Geologists should never forget that they
are vital members of the drill team.
Information from the geologist, of course, can help
ensure successful drilling and well completion without damage to
the producing formation, and with a best chance of maximizing ultimate
But the geologist's input becomes even more critical
in light of today's advanced drilling tools and techniques, including
underbalanced drilling, riserless (deep-water) drilling and drilling
using coiled tubing.
Unfortunately, many geologists fail to keep up to
speed on the drilling fluids, equipment and new engineering techniques
employed in contemporary drill programs.
Take underbalanced drilling (UBD), for example, a
procedure in which the geologist must consider not only the process
of drilling but also the type of drilling fluids used and the way
the mud interacts with drilled formations.
Shell International Exploration and Production BV
has identified critical success factors for a UBD project. The first
three of these are:
- A clear business case weighing cost and value.
- People involved in the project with the right level of expertise
- Ample time to plan and prepare.
The more expertise and preparation the geologist
can bring to a UBD project, the greater the chance of its success.
In UBD, the bottom-hole pressure exerted by the hydrostatic
head of the fluid column is less than the formation pressure being
drilled. UBD can be used:
- To protect the reservoir from mud invasion.
- To avoid lost circulation.
- To increase penetration rates.
- To obtain early information about producing zones.
Helping Appropriate Candidates
At the earliest stage, the most important role of
the geologist is selecting an appropriate reservoir candidate for
UBD, according to Graham Ritchie, manager of the Underbalanced Reservoir
Development group at Schlumberger Oilfield Services in Houston.
"Candidate selection is absolutely essential to underbalanced
drilling," Ritchie said. "You've got to have the right reservoir
"In terms of the fluids, we want to know how sensitive
the reservoir is to going overbalanced," he said. "Typically, we
use fluids that have no fluid loss and don't build a filter cake."
Selection of a drilling fluid depends on bottom-hole
pressure — a slightly depleted formation may require only a small
amount of nitrogen in fluid for UBD, Ritchie said. Foam might be
used where more depletion is evident, and gas with misting in seriously
In addition to good estimates of bottom-hole pressure
when planning a well, engineers need accurate readings while drilling,
Ritchie said. Not only is that "the only way you can know for sure
that you're drilling underbalanced," but the annular pressure readings
will provide information about hydrocarbons entering the borehole.
"The advantage of that is in giving you some idea
of the long-term productivity of the well," Ritchie added. Schlumberger
has developed a method for deriving formation pressure and permeability
profiles in real time during UBD.
Land applications of UBD may cost 10-30 percent more
than a routine, overbalanced drilling approach. "Obviously, offshore
is going to be even more expensive," Ritchie said, "and there's
quite a bit of lead time involved in planning these projects."
Difficulty in projecting the potential increase in
production from using UBD makes economic judgments difficult.
And UBD should not be considered a cure-all for drilling
problems, according to Ritchie.
"As with any new technology, you have to understand
it's not a panacea," he said. "Underbalanced drilling does not improve
rock quality. If it's a real dog, it's a real dog."
Houston consultant Bill Rehm has more than 30 years'
experience with drilling fluids and equipment. He was a vice president
of Maurer Engineering and later founded BecField Horizontal Drilling.
Rehm now teaches UBD techniques.
"The geologist's primary interest with the mud is
usually to get good cuttings," he said. "With underbalanced, you're
looking at getting logs that are badly invaded."
To the driller, UBD presents a challenge in controlling
bottom-hole pressure while preserving the borehole. The driller
may decide to avoid complications with UBD, such as drilling through
shale, Rehm said.
"Whether we admit it or not, most of these big shale
zones are overpressured, or at least overwatered," he noted.
By providing information about downhole geology,
the geologist helps other members of the drill team make good decisions
about appropriate techniques and timing. Put simply, the geologist
can help keep the driller out of trouble.
According to Rehm, these are the geologically related
questions the drill program needs to answer, or have answered:
For Reservoir Protection
- What is the cap on the reservoir? What is the depth and thickness?
What is the degree of confidence in this?
- What is the reservoir's lithology? What changes occur in
- How sensitive is the reservoir? What is the reservoir pressure?
What are the permeability and porosity?
- What type of completion is going to be required?
- Is there natural fracturing? If so, what is the orientation?
- What is the best estimate of open-hole flow?
- What is known about pore pressure? What can be determined
about fracture pressure?
- Where are the water, oil or gas tops?
- How critical are cutting samples? Where are they critical?
- What logs are absolutely essential?
All of the above, plus:
- Is there a persistent marker formation just above the target
formation, for the directional driller to check on?
- Depending upon the radius to be drilled, where is a good
hard sand or limestone as a kickoff point?
- How stable is the formation where the curve will be drilled?
Is it water sensitive? What are the pore pressures?
- How hard is the cap on the reservoir? Will the directional
drill skid along it?
- Is there faulting? If so, what is the angle and direction
of faults? What type of faults? How thick are the fault zones?
- What is the chance of finding virgin pressures?
- Key Question: How persistent is the reservoir?
- What is the lithology, horizontally and top to bottom?
- What is known and what can be determined about permeability
- What is the angle and thickness of the beds?
Avoid Lost Circulation
- How important are drill samples?
- What may cause lost circulation?
- Are the formations in the open hole water-sensitive? How long
will they stay open with underbalance and dampness?
- What is the pore pressure? What can be determined about fracture
- What is the chance of a well kick in a stray formation?
Get On Board Early
Rene St. Pierre, drilling manager for Westport Resources
in Dallas, said the geologist can help a drilling engineer understand
the implications of choices made both prior to and during drill
"I like to think we pick the best fluids for the
formations that we drill in," he said. "We can use a better examination
of what the formation damage characteristics might be, based on
the fluids we use."
St. Pierre emphasized the importance of close teamwork
and good interpersonal relationships in the drill team.
"Geologists are innovators," he said. "Most people
in drilling, if they're in drilling very long, get to be pretty
dominant personalities in terms of making spontaneous decisions.
"I think some geologists are a little bit intimidated
by drilling people. And they shouldn't be."
In UBD for horizontal wells, geologists have to understand
"the implications of the target information they give," according
to St. Pierre. Targets that are changed or revised too often during
drilling are of little use to the driller, he said. The geologist
should recognize practical limits as well as economic restraints.
"We try to drill these wells economically, as well
as damage-free," he explained. "If we get a dry hole, it's a dry
"Geologists sometimes seem to think it wouldn't have
been a dry hole if we had drilled a little straighter hole or if
we had hit our target within two or three feet. I get pounded to
drill these wells as economically as I can."
In that regard, St. Pierre believes geologists should
work with engineers early in the project-development process, and
that drillers should be consulted in AFE preparation.
"From a drilling engineer's standpoint," he said,
"it's better for us to get on board as soon as possible."
A Valuable Service
Geologists can provide information to guide decisions
about the optimal approach for drilling a well. UBD may not be effective
in conditions of poor reservoir quality or borehole instability,
"If you have a formation that's hypersensitive to
underbalanced, you might want to drill with coiled tubing," said
Scott Tinkham, product champion-CT Drilling for Schlumberger in
Sugar Land, Texas, who added that the geologist should consider
the potential for wellbore stability, especially, when evaluating
a coiled tubing-drilled project.
"That's one of the big keys to applications that
involve coiled tubing," he said. "If similar profiles haven't been
drilled with a rotary rig, you shouldn't drill it with coiled.
"We've seen a lot of projects come to a screeching
halt when the wellbore doesn't hold up."
Tinkham cited BP's experience in Colombia as an area
of extreme formation sensitivity. "The porosity to perm rates are
really sensitive. A 4 percent porosity might be 10 millidarcies
and a 5 percent might be 100 millidarcies, so drilling fines can
do a lot of damage."
His experience in North Sea drilling provided another
example where the geologist can develop information to guide the
"Resolution of seismic is such that you don't know
within 100 feet where the fault zones are. It can get really ugly
and you can lose a BHA or two," he said. "On the front end, the
geologist and geophysicist can do a lot to warn the drilling crew."
By understanding both the relevant geology and the
requirements of today's new approaches to drilling, geologists can
provide crucial information for successful and economically beneficial