Summary

Pt. McIntyre field operates an enriched-gas-injection scheme that displaces oil in a multicontact miscible (MCM) displacement achieved through a combined condensing/ vaporizing (C/V) mechanism. A range of miscible-injectant (MI)compositions, varying from the minimum miscible enrichment (MME) to substantially above the MME, is available to the project. Field-scale numerical-simulation studies for the Pt. McIntyre field show that incremental oil recovery is nearly doubled when using the richest available MI composition.

At the MME, dispersion can act to substantially reduce oil recovery by reducing the concentration of enriching components in the near-miscible zone. Increasing enrichment above the MME compensates for this action. Accurate predictions of incremental oil depend on the numerical dispersion in a simulator being able to match the impact of physical dispersion. We show how compositional core data from an MI-swept interval provide confirmation of the impact of dispersion at field scale and demonstrate the appropriateness of the simulation model. The benefit of enrichment appears to be robust to the variation of reservoir description found at Pt. McIntyre and to whether the MI application targets incremental oil through liquid- or vapor-phase recovery.

Previous studies into MI enrichment have reported that for a four-component system, the mechanism changes from C/V to purely condensing as the enrichment level approaches first-contact miscibility (FCM). We show that with the addition of a small amount of heavy component, the C/V behavior is retained with increasing enrichment until transition into FCM. This makes it unlikely that a transition to a purely condensing mechanism will occur in real reservoirs.

Introduction

The Pt. McIntyre field, located on the north coast of Alaska, was discovered in 1988 and contains an estimated 800 million bbl. Production began in 1993, and the current production rate is approximately 55,000 STB/D. The field has a gas cap, and production is by gravity drainage in the region underlying the gas cap and by waterflood in the wedge zone. From inception, it has been planned to increase recovery from the waterflood region by means of an enriched-gas-drive flood. The field has been maintained at original pressure with voidage replacement by water injection.

The productive interval in the Pt. McIntyre field is the Kuparuk formation. The reservoir has moderate permeability and typically is approximately 300 ft thick. The oil is 27 degrees API.

The Pt. McIntyre field is produced into a common export system, the Trans-Alaska Pipeline System, and is surrounded by several other producing fields. Although limited in the amount of enriching components available from its own production, additional enriching components can be obtained by agreement with nearby fields. This provides greatly increased flexibility to select the MI enrichment level that optimizes field performance.

Previous studies of MCM gas-injection schemes have reported results for field-scale simulation models that show a significant increase in oil recovery with enrichment above the MME. This is contrary to the much-studied 1Dlaboratory scale approach to miscibility that shows a negligible oil-recovery increase for enrichment above the MME. The difference between the impact of enrichment at laboratory-scale and field-scale behavior is caused by the greater dispersion in field-scale systems. Unfortunately, the magnitude of field-scale physical dispersion is uncertain. The concern exists that field-scale numerical-simulation results are reporting the impact of dispersion caused by numerical truncation errors rather than physical effects and, consequently, may overstate the benefit of MI enriched above the MME. Here, we use compositional core data to show that the dispersion assumptions in the field-scale models are appropriate.

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