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
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
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
"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
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,"
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
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
"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."