Abstract

An extensive reservoir management program in a hydrocarbon miscible flood has allowed the reservoir team to stay abreast of reservoir performance and to influence ultimate recovery of oil.

A key component of this program, operated by Canadian Hunter Exploration Ltd. at the Brassey miscible flood in British Columbia, included the downhole manufacture of tritiated methane as part of an innovative tracer injection program. Other technical elements in the reservoir management program include frequent pressure surveys, volume monitoring, and compositional analysis.

Teamwork between geologists, reservoir engineers, production/facilities engineers, and field operations staff was absolutely essential to the success of the program.

Introduction

The reservoir management program at Brassey has enabled the business unit reservoir team to evaluate performance and influence ultimate recovery of oil from the miscible flood. The program was implemented by field operator Canadian Hunter Exploration Ltd., with the assistance of its partner, BP Exploration Inc. and affiliate BP Research. The program included depletion design using full-field black oil and later compositional modelling; a regular, frequent data collection and monitoring program; and feedback of results to field operations staff and to the reservoir simulation team.

Key objectives in managing the field arc to maintain pressure above the minimum miscibility pressure, and to balance flood fronts in each pattern to minimize breatkthrough gas production. Technical elements in the reservoir management program include an innovative tracer program which involved the first downhole manufacture of tritiated methane, frequent pressure surveys, voidage monitoring, and compositional analysis.

The need for a tracer program was identified early in the design of the five-spot pattern gas miscible flood in order to determine the source of gas breakthrough. Tritiated methane (CH3T), krypton-95 (Kr-85), and sulphur hexafluoride (SF6) were selected as tracers of choice in the five-pattern flooded area. Other tracers considered were halocarbons Freon-11, Freon-12, Freon-13B1 and Freon-114. A potential problem with radiolytic decomposition of tritiated methane led to development of an on-site, downhole manufacturing method.

Tracer results to date have made it possible to identify the origin of breakthrough gas at several producing locations. This gas breakthrough has been counteracted by making adjustments to offset well production rates and by injection reallocation. Tritium and krypton have been detected at several wells, verifying the efficacy of the injection scheme. Sulphur hexafluoride has not yet been detected. Using the results of the surveillance program, an effective history match is being constructed for a full-field compositional model, and the information gleaned from the tracer results in particular has added considerable confidence to the accuracy of the match.

Reservoir Description

The Brassey field, located in northeast British Columbia, Canada (Figure 1), produces oil from the Artex member of the Triassic Charlie Lake formation. The Artex lies at a depth of 10,000 feea, forming a stratigraphic trap with average net pay thickness of 10 feet, with porosities averaging 16 percent, water saturation of less than 2 percent and permeability of 152 md.

The reservoir sand is interpreted to be an aeolian-deposited sand encased in an evaporitic platform sequence'. lateral sand pinchout forms an effective reservoir seal and renders the Brassey field a closed system. The sand is predominately quartz with minor amounts of chert, feldspar, sulfate and dolomite grains.

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