World Oil
®
Originally appeared in
JUNE 2014 issue, pgs 50-56. Posted with permission.
CORE ANALYSIS
Digital core flow simulations accelerate
evaluation of multiple recovery scenarios
Digital core flow simulations can
now replicate familiar lab results.
Consequently, operators will be able
to dramatically shorten traditional
cycle times, better understand
increasingly complex reservoirs
before making costly field decisions,
and maximize both short-term
production and long-term recovery.
ŝŝMARK A. ANDERSEN, Schlumberger
Analyses of rock samples in laboratories worldwide provide a wealth of information about key rock
and fluid properties, including lithology, porosity,
permeability, fluid saturation and multiphase flow,
enabling operators to better characterize pore systems in oil and gas reservoirs. Physical core measurements allow geoscientists and reservoir engineers to
better understand depositional environments, evaluate exploratory wells, assess field redevelopment potential, and investigate early production strategies, as
well as improved recovery scenarios.
To boost recovery in conventional sandstone and
carbonate reservoirs, it is essential to identify which
fluid types, injection rates, volumes, and other engineering parameters are likely to have the greatest
impact on performance. In unconventional shales
and mudstones with complex vertical and lateral heterogeneities, it is vital to determine which subtle rock
textures exhibit the greatest producibility. Traditional
core analysis can help, of course. However, as reservoir engineering becomes more complex to deal with
current challenges, core analysts must understand
and realistically model pore geometries and fluid behaviors at pore scales. In addition, obtaining physical
core measurements under different boundary conditions can take months, even years, to complete.
Emerging digital core technologies and pore-scale
flow simulations are beginning to complement—and
enhance—traditional laboratory techniques. IntegratUsing digital rock and digital fluid models, CoreFlow
simulation delivers flow and oil recovery information
to help optimize reservoir recovery (image courtesy of
Schlumberger).
50 JUNE 2014 / WorldOil.com
ed physical and digital rock and fluid workflows promise to accelerate the evaluation of multiple scenarios,
improve decision-making, and maximize production
and ultimate recovery from highly complex reservoirs.
DIGITAL ROCK MODELING
Traditional core analysis can be time-consuming.
Consider the steps involved in a typical relative permeability test. First, rock samples are cut from the
whole core and cleaned, which can take from days
to weeks and, in some cases, more than a month. Often, fluids must be recombined, which can also take
several weeks. Saturating rock samples with fluids,
and allowing time for core and fluids to reach equilibrium requires approximately 40 days. Most relative permeability tests analyze roughly six equilibrium points, each of which takes three to five days, or
an additional 18 to 30 days to obtain the initial set of
curves. All told, a single test can take three months
or more. To run a second test on the same core, the
sample must be cleaned again, which may require
less time, but still, roughly a week. The core must be
resaturated with fluid and equilibrated, which takes
another 40 days. Finally, obtaining a second set of
relative permeability curves requires several more
weeks to a month.
If an operator wanted to investigate multiple production strategies and improved recovery scenarios,
it could literally take years to run them all, using traditional laboratory techniques. Since few companies
have the time or budget, most settle for a base-case
scenario—an engineering evaluation to estimate the
best-case scenario, and a second lab test. Frequently,
this is the end of core analysis, at which point, an executive decision may be made—based on just two
tests—either to implement the recovery scenario, or
to suspend the project. In some cases, risky trial-anderror experiments are conducted in the field, injecting various fluids under differing boundary conditions to see what happens. The danger, of course, is
that permanent reservoir damage can occur, which
has serious, long-term economic consequences.
Ideally, most oil and gas companies would prefer to
test many more scenarios in far less time. For several
decades, therefore, researchers have been investigating
ways of using digital images of core samples to extract
useful reservoir information. In the 1980s, for example,
universities began digitizing thin sections from cores,
trying to calculate capillary pressure and relative permeability, without impressive results. Later, research
World Oil / JUNE 2014 51
CORE ANALYSIS
Fig. 1. Digital rock and fluid analytics services use results of
laboratory evaluations, such as fluid properties tests, as input to
digital core analysis. Running multiple digital scenarios provides
guidance for additional physical laboratory measurements, such
as flow studies (image courtesy of Schlumberger).
laboratories began using medical imaging technology to scan
cores and create various digital rock models for core analysis.
Only in the past five years, however, have digital rock technologies been commercialized successfully. And only within the last
year or so, have core analysts, petrophysicists and operators begun to accept the potential viability of digital approaches.
Why has there been such skepticism until recently? First,
vendors initially overstated the capabilities of digital rock modeling. Claims were made that digital core analysis would replace
traditional lab techniques, which has never been feasible. Second, digital rock models lacked sufficient validation from physical measurements to convince specialists that they could realistically simulate multi-phase fluid flow through complex pore
systems. In fact, most providers focused primarily on modeling
Fig. 2. Flow through the pore spaces of rock samples can
be visualized after running a scenario in the DHD simulator.
In the image, the grains are rendered invisible so the fluid
saturation locations can more easily be seen (image courtesy of
Schlumberger).
the rocks, not the fluids, hence the term “digital rock.” In addition, no one could actually perform both digital and physical
core analyses on the same rock sample in the same facility. Third
parties, different core plugs, and inevitable delays in obtaining
final lab results were not uncommon. Discrepancies—often significant ones—between the digital and physical measurements
were inevitable, especially in heterogeneous reservoirs.
Finally, digital modeling suffered either from oversimplified
pore geometries, oversimplified fluid properties, or both. For
example, one technique retained the overall complexity of the
rock’s pore spaces, while reducing their actual geometries to a
network of artificial tubes and spheres. A newer method successfully retained the true geometry of the pores, but fluid dynamics
remained overly simplistic. Neither digital rock approach could
realistically simulate the highly complex chemistry and physics of
fine, pore-scale rock-fluid and fluid-fluid interactions.
All these shortcomings raised genuine concerns in the minds
of astute observers. Uncertainties at the beginning of a project
could lead to unreliable results farther down the road. Therefore, to take digital core analysis to the next level, more integrated processes and more realistic flow simulations were necessary.
INTEGRATING PHYSICAL AND DIGITAL ANALYSIS
In December 2013, Schlumberger introduced CoreFlow
digital rock and fluid analytics services, Fig. 1. This new, integrated approach is unique in two ways. For one thing, physical
and digital core techniques are fully integrated in the same laboratory, using the same rock sample for both types of analysis.
Physical measurements refine the digital model, while digital
flow scenarios guide subsequent lab tests in an iterative manner,
rather than in a sequential series of steps.
More importantly, complex multiphase pore-scale flow
simulations are carried out using a new breakthrough technology known as direct hydrodynamic (DHD) simulation, Fig. 2.
World Oil / JUNE 2014 53
FIGURE 1
CORE ANALYSIS
Fig. 3. An adsorption isotherm test determines the volume of gas
that can adsorb onto the pores of a shale or coal sample. These
pore spaces in unconventional materials can be evaluated using
SEM digital imaging and analysis techniques (image courtesy of
Schlumberger).
Fig. 4. Scanning electron microscopy image shows quartz
overgrowths that have formed on quartz grains in a core sample
(image courtesy of Schlumberger).
Unlike conventional digital rock approaches, DHD simulations
correctly represent real pore geometries, real fluid properties,
and real rock-fluid/fluid-fluid behaviors—without oversimplifying any of these vital components. Thus, digital simulations
enable geoscientists and engineers to evaluate numerous production or improved recovery scenarios in a fraction of the time
required by traditional lab methods, alone.
Workflow integration. In either a conventional or unconventional reservoir, the integrated workflow typically begins
by imaging the whole 4-in.-diameter core, using a helical dualenergy computed tomography (CT) scanner, leaving the rock
and fluids sealed inside the original sleeve. Cross-sectional
images of rock, spaced as close as 0.5 mm apart, are used to
construct a slice-by-slice 3D “movie” of the whole core. Then,
heterogeneous rock types and flow units can be color-coded
to help asset geologists identify and prioritize reservoir zones
for more detailed physical and digital core analysis.
Next, sample plugs of sufficient size are taken from selected locations to allow core analysts both to physically measure rock properties and to digitally image the same plug. By
cutting a 4×8-mm sample plug from the whole core, it is no
longer necessary to test two, slightly different samples. This
enables a one-to-one comparison of physical and digital results, minimizing or eliminating the uncertainties that arise
from subtle differences between even closely spaced plugs.
Once a sample’s physical properties have been measured, if
it is a conventional sandstone or carbonate, micron-scale or
microCT imaging comes next. In an unconventional shale or
mudstone, nano-scale imaging with a scanning electron microscope (SEM) is more appropriate, Figs. 3 and 4.
MicroCT imaging effectively resolves clastic and carbonate
grains and pores in samples with micron-scale resolution. Technicians shine X-rays through each sample to obtain a backscatter
image, rotate the sample a 10th of a degree, take another image,
and continue rotating until 3,600 backscatter images provide
a full 360° view. Running a mathematical inversion on the microCT data enables them to construct a high-resolution, 3D digital image of the rock. Additional processing creates a digital rock
model through which various core flow simulations can be run.
SEM analysis of shales and mudstones provides an order-ofmagnitude higher magnification, imaging grains, pores and cap-
illaries at nanometer scales. First, the sample surface is flattened,
using a focused argon-ion beam, and a 2D image is captured.
Then, by repeatedly milling away the surface, and taking an additional 200 to 500 images of the slices, a 3D volume, about the
diameter of a human hair (less than 100 microns), is created.
Flow in unconventional reservoirs is controlled by pores associated with minerals, which are extremely tiny compared with
conventional rocks, as well as pores associated with organic material, which can be as small as 20 nanometers in diameter.
After imaging by microCT or SEM, the static 3D digital rock
model is entered into the DHD simulator, along with a dynamic
digital fluid model, derived directly from physical fluid analysis.
DHD simulation. The theory and physical principles underlying direct hydrodynamic simulation date back to the 19th
century, originally in the field of thermodynamics. In the earlyto-mid-20th century, the theory was utilized more broadly in
physics and chemistry. In the 1990s, it was finally applied to
compositional hydrodynamics by a Russian scientist, who subsequently joined the Schlumberger Research Center in Moscow.
Formal development of the DHD simulator, itself, began about
five years ago. It was commercially introduced in late 2013. This
unique technology accurately simulates real multiphase, multicomponent fluid flow through digital rock models that honor
real pore geometries, even at very fine scales. Its technical capabilities have been documented extensively in more than 20
academic papers and one book.
A combined digital and physical core analysis study can begin by establishing an initial base-case from rigorous laboratory
analysis, and confirming that the DHD core flow simulation is
a good match with the physical measurements. At this point,
the true power of the digital approach becomes apparent. In the
time that it would normally take to run one more physical core
test—to evaluate a presumed best-case scenario—analysts can
perform several—perhaps four to eight—complicated digital
rock and multiphase fluid simulations.
Scenario evaluation. To plan improved recovery operations
in conventional reservoirs, operators could rapidly compare, for
example, the relative sweep efficiencies of gas or waterflooding under different initial injection rates, wettability conditions, and pressure drop scenarios, at varying distances from
the wellbore. Digitally investigating multiple flow scenarios in
54 JUNE 2014 / WorldOil.com
CORE ANALYSIS
less time empowers the asset teams to make more confident
technical and financial decisions, to focus on the geological and
engineering parameters that represent the greatest risks, and
to optimize those risks with the greatest potential to increase
ultimate recovery. Even in the primary production stage, DHD
simulation enables operators to assess the economic impact of
moving from single-phase to two-phase fluid flow.
The value of digital DHD rock and fluid simulation in unconventional reservoirs lies in its ability to quantitatively distinguish the relative producibility of different rock textures.
Traditional core analysis often cannot differentiate subtle
heterogeneities in shales, due to extremely small sample sizes.
Thus, physical measurements tend to average flow characteristics across several distinct textures. Arming engineers with the
relative producibility of specific argillaceous, calcareous and siliceous zones would allow them to design better hydraulic fracture completions. Once petrophysicists know which textures
matter, these textures can be located elsewhere in the reservoir
through optical examination of drill cuttings.
DIGITAL ROCK AND FLUID MODEL VALIDATION
Prior to commercialization, DHD core flow simulations
through pore-scale digital rock models were rigorously benchmarked against both physical models and real rocks. Comparisons have been made with experimental flow studies using 2D
micromodels, standard reference curves from core analysis in
the classic literature, and pore flooding of actual rocks, imaged
in real time with advanced X-ray microtomography.
The actual behavior of multi-phase fluids through various
pore and capillary geometries is well known from experimental studies flooding etched glass micromodels in the lab. Digital DHD simulations have properly replicated pore filling and
displacement behaviors of wetting and non-wetting phases in
simple models, including crossing flow paths, narrow capillaries connecting two pores, pore doublets or capillaries that
split into channels of two different sizes before reconnecting,
and networks of pores and flow paths.
Analysis of improved recovery scenarios aims at achieving
the lowest possible residual oil saturation, which depends,
in part, on different capillary numbers—the ratio of viscous
forces to capillary forces. To validate DHD simulations against
data from the literature, waterflooding of an oil-saturated digital model of a real sandstone was modeled at different capillary numbers until residual oil saturation was negligible. The
resulting capillary desaturation curve corresponded well with
published curves, measured physically in very similar sandstone samples.
Finally, Shell researchers used fast, synchrotron-based Xray microCT imaging to validate DHD modeling of digital
two-phase fluid dynamics. First, the distribution of oil in a
physical rock sample was imaged before, during and after a
series of pore filling events. Next, the simulator reproduced
selected events using the same initial conditions as the experiment—fluid properties, pore geometries and flow regimes.
The resulting experimental and digital geometries of oil distribution, as well as displaced fluid volumes, were in quite good
agreement. In fact, the digital simulation was able to visualize
even more complexities, at more time steps during pore filling
events, than fast microCT imaging.
FUTURE IMPLICATIONS
By combining rigorous physical testing in the laboratory
with advanced 3D imaging and direct hydrodynamic simulation technologies, core analysts are finally demonstrating the
validity of digital rock and fluid analytics. There is no reason,
however, to believe that digital core analysis can, or ever will,
replace physical analysis. Laboratory testing will always be
essential to anchor digital models in the ground truth, found
only in real rocks and fluids.
Nevertheless, as more petrophysicists, geologists and
reservoir engineers see that digital core flow simulations
can actually replicate familiar lab results, they may decide
to forego further, time-consuming physical tests, when they
need to evaluate additional recovery strategies. As a result,
operators will begin to dramatically shorten traditional cycle
times, better understand increasingly complex reservoirs before making costly field decisions, and maximize both shortterm production and long-term recovery from oil and gas
assets worldwide.
MARK A. ANDERSEN is Schlumberger’s domain head
for core physics and manager of the digital core
analysis group at Schlumberger Reservoir Laboratory
in Houston. Dr. Andersen is the author of many
technical papers focused on wettability and production
rock mechanics, and is the author of a book on
reservoir properties of North Sea chalks. He earned a
BS degree in engineering physics from the University of Oklahoma at
Norman, Okla., and MS and PhD degrees in physics from The Johns
Hopkins University in Baltimore, Md.
Article copyright © 2014 by Gulf Publishing Company. All rights reserved.
Printed in U.S.A.
56 JUNE 2014 / WorldOil.com
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