From the President

Dick Merkel
EnCana

Next month (April) we will be having our second workshop of the year; once again hosted by the Colorado School of Mines. The topic of the course will be on core measurements and how these data can be used in log analysis modeling. As you will note on the announcements, this will be on April 14 (which is before our April meeting).

The course will include (but not limited to) core measurements and analysis of tight gas sands, shale gas, and coal bed methane. We will have experts from the industry such as Matt Mavor (TICORA) and Robert Lee (Core Lab) as speakers discussing how the various measurements are made, and the accuracy of these measurements. Experience has shown that depending upon the desired core measurement, there is considerable variability in coring procedures, transportation, storage, and sample preparation. Since we plan to have a round table discussion at the end of this course, it will be an excellent opportunity for you to get input from industry experts on both why and how you do your coring, as well as what measurements are required for a desired final product. 

From the Editor

Dominic Holmes
Digital Formation

As Dick mentioned above, the 2008 DWLS Spring Workshop on Special Core Analysis is coming up on April 14^th.  We have a preliminary announcement posted now, which has all the basic information.

If you're interested in signing up for this, please contact me at Dominic Holmes or 720-240-0603.  And please tell your colleagues about this as well; last time our #1 method for attracting non-members was through word of mouth.

The deadline for accepting reservations is March 28^th, so don't delay.

"The state of fluid saturation in tight-gas reservoirs: Insights and implications
from the Rocky Mountain basins"

Keith W. Shanley
The Discovery Group

Log analysis techniques attempt to discern the relative proportions of formation water and hydrocarbons within reservoir rocks in the hopes of guiding completion decisions, aiding in volumetric assessments of hydrocarbons, and deciphering production trends. In most conventional petroleum provinces there are close correlations between log calculated water saturations, hydrocarbon producibility, and hydrocarbon column height. In most tight-gas basins, however, these relationships do not evident.

Implicit to the interpretation of water saturation and hydrocarbon productivity is the assumption that the reservoirs are in a state of primary drainage equilibrium with respect to capillary pressure. We find that in many tight-gas fields and adjacent non-productive wells there are inconsistent relationships between saturation and height, productivity, and water production. Water saturation-height trends often differ by little more than 5-10 points from top to bottom of a reservoir interval, in some cases over several thousand feet, and we rarely observe truly high water saturations associated with water-bearing intevals. Log calculations in many waterbearing zones imply significant hydrocarbon saturation. Likewise, capillary-pressure curves provide little assistance in terms of predicting fluid distribution and petrophysical rock-typing
often yields conflicting insights to performance. Because of these poor relationships it has been suggested that calculated water saturations must be in error due to incorrect electrical parameters, errors in water resistivities, excess conductivity due to clays, etc. Implicitly, if we just had the 'right' parameters or used the correct model, we could better describe the saturation state of the subsurface and achieve a better correlation to production.

Perhaps the fundamental problem lies not in our choice of saturation model or the various parameters required, perhaps the fundamental problem lies in the assumption that tight gas reservoirs are in primary drainage equilibrium with respect to capillary pressure. Basin history models suggest reservoirs and fields were charged with hydrocarbons when porosities were greater, perhaps by a factor of 2, and permeabilities were much greater, perhaps by a factor of 10
- 1000 times, the values found today. This initial charge was likely a drainage capillary process. During subsequent uplift and structural reorganization, many (most?) reservoirs departed from their primary drainage equilibrium state and are better described by a state of imbibition (primary or higher) or a state of secondary (or higher) drainage equilibrium. Outside of trap geometries, many reservoir intervals approach residual gas saturation as a result of gas re-migration and trapreorientation.  Residual gas saturation is largely controlled by initial saturation and pore geometry and may vary from approximately 25% Sw to as much as 80% Sw. As a result, it becomes extremely difficult to distinguish near residual gas saturation from saturations associated with economically attractive gas columns. The key to improving tight-gas formation evaluation is understanding and recognizing the geologic history that leads to a non-primary drainage equilibrium state and developing new methods to identify residual (near residual) saturation.

About the Speaker

Keith W. Shanley is a consulting geologist with the Discovery Group in Denver, Colorado with more than 25 years of experience in petroleum exploration, development,and research. He has worked in a variety of basins around the world for both major andindependent oil and gas companies. Keith has published numerous papers, editedvolumes, and organized conferences and seminars dealing with sequence stratigraphy, it'sapplication to reservoir characterization and prediction, non-marine sedimentology and stratigraphy, and tight-gas resources and petrophysics. His research interests include sequence stratigraphy and reservoir architecture, the integration of petrophysics, risk analysis and unconventional hydrocarbon resources. In 2004 John Robinson and Keith co-edited an AAPG Studies in Geology volume dealing with Jonah Field and in 2005 Keith was a co-convenor of the Vail Tight Gas Hedberg Conference. Keith's work on tight-gas resources has been recognized by the American Association of Petroleum Geologiststs which awarded him and his co-authors the 2006 Pratt Award for best paper, and by the Canadian Society of Petroleum Geologists which awarded them the 2005 Medal of Merit for the most significant paper pertaining to the petroleum geology of Canada.

Keith Shanley was born in The Hague, The Netherlands and moved to the United States to attend University. He received his BA degree in Geology from Rice University, Houston, Texas in 1978 and his MSc (1983) and PhD (1991) degrees in Geology and Geophysics from the Colorado School of Mines in Golden, Colorado. He is a member of the AAPG, SEPM, SPE, RMAG, and SPWLA and is a registered petroleum geologist in several states within the United States.