Abstract

During 1986, PanCanadian Petroleum Limited completed 57 infill development wells within the Weyburn Unit.

The average initial oil production per well was 24 m3/d. compared to an expected rate of B to 10 m3/d.

This paper reviews the development of the Weyburn Unit, and, more specifically, details the evolution of the completion practices and procedures through the 1986 program. The stimulation program featured rigorous quality control and an acid pickling procedure. The application of this pickling procedure increased initial oil production by approximately 20% for a relatively minor incremental expense.

Introduction

The Weyburn Field is geographically situated in the north-west quarter of the Williston Basin (see Figure 1). The discovery well, Central del RIO Ralph 14–06-07-13 W2M, was drilled in 1955 to a total depth of 1 562 m. The productive formations are the Midale Beds (Mississippian Age) at approximately 1 450 m.

The field was unitized in 1963, and an inverted nine-spot waterflood system commenced operating in 1964. PanCanadian operates the unit which involves a total of 57 partner companies and 1ndlv1duals. PanCanadian holds a 20.56% working interest in the unit.

DISCUSSION
Geology

The Midale Beds can be separated into three lithological sections. In descending order, these units are termed as the Midale "Marly", Midale "Vuggy", and the Frobisher Evaporite (see Figures 2 and 3).

The Marly's composed of thin bedded, microcrystalline. slightly argillaceous dolomite. The Vuggy consists of interbedded, occasionally porous pack5tones and thin, tight layers of mudstone and wackestone. The Frobisher Evaporite varies from a massive anhydrite to a tight, dolomitic limestone.

The lateral continuity of reservoir rock ranges from good in the Marly to erratic in the Vuggy_ An important factor influencing reservoir properties in the Vuggy is the presence of vertical microfractures.

2. Completion Procedures
2.1 Perforating

Perforating techniques have been designed to optimize selective acid 1njection. To this end, the length of the perforated section is normally restricted to 2 m, with a minimum of 1 m between perforated 1ntervals. This technique enables each interval to be selectively acidized, and reduces communication of the acid behind casing.

The perforation shot density normally used is 13 shots per metre (SPM). Where total fluid 1S expected to be 50 m3/d or higher, a shot density of 19 SPM is considered.

2.3 Cement Bond Logs

As a general practice, PanCanadian does not normally run cement bond logs (CBL). Instead, Temperature/tracer logs are run when annular isolation problems are suspected.

This type of log has two distinct advantages. Firstly, it is a more diagnostic tool than the CBL, and can usually pinpoint the water source, and secondly, it is run only after problems are encountered. This means that the cost of this log (which is comparable to a CBL) is only incurred on problem wells.

2.3 Acidizing

The Midale Beds are carbonate members which are normally stimulated with hydrochloric acid (HCl).

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