Abstract
Reciprocal Productivity Index (RPI) is graphical production analysis method that provides us with the technique/ability to estimate descriptive engineering parameters, such as permeability, skin, drainage area, and to generate production forecasts and reserves based on those parameters, without the cost of full numerical simulation or extended buildup tests.
The method's theoretical basis arises from the fact that traditional constant rate or constant pressure (potential) boundary conditions are sufficient, but that the necessary boundary condition only requires that the sandface transmissibility remain constant over time. With that difference, it is possible to accurately evaluate production histories, in which both the rate and pressure are varying over time, using traditional well testing methods.
The reservoir average pressure can also be evaluated from routinely available rate and flowing pressure production data, using an extension of this reciprocal productivity index method. Traditionally, reservoir average pressure could only be determined from an extended duration build-up test. This procedure generally tends to underestimate the pressure, due to practical limitations on shut-in times. In addition, an error in the reservoir average pressure determination results in an error in the computed skin for the well. However, this new procedure provides an independent evaluation of skin and pressure so that they are not dependent on one another. The theory for the method is in the appendix.
This paper explains how we used this technique to evaluate engineering parameters and validate the estimated reservoir pressure using reverse material balance.
