(415a) New Reservoir Opportunities for Ccus: The Bio-Geochemistry of Mobile Water Intervals with Residual Oil: A Permian Basin Case History

The Bio-Geochemistry of Mobile Water Intervals with Residual Oil:

A Permian Basin Case History

L. Stephen Melzer[i], Robert Trentham[ii] and David Vance[iii]

ASChE Annual Meeting

Session on CO₂ Utilization/EOR

Pittsburgh, PA

October 28 – November 2, 2012

Abstract

An entirely new set of CO₂ EOR targets has exploded on the scene in the Permian Basin region of the SW U.S.   The commercial success of this group of reservoirs called residual oil zones (ROZs) has prompted a systematic reexamination of the science and distribution of their origins.  Three types of ROZs have been identified.  Much of the work to date has focused on carbonate reservoirs and the lateral sweep of a paleo-oil entrapment of dramatically larger thickness and lateral extent that would be suggested by looking only at the “conventional” main pay zone (MPZ) fields with a history of oil production.  The work is demonstrating that the ROZ rock properties display a slightly higher porosity and permeability when likened to the nearby or overlying MPZs.  A characteristic “bow” shape on the resistivity and porosity logs is generally present in the ROZs with decreasing resistivity and increasing porosity below the oil/water contacts.  The long held explanation of a downward transition from oil saturations of 75-85% in the MPZs linearly grading to zero oil saturation fails to explain both the shapes of many ROZ saturation profiles and the rock property changes observed in the ROZs.

Evidence will be shown to support a late stage diagenetic event(s) creating more pervasive dolomitization in the ROZs.  This overprint of dolomitization is accompanied by an otherwise uncharacteristic lack of disseminated anhydrite; evidenced not only in the observed cores but also in both the photoelectric cross section and spectral gamma ray logs.  The implicated process is sulfur extraction from the anhydrites by anaerobic (sulfate reducing) microbes (with consequential effects on porosity and permeability).  Occasional observation of free sulfur in vugs, sour oils and gas, and in sulfur-rich formation waters adds additional credence to the microbial process at work.

Finally, the combination of understanding the lateral sweep origins, successful modeling of the hydrodynamics, the rock property changes, and the success of CO₂ EOR ROZ projects is leading to a belief that the opportunities for CCUS may be much larger than previously believed including greenfields that are not associated with primary production.  The large reservoir targets for CO₂ storage and oil production identified by NETL and DOE in previous studies may be understated when the ROZ targets are more fully defined and characterized.