Abstract

This paper presents a case history study of a relatively small but highly productive Triassic sandstone oil pool under waterflood. The study indicated that several unusual factors respecting reservoir performance have come to light since production commenced in 1962. These include gas coning and fingering as well as water coning and tonguing. Gas fingering and gas and water coning have resulted in poor primary performance. Also water tonguing has caused more than 5 million barrels of calculated recoverable oil being trapped within the present water flooded area. Evaluation of the causes of these phenomena are discussed and particular emphasis is placed on the effect these will have on future operations in the pool.

The paper discusses the development history of Milligan Creek and the subsequent initiation of a waterflood scheme. Several model studies of the pool have been conducted. Two laboratory models and two numerical simulators have been utilized in evaluating past and future behaviour. Because the Halfway reservoir is essentially a discontinuous sand deposit and reservoir boundary is dependent upon pinchouts due to lateral facies changes, the combination of reservoir configuration and unusual reservoir performance has made this a difficult reservoir to simulate.

Introduction

The Triassic beds of northeastern British Columbia contain important reservoirs of oil and gas and may have total reserves surpassing Triassic sources elsewhere in North America. The hydrocarbon accumulations in this area can be related to the depositional history and post depositional deformation of the Triassic. Oil production in the Triassic is dependent upon porosity pinchouts due to lateral facies changes. The oil pools of the Milligan area produce from discrete sands which were deposited in either a barrier or tong-shore bar type of environment. In the Beatton to Bulrush trend, the largest sandstone oil pool – Milligan Creek field originally contained about 100 MMSTB of oil in place. This field is located approximately 70 miles north of the town of Fort St. John, British Columbia, as shown on Figure 1. The Halfway reservoir is a stratigraphic trap with two separate gas caps overlying the oil column and also underlain by one or more limited aquifers. The eastern part of the pool (Milligan Creek Unit No.1) Which initially contained about 90 MMSTB of oil in place, was unitized in May, 1964. The Unit cumulative oil production to Dcembcr 31,1972 of nearly 30 MMSTB represents about 33 per cent of the original oil in place. The development history)C, engineering detail of construction of facilities, and the results of four years of unitized operation of Milligan Creek Unit No. 1 were discussed in prior articles in the literature 2,3. Although this waterflood scheme is a successful operation, certain unusual reservoir performance factors have been indicated during the pool production history. This paper presents the review of pool performance and the results of reservoir studies of the Milligan Greek Unit No. 1 in an effort to provide a basis for evaluating and controlling field operations in this type of reservoir.

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