American Oil and Gas Reporter - October 2015 - 91

SpecialReport: Drilling Fluids Technology
FIGURE 3A

FIGURE 3B

Location Construction for OBM

OBM Vacuum System

400'

Pad Slope
(Area 1"/10"
Minimum Fall
to Back of Pad

Reserve Pit

Trailer Lot Cement
Only "No Rock"

400'

Mud Tanks

12" Soil
Cement
180' L x
60' W

Mud
pump
#1

Upright
going to
rig floor

Mud
pump
#2

Substructure

Substructure

Soil Cement/Crushed
Rock

OBM
Cuttings
Pad (1x3)
Rock

Discharge

Legend

Location Entry,
(Varies by
Location)

hole stability in thick, water-sensitive
formations.
However, the higher incidence of massive lost circulation when drilling overbalanced with OBM precluded its use in
the fractured carbonate sections of the
hole. The operator already had determined
that running an intermediate string deep
enough to cover the Dexter was very expensive and was not always successful
because Dexter degradation meant the
intermediate casing could not always be
run to bottom. While recognizing that
underbalanced MPD was a potential solution to simultaneously fight lost circulation and improve well control in fractured
carbonates, a major roadblock to using
MPD was the perceived nature of the
Dexter Shale.

Line-up valves
Suction inlet valves
Suction line
Discharge line

The operator ultimately concluded that
assuming Dexter was overpressured was
false. A review of available literature indicated that large gas shows seen in the
Dexter might be from porous, extremely
low-permeability stringers with no commercial deliverability. Moreover, these

Vacuum
tank

shows also could be from gas released
from tiny pores in overpressured shale
fragments that move up the hole and decompress explosively. Based on these
hypotheses, the operator lowered the
OBM mud weight to as low as 8.5
pounds/gallon (8.7 PPGE).

Challenging Assumptions
Conventional wisdom held that the
Dexter was overpressured. Historically,
strong gas shows often were encountered
while drilling through the Dexter, even
with relatively high mud weights. Assuming the Dexter was overpressured,
the operator experimented with mud
weights as high as 10 pounds per gallon
in an attempt to control shale sloughing
and reduce what was thought to be overpressured gas influx that potentially could
create well control problems.
But mud weights greater than 9.2
pounds/gallon consistently caused severe
lost circulation at the weak interface between the Harris Delta and Dexter Shale.
Moreover, when the BHA drilled into
the Buda/Georgetown with high mud
weights, massive amounts of mud were
sometimes lost into Buda fractures.
OCTOBER 2015 91



American Oil and Gas Reporter - October 2015

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

Contents
American Oil and Gas Reporter - October 2015 - Cover1
American Oil and Gas Reporter - October 2015 - Cover2
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