American Oil and Gas Reporter - October 2015 - 89

SpecialReport: Drilling Fluids Technology

OBM MPD Solves Drilling Challenges
By Harold E. McGowen III

BULLARD, TX-Managed pressure
drilling (MPD) was used successfully to
exploit the lower Cretaceous Buda,
Georgetown, Edwards, and Glen Rose
formations in Houston and Madison counties in South Texas, with the objective of
economically stimulating and commingling all the drilled zones.
A comprehensive solution was developed to combat the principal causes of
cost overruns related to weak zones in
the shallow formations above the productive intervals, wellbore stability issues
caused by sensitive shale formations, and
lost circulation related to natural fractures
in the productive interval. The operator
was faced with several shallow, low-fracture, gradient intervals above the Dexter
Shale, an extremely thick and sensitive
shale sequence overlying a 1,400-foot
pay interval consisting of the multiple
naturally fractured carbonate horizons.
Moreover, the lowermost productive
interval had a significantly higher pore
pressure and the potential for significant
gas deliverability, while the shallower
productive horizons had lower pore pressure and were primarily oil-productive
zones with associated gas. Problems encountered included excessive shale production, wellbore collapse, bottom-hole
assembly sticking, and severe lost circulation.
The operator drilled multiple wells
overbalanced with various water-based
mud systems in certain parts of the field.
Success with WBM required a great deal
of finesse, and drilling performance in
the most prolific producing areas was inconsistent and unpredictable. Unexpected
cost overruns occurred and several wellbores were lost and required re-drilling
with sidetracks.
MPD allowed the operator to use oilbased mud to control the sensitive shale
formations while drilling with a low
enough mud weight to prevent lost circulation. MPD dramatically reduced OBM
losses into extremely high-permeability
fractures in the productive strata. A rotating
control head along with a gas buster,
choke manifold, fluid handling system,
and flare system were used to flow while
drilling the deeper high-pressure/high-deliverability zones and maintain an "atbalance" bottom-hole pressure in the shal-

lower/lower pressure zones by judiciously
applying surface backpressure.
The OBM eliminated all issues with
the sensitive shale formations. A mud
weight (including equivalent circulating
density) of about 8.95 pounds per gallon
equivalent for most of the hole and a
base-line lost circulation material (LCM)
load proved sufficient to control lost circulation in most instances. Slugs containing a higher concentration of LCM
(cottonseed hulls along with graphite,
cellulosic fiber, and calcium carbonate)
were used while drilling intervals with
intense natural fracturing to prevent partial
lost circulation. In some extreme cases,
freshwater gel pills were used to combat
total lost circulation.
The operator developed a "mud cap"
technique to pull the BHA when total
depth had been achieved, which improved
well control and reduced OBM losses
while running and cementing casing. By
producing consistent and economic results
over a large number of wells across a
large area, the OBM MPD program made
the continuous and successful development
of the entire field possible.
Geologic Setting
As shown in Figure 1, the OBM MPD
well sites are located in an area between
the Angelina-Caldwell Flexure to the
northwest, the San Marcos Arch to the
southwest, the Sabine Uplift to the north-

east, and the Edwards-Sligo Reef to the
south. The play is situated uniquely in
the Gulf Coast Cretaceous fractured carbonate trend that runs onshore parallel to
the Gulf of Mexico, and below the southern edge of the East Texas Basin, where
the siliciclastic sediments poured into the
Brazos Basin from updip erosion of the
Quachita Mountains.
Figure 2 shows the stratigraphic column
from the base Georgetown through the
Glen Rose. The overarching goal of the
project was to drill economically viable
wells to prove up a leasehold of almost
100,000 gross acres. All the wells in the
drilling program had the same basic
drilling and completion objectives:
* Drill a surface hole and set surface
casing;
* Drill a vertical hole to a total depth
below the deepest Glen Rose formation;
* Run 5½-inch production casing;
* Bring the top of cement in the 5½inch production casing annulus to a point
above the Harris Delta/Woodbine formation; and
* Hydraulically fracture all the productive horizons (Buda/Georgetown/Edwards/Glen Rose A-E) in stages using
the plug-and-perforate method.
Subsequent to hydraulic fracturing,
the objective was to commingle all 1,400
feet of pay into a single completion by
drilling out the composite frac plugs. A
good cement job over all intervals was

FIGURE 1
Geologic Setting

OCTOBER 2015 89
American Oil and Gas Reporter - October 2015

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