Rock-Fluid Relationship Studies On the Windfall D-3A Reservoir AndTheir Application In Evaluating Gas Cycling Effectiveness
Authors: MacFadzean T.B. 
Published: March 2010

Abstract
The Windfall D3A reservoir in north-central Alberta is a Devonian Leduc D-3 reef structure containing sour gas, condensate and oil. This paper describes the results of recent studies on rock-fluid relationships for the Windfall reservoir which led to several conclusions regarding the effectiveness of the past 15 years of cycling and the importance of maintaining high voidage replacement ratios.
It is postulated that the sweep efficiency of the cycled gas in Windfall is low due to the heterogeneous permeability distribution within the reservoir. It is concluded that liquid recoveries can be further enhanced by altering production-injection flow patterns aimed more at main-taining or increasing energy in certain segments of the reservoir rather than by depending entirely on a high sweep efficiency of cycled gas from a fixed injection-production well pattern.

Introduction
DRY GAS CYCLING PROJECTS are an important means of enhancing liquid recoveries from gas condensate reservoirs. Many factors, such as injection patterns, replacement ratios, displacement efficiendes, water influx, rock and fluid characteristics, etc., have an effect on the recovery efficiency of the cycling project. Over the past 15 years, the Windfall reservoir has been cycled under a variety of conditions that were altered as knowledge of the reservoir increased and as external factors changed. In 1974–75, a detailed petrophysical study of the Windfall field was undertaken to determine if liquid hydrocarbon recoveries could be further enhanced. Several conclusions were reached which indicate that voidage replacement ratios of one are required to inhibitwater influx and to obtain maximum liquid hydrocarbon recoveries. The rock properties and rock-fluid relationships and their influence on the effectiveness of the cycling project are discussed in this paper.

Windfall Production History
The Windfall field was discovered in 1955. It is a dolomitized Devonian Leduc D-3 reef structure containing sour gas, condensate and oil. Between 1955 and 1962, the field was developed and a pilot cycling operation was tested"'. In 1962, a full-scale gas cycling project was put into operation. Dry sour injection gas is obtained from the Pine Creek D-1, Pine Creek D-3 and Pine NW D-3 fields (Fig. 1). It is injected into the Windfall D-3 reservoir to maintain pressure and to sweep condensate-rich gas to the producing wells. Wet sour gas is produced from Windfall to the West White court gas plant. Condensate and sulphur are stripped out of the wet sour gas, and the sweet gas, condensate and sulphur are sold.
A structure map of the Windfall D-3A reaf above the original designated water line is shown in Figure 2. A schematic cross section of the reef is shown in Figure 3.
Replacement ratios in Windfall were kept high (80 to 100 per cent) until mid-1967, at which time deliverability problems from the source gas fields began to develop. Revaporization studies carried out in the mid-1960's'" indicated that there would be a negligible loss in ultimate liquid recoveries through retrograde condensation if a 60% voidage-replacement ratio was maintained through controlled cycling.