Design of a Definitive Field Pilot Test of Rich Gas Drive Recovery, Willmar Field, Saskatchewan

Abstract

Implementation of hydrocarbon miscible flood operations in the Willmar Field, Saskatchewan, requires a preliminary field test to define the design parameters, such as slug size and enrichment required, as well as to estimate how much additional oil may be recovered. The planning and initiation of a 4-hafive-spot pilot test is discussed, including the gathering of in-situ reservoir data, design of facilities having a high data acquisition frequency, reservoir engineering calculations and operating plans.

The project work has already revealed some unexpected. results, and greater insight into reservoir performance under miscible displacement is expected.

Introduction

The implementation of a field test blends business and technical viewpoints. For the test to be useful, results must be derived to explain how the miscible process works, indicate how it can be optimized and define design parameters (slug size, enrichment, gas/water ratio, etc.) for use in installing a full-scale scheme. From the business viewpoint, cost-effective equipment design and operation and effective use of government incentives must play significant roles. Planning must also include derivation of a reservoir performance base for the pilot, against which the final results can be compared quantitatively.

Miscible flood is the only type of enhanced recovery process currently attractive for Western Canada carbonate reservoirs, because commercial thermal, chemical and polymer processes are not predicted to improve reservoir performance. Sources of natural gas liquids, non-associated gas and pure carbon dioxide are limited in southeastern Saskatchewan, so plans were made to generate miscible fluid from the produced oil. Free gas saturations can be expected to develop as a result of replacement of methane by intermediates, so -alternate water and gas injection is planned to control gas breakthrough. After initial waterflood to provide a data base and placement of the initial rich gas slug, dry chase gas injection is planned to enable natural gas liquids to be recovered, followed by waterflood to depletion.

Reservoir Characterization

The Willmar Field comprises several pools containing under saturated oil trapped beneath the regional Mississippian unconformity. The most southwesterly reservoir underlying a 400-ha area in Sections 31 and 32-5-3W2, shown ill Figure 1, was chosen for further study, The reservoir quality and performance, described in Table 1, suggested a high likelihood for obtaining meaningful pilot results ⁽¹⁾. Development on 30-ha spacing has yielded 14 producers, completed primarily in the Lower Porosity ⁽²⁾, shown in Figure 2. Water production has steadily increased in Sections 31 and 32-5-3W2, as shown in Figure 3, and all wells now produce water. No substantial increases in gas-oil ratio have occurred.

Pool History

The early production history for the pool indicates depletion drive to predominate, with pressure decline from 11.7 MPa ⁽³⁾ initially to 9.2 MPa during the first six years. Natural water influx\ has since provided 80% of the reservoir energy. Pool production rates increased with development to 100 m ³ /d ⁽⁴⁾ in 1969, but have since declined as water-oil ratios increased. The water influx has been primarily from the northwest, where water-oil ratios average 5.6 m ³/m ³ compared to 0.6 m ³/m ³ in the southeast part of the pool.