The science of
geology has always been about "seeing" into the earth
— a discipline founded on the skill of early practitioners to divine
the subsurface by studying topographical features.
While surface mapping was effective, pioneering geologists
longed for methods that would allow them the chance to actually
study the rocks. Technological innovations in the petroleum industry
afforded those early geologists with their first look at the subsurface
— not seismic, but rather the first well logging techniques.
In 1927 a quantum change in geologists' ability to
see into the earth occurred when Henri Doll and the Schlumberger
brothers made the first downhole electrical measurements. Using
a wire cable with electrodes, they were able to measure formation
resistivity.
"The very first hand-drawn log listed the geologic
layers next to the resistivity readings, so from the beginning well
logs and geology have gone hand in hand," said Brian Clark, technology
center manager for Schlumberger Oilfield Services.
"The motivation for their experiment was to help interpret
surface electrical measurements," he continued. "At the time, few
envisioned the impact that resistivity logging would have on hydrocarbon
detection."
Spontaneous potential was discovered shortly after
the first successful resistivity logs were achieved and SP logs
were used to indicate porous formations. Together SP and resistivity
well logs were used for well-to-well correlations, allowing geologists
to draw the big picture.
Quantum Leaps
The next quantum leap in downhole geology technologies
came in the 1960s, when the first borehole images were captured
with televiewers that measure reflected acoustic waves.
The amplitudes of these reflected waves showed fractures,
vugs and lithology changes.
Computer processed log interpretation also started
in the 1960s with the advent of digital computers, Clark said.
"The integrated interpretation of all available log
data, such as resistivity, SP, natural gamma ray, sonic, neutron
and density was treated as an optimization problem," he said. "This
technique gave a mineralogical interpretation of the subsurface."
Check shots and vertical seismic profiles, or VSPs,
were the first downhole techniques that gave geologists the ability
to look away from the borehole, using surface sources and downhole
receivers. Amazingly, these methods were proposed in 1917, but were
first developed in Russia in the 1960s.
"VSP images add definition to seismic reflectors near
the wellbore," Clark added, "and improve the quality of correlation
between surface seismic and well logs."
Downhole geology technology has come a long way since
those advancements in the first part of the century through the
1960s, thanks largely to a rapid acceleration in the evolution of
downhole measurements since the late 1980s.
Many factors have contributed to this technology explosion,
Clark said, including:
- Greater computing power.
- Electronics that can survive severe operating conditions.
- Enhanced sensor physics.
- Communication and data-delivery systems that offer real-time
capabilities.
"The result," he said, "has been a broader spectrum
of measurements and improved accuracy, reliability and ease of acquisition
and interpretation."
Technological Advances
Today high-resolution electrical images of the borehole
are routinely obtained on wireline, yielding a resolution of 0.2
inches. In water-based muds, such images are used to see lithology,
sand count, unconformities and rock texture. Electrical imaging
also is widely used to see fractures.
Major steps in downhole geology include:
➤
Logging while drilling, or LWD, where the formation properties are
measured just after they have been drilled.
Electrodes on a rotating LWD collar produce 360-degree
images of the formation reistivity. These images can be transmitted
to the rig floor and even to the office in real time.
"Such real time images help geologists detect and
identify structures and features, and their interpretation helps
the driller make timely decisions about the well trajectory," Clark
said.
Another feature: LWD images give quantitative resistivities.
In contrast to resistivity images, which require a
conductive, water based mud, real-time logging while drilling density
images can be obtained in any mud type. A rotating LWD collar measures
density in 16 azimuthal sectors, with one-foot axial resolution.
"This resolution can be sufficient to identify dipping
beds and structural dip," Clark said. "This characteristic 'U' indicates
that the borehole is passing upward through a higher density bed."
➤ Geosteering, which allows scientists to use real time geological
interpretation to steer a well.
Geosteering can place the wellbore with an accuracy
of about one foot relative to the formation boundaries at horizontal
drilling distances exceeding 10 kilometers, he said.
➤
Magnetic resonance logging, which is "a major step forward in seeing
into the earth," Clark said.
"It is a penetrating microscope that can detect geometric
features down to the micron level, he continued.
The distribution of transverse relaxation times, or
T2s, is related to the pore size distribution and to how fluid molecules
are distributed. In small pores, molecules in the pore fluids collide
quickly with the matrix and their magnetization is short lived.
Clay-bound water has the shortest T2s, typically less
than three milliseconds. Capillary bound water has T2s between three
and 33 milliseconds, while oil and free water typically have T2s
greater than 33 milliseconds and are producible fluids.
"Geologists and petrophysicists can improve their
understanding of pore geometry for depositional analysis from T2
distributions," Clark said. "Reservoir engineers can use high-resolution
magnetic resonance logs to locate vertical permeability barriers.
Hydrocarbon characterization has improved by interpreting magnetic
resonance logs in combination with other logging measurements."
➤
Quantitative formation lithology can be obtained using nuclear spectroscopy,
where the elements silicon, calcium, iron, sulfur, titanium and
gadolinium are identified by the characteristic gamma-rays they
emit after capturing neutrons.
This nuclear activation process provides the basis
for accurate mineralogical analysis used to identify and quantify
clays, quartz, carbonates, pyrites, salt, coal and other minerals.
"This technology greatly reduces the uncertainty in
log interpretation," Clark said, "and provides a powerful new tool
for cross-well correlation."
Looking Ahead
New downhole technologies are expected in the near
future, Clark said.
In highly deviated and horizontal wells it is important
to see geologic features that do not intersect the borehole, and
there is a need for downhole measurements that can see much deeper
than conventional logging tools.
➤
"One approach is deep acoustic imaging using a wireline tool with
an array of acoustic transducers," he said. "The resulting traces
are processed in a manner similar to seismic to identify reflectors,
but direct arrivals and borehole modes are significant noise sources
and must be handled carefully."
One example from the Toll Field in the North Sea showed
a calcite stringer imaged about 30 feet away from the wellbore.
Such calcite stringers are barriers to vertical flow, Clark said,
and affect the well's ability to produce.
➤
In exploration drilling it can be critical to know the correct time-to-depth
conversion for the seismic section so that the driller can plan
the right well trajectory to hit the target formation or to stop
and case before drilling into a high pressure zone. The conventional
approach would be to trip out of the well and run a wireline checkshot,
or VSP — at significant rig time and additional expense.
A new LWD service uses a surface air gun and downhole
receivers to provide real time checkshots. When adding a 90-foot
stand of drill pipe, the measurement is made without disrupting
the drilling process.
The LWD tool detects the seismic waves, processes
the data to obtain an arrival time and transmits it to the surface.
"In one example, an LWD checkshot obtained at 1,870
meters was used to improve the time-to-depth conversion, and this
placed a high-pressure target at 2,580 meters, with an uncertainty
of 120 meters," Clark said. "Subsequent real time checkshots were
used to improve the target depth estimates, which changed downward
by 90 meters, and to reduce the uncertainty of its position."
This technique can have a significant impact on drilling
costs and drilling safety, especially in deepwater and under salt.
"In some cases reflectors can be seen hundreds of
meters ahead of the drill bit," he said, "indicating that real time
VSPs are one the horizon."
➤
While oil-based and synthetic muds are in wide use because they
improve borehole stability, reduce formation damage in water sensitive
formations, and increase the rate of penetration, they have prohibited
high-resolution electrical imaging — until now.
Clark said a new wireline imager works in non-conductive
muds and provides a resolution of 0.4 inches. A side benefit of
this is that it provides quantitative resistivities.
➤
Modern computing technology and visualization methods continue to
evolve — and improve.
Analogs derived from outcrops, photos, cores, ground
penetrating radar, GPS data and other sources will be stored and
manipulated as digital objects and used to create realistic reservoir
models.
"The oil and gas industry faces mounting challenges
to reduce finding and producing costs, to increase recoverable reserves
from the traditional average of 35 percent to 65 percent or higher
and to maximize asset value," Clark said. "These challenges are
all the more difficult because most new prospects are in deepwater
or remote areas where costs and risks are high. These new fields
tend to be smaller and more complex.
"Meanwhile, the world's existing reservoirs are rapidly
aging and developing problems," he continued. "Plus, our industry
is being asked to do more with less, particularly with the worldwide
downsizing of the oilfield workforce.
In other words, new technologies are critical to overcoming
these challenges — and will fundamentally change the way reservoirs
are managed.
"Rather than reacting to problems, we will anticipate
and prevent problems and will optimize the flow and sweep of hydrocarbons
and overall recovery," Clark said.
"The prize is enormous — each 1 percent increase
in oil recovery equals one year's consumption at current demand."