American Oil and Gas Reporter - October 2015 - 93

SpecialReport: Drilling Fluids Technology
With the Glen Rose slightly overpressured
at a 9.1-9.4 PPGE pore pressure, the normal approach was to spot a heavy trip
margin pill (TMP) on bottom to stop all
flow prior to pulling the drilling BHA
out of the hole. This often was accomplished using a PBL tool to circulate a
massive amount of lost circulation material
in the TMP to plug the Glen Rose natural
fractures.
The problem with this approach was
that in order to cement the well, the
heavy pill had to be circulated up the annulus and the height of the heavy pill
column sometimes exceeded the fracture
gradient of the Glen Rose (even with
LCM in the hole), and a significant
amount of OBM would be lost into the
formation. Moreover, well control became
an issue when the fluid level dropped in
the annulus.
In response, the operator developed a
method of spotting a heavy "top kill" or
"mud cap" TMP, which also often was
set using a PBL, but at a much shallower
depth (±400 feet from surface) after
drilling through the top of the Glen Rose
B. A typical TMP was 31 barrels of 13.3
pounds/gallon OBM to yield a 0.15 PPGE
increase in bottom-hole weight. In situations in which the fracture and pore pressure gradients were extremely close, the
mud cap TMP was added in small increments while pulling out of the hole. As a
section of pipe was pulled, the volume
of fluid displaced by that length of pipe
was replaced with a TMP.
In some cases, as dictated by pressures
and deliverability potential, a small TMP
with LCM was set on bottom and a mud
cap TMP also was set on top as the pipe
was being pulled out of the hole. This
approach proved very effective in terms
of both maintaining well control and conserving OBM in order to reduce costs.

Controlling Lost Circulation
Controlling mud losses is one of the
biggest challenges in using MPD and
OBM to drill fractured carbonates. By
implementing an organizational learning
program that included a daily "lessons
learned" capture process, the operator
was able to develop a highly effective
LCM control system.
The first line of defense is maintaining
ECD at or below the formation fracture
pressure, and in the case of these naturally
fractured formations, the fracture gradient
was very close to reservoir pressure.
From time to time, partially depleted

fracture systems also were encountered.
In addition, fluid swapping sometimes
occurred in highly permeable, nearly vertical, tall, natural fractures, causing OBM
to be lost into the fracture system.
Specific recipes for OBM sweeps,
OBM-based LCM, and as a last resort,
WBM "reverse gunk" squeeze pills were
developed. The crew was trained to rapidly
implement preset procedures based on
the type of mud losses encountered, and
the LCM response process was reduced
to a series of decision trees based on five
defined categories of fluid loss (ranging
from minor seepage to complete loss).
As an example, Figure 4 shows the decision tree flow chart for handling more
severe losses (categories IV and V)
Figure 5 shows OBM MPD cost per
well as the drilling program proceeded.
Drilling cost performance was somewhat
erratic between the first and 12th wells
(the latter having 25 percent higher cost).
These wells were drilled before the operator had fully implemented its organizational learning and continuous improvement program. Moreover, the 12th well
was an outlier. The high cost was the
result of encountering a very shallow
fault/weak zone that led to massive OBM
losses and required a sidetrack.
After that event, the two-string casing
design was replaced with a three-string
program to achieve consistent incremental
drilling performance improvement. The
three-string design included running larger
surface casing and a 95⁄8- or 85⁄8-inch intermediate string to cover the very shallow
weak zones. This allowed the operator to
drill a longer interval with WBM (rather
than just the surface hole) before switching
to the OBM.
This shallow intermediate shoe depth
(about 6,000 feet) did not require the 7inch, 29-pound P-110 casing required if
the relatively low-grade intermediate
string was run over the Dexter Shale and
set in the deeper Buda. Moreover, at the
same time the low-cost intermediate
casing was being run, the mud handling
system could be converted from WBM
to OBM. Counterintuitively, over time,
the three-string design proved to be more
economical than the two-string design
since drilling inconsistencies were virtually
eliminated. Moreover, OBM losses resulting from seepage into the Wilcox
were eliminated.
By systematically developing operational best practices and leveraging a disciplined organizational learning and con-

tinuous improvement process, MPD with
OBM was implemented successfully to
effectively control lost circulation into
the weak shallow zones above the productive intervals, completely eliminate
wellbore stability issues caused by sensitive shale formations, and adequately
control lost circulation caused by natural
fractures in the productive intervals.
This approach allowed the operator to
achieve its objective of drilling a sufficient
number of economically viable wells to
partially prove up and hold a large acreage
position. The most productive core of the
field with the greatest natural fracture
density was developed using OBM and
MPD. By generating consistent economic
results over a large number of wells across
a large area, the OBM MPD program
made the successful development of the
entire field possible.
r

HAROLD E.
MCGOWEN III

Harold E. McGowen III is president
and chief executive officer of Navidad
Resources LLC, an independent operating company, and Navidad Energy
Partners LLC, a company that generates
investment opportunities and provides
consulting services to investors in the
exploration and production business.
At Navidad Resources, McGowen successfully led a multidisciplinary team
in discovering and developing the BudaRose stacked, tight, fractured carbonate
play in the East Texas Basin, which
included applying OBM MPD, staged
slickwater hydraulic acid-fracturing
treatments, and commingling production
from multiple zones. Before establishing
Navidad Resources in 2001, McGowen
served as senior vice president of engineering at SIGNA Engineering Corp.,
president of Navidata Systems Inc., engineering manager at Trinity Resources
Inc., and petroleum engineer at Union
Pacific Resources Co. He holds a B.S.
in mechanical engineering from Texas
A&M University.
OCTOBER 2015 93



American Oil and Gas Reporter - October 2015

Table of Contents for the Digital Edition of American Oil and Gas Reporter - October 2015

Contents
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