Analyzing Waterflood Responses for Pekisko B
Authors: Fedenczuk L. , Pedersen P. , Marshall M. 
Published: March 2010

This paper focuses on the fluid communication and injection responses in the Pekisko B Pool between January 1990 and September 1997. The analysis was based on responses of producers to changes in the injection rates. The strength of oil response is measured as the correlation between the oil rate and the injection rate. In a similar fashion, the strength of the total fluid responses is measured as the correlation between the total fluid rate and the injection rate. These responses and the corresponding time lags are presented in the form of special XY-diagrams. One form of these diagrams shows normalised responses of wells around each injector. The second form shows the long distance communication between injectors and producers that are characterised by strong responses above the predefined level.

Results indicate very good short and long distance communication between injectors and producers. Waterflood responses can be segmented into three geographical areas: northern, central and southern. The oil responses correlate strongly to the injection in the central part of the pool. Weaker responses and longer time lag response was observed in the southern pool area, and strong interference between injectors was detected in the northern pool area. A more detailed analysis, involving pressure data, geology and completion intervals, demonstrate that the existing waterflood inefficiencies are related to inadequate injection volumes in the southern pool area and poor injection well distribution in the northern part of the pool. The study proved that the applied methodology is very efficient in diagnosing the waterflood performance and helping to optimise waterflood design.

This analysis very quickly identified communication or lack there of, which had only partially been understood through lengthy geological study of the pool. This method should be used in the early stages of study to very quickly "zero in" on areas of difficulty due to complicated configuration or geological/ depositional complexities.

Introduction

Location and History

The Pekisko B Pool is located in Township 38–03W5. Discovered in 1962, it is the largest oil pool in the Sylvan Lake Field and the third largest oil pool in the Gilby, Medicine River, and Sylvan Lake Field areas. The depletion mechanism up to 1994 was primary depletion, with produced water being reinjected in the downdip portion of the pool. The pool pressure declined from approximately 17,000 kPa to approximately 11,000 kPa by 1994. Cumulative production at that time was 14.9 MMstb oil, 21.0 Bcf gas and 21.3 MMstb water. Injection was 23.6 MMstb. OOIP is estimated at 70 - 100 MMstb. It is believed there is no associated gas cap and there is a weak water drive due to the Pekisko water leg.

Ocelot purchased controlling ownership in the pool in 1993 and recognised that huge potential existed from secondary (waterflood) recovery. Waterflooding was implemented in July 1994. A eservoir simulation study, with the ultimate aim of maximising secondary recovery, was initiated in late 1993. It soon became evident that the reservoir was far more complex than initially thought.