Infill Well Water-Cut Estimates Based on Open Hole Log Data in a Mineralogically Complex Reservoir: Kuparuk River Field Alaska
Authors: Hedges P.L. , Moothart S. 
Published: December 2007

A method of predicting the initial water-cut for an infill well was derived by combining estimates of effective porosity and of the water saturation within the effective pore space with relative permeability data. Estimates of initial water-cut from wells drilled in active waterflood areas were compared to production tests to judge the efficacy of the technique. Input from production, geological, and petrophysical reviews were used to refine the model.

Estimates of water-cut from open hole log data have been used to guide well completion strategies. Wells with high predicted water-cuts are completed as injectors. This saves the costs that would have been incurred by re-completing a producing well with uneconomic water-cuts.

Introduction

The Kuparuk River Field is located west of the Prudhoe Bay Unit on the North Slope of Alaska. The field is undergoing active waterflood An infill drilling program is underway in selective areas to improve sweep and flood conformance. The Kuparuk C sand is often the primary target.

Complex stratigraphy and faulting often make it difficult to optimize infill well locations. Well locations are selected after detailed review of an area by a team of geophysicists, geologists, production engineers and reservoir engineers. Infill well results sometimes change interpretation of local reservoir continuity and modify flood management plans. A method to reasonably predict the water-cuts from the C sand in infill wells was required to help guide well completion decisions.

The C sand interval of the Kuparuk River formation is a reservoir with complex mineral and pore distributions. In particular, grains of microporous glauconite add to the difficulty in estimating water saturations. A detailed understanding of the geology, petrology, and log responses was required to improve water saturation calculations so that they could be used to estimate water-cut.

Reservoir Geology. The Kuparuk River Formation is divided into upper and lower members separated by a regional unconformity (Fig. 1). The lower member consists of units 'A' and 'B', the upper of Units 'C' and 'D'. Sands in the A and C units along with minor sands in the B unit are productive at the Kuparuk River Field. The C sand is the most productive interval. Although the C sand has about one third of the field's reserves, it has produced over half of the oil to date. Nearly all waterflood breakthrough has occurred in the C sand.

The Kuparuk C sand is a glauconitic, siderite-cemented sandstone interbedded with mudstone and siltstone. Extensive bioturbation and secondary diagenisis has destroyed and/or masked most of the primary sedimentary structures. The more important diagenetic events include siderite cementation and later dissolution of siderite and glauconite. The syngenetic and diagenetic events have resulted in a reservoir with complex mineral and pore distributions.

C Sand Petrology. The major components in the C sand are quartzose grains, microporous glauconite grains, clay matrix, siderite, and porosity. Previous studies have provided a petrologic basis for the complex mineralogy and pore types and the effect on reservoir quality.

Matrix Clay. Clay in the reservoir consists mostly of mixed-layer illite/smectite, illite, and kaolinite It occurs as a dispersed matrix and as burrow fills and laminations. Water saturation increases with clay content due to the high surface area and bound water. Permeability and porosity degrade with increasing clay content.

Glauconite. Glauconite composition is extremely variable. Glauconite occurs as microporous grains. These grains consist of several forms of glauconite as well as many accessory minerals. The grains have significant intraparticle porosity and high surface area. The micropores have high capillarity and are believed to be filled with water in the Kuparuk River Field. Both microporosity and high surface area cause water saturation to increase with increasing glauconite content. Dissolution of these grains can lead to larger micropores and eventually to macroporosity.

P. 343