By KATHY SHIRLEY
EXPLORER Correspondent

Study Compares Reservoirs

Deepwater Pools Share Attributes

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How much influence does deepwater depositional environment have on hydrocarbon recovery?

An important question, given the fact that oil companies are spending billions of dollars all over the world exploring and developing deepwater reservoirs, which can range from relatively simple sheet-like deposits to more complex channel deposits.

Such projects are hugely expensive, meaning that business decisions should be based on a clear understanding of the productive reservoirs.

Considering the question was the challenge for Dave Larue, senior staff research geologist with ChevronTexaco E&P Technology in Bakersfield, Calif., who with co-author Yongjun Yue tried to find answers.

"This is a multi-billion dollar question," Larue said. "The petroleum industry is spending tens of billions of dollars developing deepwater reservoirs, yet we, as an industry, don't have the experience in these settings that we do in fluvial or shoreface reservoirs."

To address these issues, he and co-workers began constructing geologic models and performing flow simulation studies to better understand the unique difficulties associated with deepwater reservoirs. They wanted to know:

• How deepwater reservoirs produce.
• What factors might limit their production.
• What geologic characteristics might influence recovery.

"I am a stratigrapher and, of course, I always felt depositional environment would be a key to recovery efficiencies," Larue said. "I wasn't alone. Before we started work I talked to both engineers and geologists, and everybody agreed that radically changing the stratigraphy in a model would dramatically impact recovery."

The studies, however, told a different tale: The geologist and engineer perceptions about what most strongly affects recovery are not always the dominant factors.

"We did see some differences in recovery factors associated with simulating different geologic models, ones in which the depositional environment was visibly altered," he said, "but they were more subtle than we expected."

It's a Puzzlement

Puzzled, Larue combed the literature to try and understand what factors contribute to recovery efficiencies.

A paper by Noel Tyler and others at the Bureau of Economic Geology in Austin, Texas, is the most widely known and cited study on the relationship of stratigraphy to recovery efficiency, he said. That study looked at 450 major Texas petroleum reservoirs, and the results indicated that average recovery efficiency could be closely tied to depositional environment and drive mechanism.

Depositional environments studied ranged from fluvial to shoreface to deltaic to submarine fan. Particularly troubling in that study was the data for mud-rich submarine fans, which attained average recovery efficiencies less than 20 percent.

However, because the study plotted recovery efficiency against drive mechanism and depositional system, it was difficult to pinpoint which of these two factors was more influential. Nor did it address how other factors like fluid properties, permeability architecture, well count or well placement might influence recovery efficiencies.

Larue also discovered a 1998 study that took an integrated engineering approach to analyze factors affecting recovery efficiency mostly via waterflooding, and learned that factors like permeability heterogeneity, fluid mobility ratio, the oil gravity and early onset of waterflooding had significant impact on recovery efficiencies.

"Because we had these puzzling results in our geologic model and flow simulation studies compared to previous literature, I decided to go back into the actual field data to see if I could support what Tyler and others had indicated," he said, "or if I saw something different."

That's what ultimately prompted a comparative reservoir database study by Larue and Yue, a post doctoral student at Stanford University who is examining how stratigraphy influences oil recovery in deepwater reservoirs.

The two geoscientists used six reservoir databases to assess the relationship between recovery efficiency and depositional environments, with the emphasis on deepwater environments.

Due to the incomplete and unevenly distributed nature of the data in the reservoir databases, the pair relied on semi-quantitative trend analysis, logic and their experience -- much like Tyler and others did in their study.

Larue and Yue used the databases to address several questions regarding uncertainty in recovery from deepwater reservoirs:

• They examined what types of recoveries are expected from deepwater environment reservoirs, combining channelized and sheet-like reservoirs.
• They looked at whether recoveries from deepwater environment reservoirs are different from other depositional systems, such as delatic, fluvial and shelf reservoirs.
• They studied what other properties in the database can be used to predict recovery efficiency.

Regarding recovery efficiency versus drive mechanism for deepwater environment reservoirs, they found that the lowest recovery efficiencies are associated with primary depletion with no aquifer support, or solution gas.

Fiftieth percentile values for recovery efficiency for this type of depletion range from 12 to 25 percent. A fiftieth percentile range of 26 to 36 percent typifies recovery efficiencies associated with drives other than pressure depletion/solution gas.

"Clearly, this aspect of the study demonstrates the well-known observation that drive mechanism affects recovery efficiency," he said.

They then kept the reservoir drive mechanism constant and varied the depositional environments. For reservoirs produced by waterflooding or with a strong aquifer, the 50th percentile recovery efficiency for all environments shows ranges from 28 to 48 percent, averaging about 33 percent.

Deepwater environment reservoirs produce grossly similar recoveries as other depositional systems.

"Based on our comparative database study," Larue concluded, "there is no clear indication that depositional system is a strong factor in determining recovery efficiency."

The Subtle Relationship

While these findings seem to be in conflict with the well-known Tyler study, Larue said that is not necessarily true; the first study did not break down the impact of depositional systems independent of the drive mechanism, making it difficult to precisely determine the affect of depositional environment alone on recovery efficiencies.

In addition, the newer databases have many more examples than used in the Tyler study.

The study indicated, however, a relationship between recovery efficiency and reservoir permeability, API gravity and field location offshore or onshore. There is a general trend of increased recovery efficiency with an increase in average permeability and higher API gravity -- something documented in other studies.

But while the origin of the relationship between average permeability and recovery efficiency may seem obvious initially, it is actually more subtle, Larue said.

"It is well known that permeability heterogeneity influences recovery efficiency, but permeability heterogeneity is a measure of variance of permeability values and is different than average permeability," he said.

"Average permeability should only affect recovery efficiency if there are rate constraints or time limitations -- that is, the relationship between average permeability and recovery efficiency, with no knowledge of permeability heterogeneity, is a factor of project economics and time constraints on production.

"Apparently, recovery in an offshore setting is better than an onshore setting," he continued, "but this relationship may reflect project economics as well -- that reservoirs produced in offshore settings are governed by different economics than onshore reservoirs, and that higher recoveries are required for a reservoir to be economic."

Coming to a Conclusion

Larue and Yue also examined whether these databases could define downsides (worst-case scenarios) and upsides (best case) for reservoir analogues, probing all the databases for deepwater reservoirs produced by waterflooding or water drive that had produced poorly (less than 20 percent recovery efficiency) or superbly (greater than 50 percent recoveries).

The exercise did not prove particularly useful.

"Either the reason the waterflood performed poorly was obvious -- heavy oil, fractured low permeability reservoir, diatomite reservoir -- or could not be discerned from the available information," Larue said.

The conclusion: "If there is a difference in recovery efficiency associated with different depositional environments, we were not able to capture it," Larue said. "We are not saying there isn't a relationship between the two, we are just saying there are a lot of other features that are more directly related to recovery, such as permeability, fluid type, relative permeability and connectivity of the reservoir.

"As a stratigrapher I expect stratigraphy to dominate," he continued, "(but) information from these databases indicates that any stratigraphic component is clouded in other factors."

A positive aspect of this finding is that companies can develop deepwater fields and not worry that deep marine environments are going to have significantly different recoveries than other environments.

"As a businessman, this overview study gives us faith," he said. "The impact of this study is significant for business decisions.

"Of course, the study gives the big picture and we all know the devil is in the details," he added. "Deepwater fields take different reservoir management techniques due to different reservoir architecture, wider well spacing and the need for more robust, trouble-free wells -- just to name a few issues.

"But this study does indicate that recovery efficiency from deep marine reservoirs is not fundamentally different than other depositional systems."