Nuclear magnetic resonance tools
are becoming a standard part of the logging suite for many
oil companies, and for good reason.
Traditional methods of acquiring formation
permeability measurements - whole core, rotary sidewall
and percussion cores, pressure transient analysis and production
history matching - can be expensive and time consuming.
On the other hand, NMR logging technology
developed over the last decade makes it possible to achieve
a continuous permeability measurement through wireline logging
that is available immediately and can be used to:
- Proactively estimate reserves and initial production.
- Design and evaluate the effectiveness of various stimulation
programs.
What's next, you ask?
Service companies realize they must demonstrate
the superior capabilities of the new technology - and to
achieve that goal they are teaming up with clients on studies
to test just how valuable an addition NMR is to the wireline
logging suite.
Recently Schlumberger joined Conoco, for
example, to test wireline logging methods that included
NMR tools to develop a continuous permeability measurement.
Stacy Kerchner with Conoco and Bruce Kaiser,
Michael Donovan and Ruben Villarreal with Schlumberger Oilfield
Services presented a paper on the study at the recent Society
of Petroleum Engineers annual meeting. The paper was titled
"Development of a Continuous Permeability Measurement Utilizing
Wireline Logging Methods and the Resulting Impact on Completion
Design and Post Completion."
The project was conducted in Conoco's Lobo
Trend in South Texas, and the objectives were:
- Develop a field-specific method to reliably estimate
permeability using wireline logging methods.
- Demonstrate how this information impacts economic
and engineering considerations related to the hydraulic
stimulation program.
- Describe the importance of knowledge of permeability
in post-audit analysis.
- Recommend "best practice" guidelines for use of a
particular method to obtain permeability.
Well, Well, Well
Advances in 3-D seismic interpretation have
improved reservoir descriptions within the depletion drive
reservoirs in the Lobo Trend, according to Kerchner, particularly
with respect to reservoir boundaries.
Reserve estimates in the trend, however,
are frequently calculated on a statistical basis, because
on a program basis, statistical estimates continue to provide
better estimates of ultimate recovery.
The team members decided that:
- The continuous permeability measurement model had to
be portable.
- The permeability output had to be valid in all areas
of the Lobo Trend and available for real time delivery.
- The final logging program needed to be cost-effective
over other permeability measurement methods.
- The permeability output had to match core permeability
measured from rotary core plugs taken from the test
wells.
The study included three evaluation wells
followed by a control or project well.
- The logging program for Well 1 included a typical suite
of array induction tool, density tool, compensated neutron
log and gamma ray to establish petrophysical correlation
with offset wells.
The NMR tool was run for pore size distribution
to relate to permeability, and an elemental capture spectroscopy
tool was run for clay typing to be used in petrophysical
analysis and completion design.
A dipole shear imaging tool was run for mechanical
properties to aid in the hydraulic fracture design.
- Well 2 was to be a test and refinement of the model
built from Well 1, but borehole conditions prevented
the collective of adequate data.
- In Well 3, the logging program was optimized from Well
1 based on results from the first well. The same tool
string was run for primary petrophysical analysis, elemental
capture spectroscopy was run for clay typing, and the
NMR tool was again run for pore size distributions.
Rotary cores were taken to provide the control for the
permeability measurement.
In Well 3 the core coverage was increased
to improve the model, but the initial NMR results still
were not consistent, much like Well 1. The NMR permeability
matched core permeability in the upper pay intervals, but
not in the lower pay interval.
Several evaluations of permeability equations
were made to combat this problem. A robust expression of
NMR permeability was developed by using the nuclear magnetic
resonance form of the K-Lambda equation. To provide further
validation, Well 1 was reprocessed with this new NMR permeability
expression.
A subsequent test well was logged with the
new expression and the results compared with core permeability.
The results gave Conoco the confidence to remove rotary
sidewall cores from the logging program in the study area.
Additional wells have since been logged using the NMR permeability
as input to the completion design.
Comparisons Are Made
The continuous measurement of permeability
was used directly in the fracture optimization process.
Conoco and Schlumberger engineers used one case to compare
a composite permeability model with the continuous permeability
model.
Proppant selection, fluid selection, optimum
rate and optimum max proppant concentration were the main
parameters required for the optimum fracture stimulation.
Once these main parameters were selected, the net present
value was analyzed to determine the optimum fracture geometry
that meets the composite system permeability of 0.003 millidarcies.
Based on these parameters and the analysis,
engineers noted that to achieve the optimum frac half-length
of 795 feet, a considerable amount of proppant and fluid
would be wasted without any additional benefit due to the
low stress contrast between layers.
"From a completion standpoint, it appeared
that only three options were available for completing the
wells," Kerchner said, "proceed with frac design to achieve
optimum frac, reduce frac volumes to meet budget, or produce
well until enough capital is raised for frac."
However, when a closer look was taken at
the log-derived continuous measured permeability and stress
contrast, engineers found that some layers have permeability
values of about 0.238 millidarcies. To take advantage of
this they could design an optimum frac for 0.238 millidarcies,
perform the fracture treatment and model if refracing later
provides additional benefit.
"By targeting the high permeability zone
first, the optimum frac half length required is 388 feet
and the fluid volume and proppant requirement are considerably
less than the pervious net present value run," the authors
wrote. "By reducing the volume of fluid and proppant mass
needed, a reduction in net pressure is expected and a lower
value for frac height is also expected. However, this will
only take advantage of the high permeability layers, leaving
the low permeability layers under stimulated."
A solution to the problem of under stimulating
lower permeability layers was to refrac later to improve
the production from those zones without sacrificing production.
"Stimulating the high permeability layers
and producing them will increase the stress contrast between
layers due to depletion," they wrote. "The lower permeability
layers can then be stimulated much more efficiently - all
we need to find is when to refrac."
Results
Conoco and Schlumberger compared the composite
permeability and continuous measured permeability methods
by determining the differential cumulative production between
the two options.
A three-year cumulative production comparison
showed that the frac/refrac using the continuous permeability
model provided better return at a lower overall cost. Cost
differential was $65,291, and the three-year cumulative
gas differential was 292 million cubic feet.
"Obtaining permeability data within the Lobo
Trend allows engineers to complete the most economical stimulation
design for a given 'tank size' and a calculated recovery
factor based on the permeability," Kerchner said.
"For a typical 40-acre block with 40 feet
of net pay interval, the cost estimate for a Conoco stimulation
design for a permeability of 0.005 millidarcies is $78 thousand,
while the same design for a permeability of 0.1 millidarcies
is $85 thousand," he added.
The identical design for a permeability of
2.0 millidarcies is $91 million.
"The cost difference justifies the cost of
the NMR for areas where permeability is unknown or highly
variable," Kerchner said.
The study's conclusion? Continuous permeability
measurement using magnetic resonance techniques can greatly
improve well efficiencies by better understanding the permeability
distribution across the zone of interest, Kerchner said,
which not only leads to better completion but also enhances
the overall hydrocarbon recovery.
"Using gas prices of about $3 per thousand
cubic feet," she said, "this completion process using the
continuous measured permeability model brings to the bottom
line $65,291 in cost savings plus $876,000 in additional
revenue from gas production."