sands (Figure 2). The A Sands contain about 3692 MMSTBOOIP and the C Sands contain about 2163 MMSTBOOIP. Ultimate waterflood recovery is estimated to be about 40% with an additional 10% from EOR.

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