Monitoring waterflood operations: Hall's method revisited

Hall’s method is a tool for evaluation of an injecting well performance. It is based on the assumption of radial steady-state flow. Besides times series of historical injection pressures and rates, rigorous implementation of Hall’s method requires information about the ambient reservoir pressure. In addition, it is assumed that the influence domain radius is constant during the observation period. Neither of these parameters can be measured directly.

This paper discusses a new method called slope analysis. It is based on analysis of the variations of the slope of the plot of the time integral of pressures versus cumulative injection volume. In particular, it produces an estimate of an apparent average reservoir pressure. This method requires only injection pressures and rates data, which are routinely collected in the course of a waterflood. Thus, slope analysis requires no interruption of regular field operations.

The method has been verified with numerically generated pressure and rate data, and tested with field data. In both cases it proved to be accurate, efficient and simple. The estimated ambient reservoir pressure can be used to correct the Hall plot analysis or to map the average reservoir pressure over several patterns or an entire waterflood project. Such maps can then be used to develop an efficient waterflood policy, which will help to arrest subsidence and improve oil recovery.