Reservoir Management For Waterfloods

Abstract

Abstract Waterflood projects account for over half the current Canadian and U.S. oil production, so the reservoir management of waterfloods is a key issue. There are numerous published textbooks and simulation methods for the design of waterfloods, however the literature has to a great extent been silent on reservoir surveillance to help monitor and improve existing waterfloods. Often the "operating" engineer has a rate and reserve forecast that often over estimates performance. When comparing actual to predicted waterflood performance, the typical conclusion is that the forecast input data is based on averaged data and is therefore too homogeneous. Consequently, the forecast can be of limited use to the reservoir management team. The methods presented here emphasize practical uses and their ties to field data and geology. Production and pressure surveillance data can implicitly account for a useful scale of heterogeneity. Therefore this data can be extremely useful, if used properly, in developing changes in operational strategy that can maximize recovery. This paper describes a simple, direct approach to the reservoir management and analysis of waterfloods. This approach is used in preparation for simulation studies, to quantify the factors limiting recovery and determine if the oil recovery can be improved. Typical Objectives for Analytical Work In general the questions that need to be addressed in order are:What is the OOIP?Where is the current OIP?What are the factors limiting recovery?Can we improve oil recovery economically?How do we improve recovery? There has been a tendency for engineers to proceed with points four and five first and bypass points one to three. This is a major mistake. Most often, reservoir or simulation studies can have non unique solutions. For example, it is easy to interpret a waterflood failure as being due to poor displacement efficiency when actually poor volumetric sweep efficiency may be the primary reason for the problems. Therefore, to reduce the chances of misinterpretation it is important to understand the amount and distribution of original and current oil in place. The understanding of flow patterns and the distribution of movable oil saturations are key to limiting the chances of misinterpretation. A fundamental geological/petrophysical analysis is a cornerstone of good reservoir engineering analysis. However, geological studies alone do not conclusively quantify the reserve and oil rate increases that can be achieved by optimizing the existing waterfloods. While this paper concentrates on the engineering criteria, it is implicitly assumed that a thorough geological/petrophysical study is either done or being done concurrently. A geological/ petrophysical study is key in understanding the initial question: What is the OOIP? It is absolutely critical that the engineer develops an understanding of the reservoir geology as they proceed. In particular the engineer should concentrate on megascopic permeability and porosity trends, as well as reservoir continuity. In other words the engineer should concentrate on hydraulic flow units. Surveillance Level This level of analysis should start from the large scale and proceed to the smaller scale.