An Analysis of Cyclic Steam Stimulation Projects in the California Opal A Diatomite

Abstract

Cyclic steam stimulation (CSS) in the California Opal A diatomite has been a successful commercial oil recovery technique since the 1990s. This paper analyzes the production performance of several California CSS diatomite projects comparing actual and calculated steam-oil ratios, SOR, as a function of oil recovery to ascertain whether a project is performing up to its full potential. SORs are computed using simplified Buckley-Leverett fractional flow theory applied to steam injection. SOR, or its reciprocal oil-steam ratio (OSR), have always been useful qualitative indicators of CSS project performance; but have not been used to optimize production performance other than to say operators should strive to keep the SOR low, or the OSR high. We show that an SOR equal to about 1 is what a CSS diatomite project should achieve through the recovery of about 20% of the oil-in-place, or more, to yield the highest oil production for a given set of conditions. A corresponding “optimum” steam injection rate is discussed. Projects that are managed this way have the highest average oil production rate per well and the lowest instantaneous SOR. This study shows that steam over-injection in the diatomite is common even when the injected cold water equivalent (CWE) steam is “in balance” with the oil and water that are produced. Although not discussed in detail, we postulate that the low permeability diatomite has an inherent low rate of heat absorption compared to other oil bearing rocks that can cause too much vapor to accumulate around the well when steam injection becomes excessive. Relative permeability effects develop during flow back that give way to (a) the preferential flow of steam to the producer and (b) constrained re-saturation of the heated zone. Avoiding this tendency helps insure good project performance including high oil flow rates and high oil recovery. Although the calculation method we discuss is used for CSS in the low permeability diatomite, it should be helpful in all CSS applications. Improving CSS steam practices will have the added benefit of lowering the carbon intensity of the oil that is produced. Our analysis uses published data for the needed input parameters including information from the California Division of Oil, Gas and Geothermal Resources (DOGGR) internet web data base.