Observations on the Mechanisms of Solvent-Additive SAGD Processes
Paper Number: SPE 13 HOCC-P-324-SPE
Authors: Neil R. Edmunds – Director, EOR Advisory - Laricina Energy Ltd.
Source: This paper was prepared for presentation at the SPE Heavy Oil Conference held in Calgary, Alberta, Canada, June 11-13, 2013.
Solvent-additive processes (SAP) are a promising, but challenging technology. Perhaps thebiggest challenge from an engineering point of view, is that simulators probably work some of thetime, but not all of the time; and there is no information about where the line between occurs, orwhat the correct answer should be, after the line in crossed. Other serious problems are the manydegrees of freedom in SAP design process, and the non-linear relationships between processesinputs and economic results. There are too many possible designs to try randomly for even asingle reservoir, and there is limited theory to interpolate or scale available experimental data.This paper attempts to assemble some known pieces of the puzzle, and to explore how they mayfit together to explain and predict SAP performance characteristics.First, some familiar PVT relationships are presented, with examples using temperature as theindependent variable. This helps to clarify the choice of solvent, as a function of the reservoirpressure, and also to understand the effect of the increasing solvent “dose”. It is shown that SAPwill create a double front, one where water is condensed, and a second where the solvent isabsorbed by, and drains with, the oil. A vapor blanket separates the two fronts.Secondly, simple estimates are given for the temperature distribution in the vapor blanket (i.e.solvent-active zone). Together with the PVT data for the same pressure, these allow thethickness of a vapor blanket to be estimated.Finally, SAP mass transport limits are considered, by observing that the second font essentiallyconstitutes VAPEX. The Butler-Mokrys theory is discussed, in view of its failure to predict certainexperimental results; it is argued that this results from neglect of capillary pressure effects, whichin fact are dominant at the front. A purely empirical correlation by Nenninger is introduced, whichcan be rearranged to predict the maximum solvent speed, also as a function of temperature.