Evaluating Waterflood Feasibility in a Complex Reservoir System

Abstract

Abstract The merging of ideas from inter-dependent geological, geophysical und reservoir engineering interpretations is described as the first step in the study of a complex pinnacle reef reservoir system. The reservoir study process is then followed through the model program selection, input data generation and model testing phases. The interpretation of the 1results from the performance match is described, leading to conclusions regarding reservoir operation in the model space. The interpretation of the results from the subsequent model depletion studies in terms of the real world then leads to determination of the reservoir qualities required to yield a reasonable chance of water-flood success. The analysis of the study techniques described is accompanied by examples of the specific study methods applied in performing the water-flood feasibility study on some of the pinnacle reef pools in. the northern portion of the Zama Basin in northwestern Alberta. Introduction THE PROBLEM of evaluating the waterflood feasibility of a given pool within a complex reservoir system can be very difficult, particularly in a reef environment with many small pools (which can contain only one or two wells). The purpose of this discussion is to describe the techniques developed in formulating a numerical model, together with the independent analysis of reservoir performance factors to define the correspondence between the model space and the real world, The logical operations leading to conclusions regarding probable waterflood performance of given pools will be amplified by reference to the studies performed by Gulf Canada in the Zama Basin of northwestern Alberta. Underlying Philosophy Development of a mental picture of the reservoir-aquifer system is considered to be the most important aspect of commencing a multi-pool study, along with the assembly of a set of tentative theories regarding the courses and relative effects of fluid movement within and between hydrocarbon-bearing pinnacles. The standardization or "history match" phase of the numerical simulation studies can then be used as a testing ground for these hypotheses, within the framework of the continuity equation, the physical description of the system and the well performance. A sequence of revisions to these theories should then be based on the results of the initial series of model runs. The final goal in the reservoir modelling phase of the study is the development of a set of working hypotheses which agree with all aspects of the observable reservoir data, with as few unexplainable exceptions as possible. The consensus of reservoir engineering opinion is usually the basis for the initial set of theories. The usual assumption in a multi-well reef development is that each reef can be geometrically defined with reasonable accuracy, and the primary concern early in the study is to derive an adequate reservoir description. The assumption is usually made that water or excess gas production early in the life of each reservoir can be attributed to coning, with saturation effects becoming more important later.