Despite the widespread application of reservoir simulation to study waterflood reservoirs, petroleum engineers still need simple predictive tools to forecast production decline, estimate ultimate oil recovery, and diagnose the production performance from the historical field data.
On the basis of the Buckley-Leverett equation and the assumption of a semilog relationship between the oil-to-water relative permeability ratio and water saturation, a consistent analytical solution can be derived as:
where fo is the oil fractional flow, tD is the fraction of cumulative liquid production to related formation volume, B is the relative permeability ratio parameter, and EV is the volumetric sweep efficiency. Two equivalent linear plots can be developed: a log-log plot and a reciprocal time plot. The log-log plot has a slope of -1 and intercept of EV/B. The reciprocal time plot has a slope of EV/B and an intercept value of 0. Both plots can be applied for the diagnostic analysis of waterflood reservoirs.
Model and field case studies show the benefits of this technique as a production-decline analysis tool in forecasting the waterflood production decline and the ultimate oil recovery. This method can also be applied as a diagnostic tool to evaluate various aspects of waterflood performance. Examples include assessing waterflood maturity, calculating volumetric sweep efficiency, distinguishing the normal waterflood breakthrough from the premature water breakthrough through hydraulic fractures, and examining the consequences of operational changes. The appropriate use of this analytical method will help to optimize the field waterflood operation.
PZhengming Yang is a senior reservoir engineer with Aera Energy LLC in Bakersfield, California, USA. He works on reservoir simulation projects, performance diagnosis and forecasts on the basis of field production data. Before joining Aera Energy LLC, he worked on reservoir simulation projects for heavy and conventional oils in USA, Venezuela, and China. He holds a diploma in chemical engineering from Tianjin University in Tianjin, China; an MS degree in petroleum engineering from the Research Institute of Petroleum Exploration and Development in Beijing, China; and a PhD degree in petroleum engineering from the University of Southern California, USA.
Katherine Wallgren, Forum Chairperson, Katherine_Wallgren@oxy.com