American Oil and Gas Reporter - October 2015 - 90

SpecialReport: Drilling Fluids Technology
FIGURE 2
Stratigraphic Column

Base Georgetown
(Duck Creek Lime)
Kiamichi Shale

Type Log:
NAVIDAD: Well A
Base Georgetown
through Glen Rose

Fort Trinidad area
multiple producing
formations

Edwards

Paluxy

Glen Rose

Multiple pays within
producting formations

Field pay intervals

critical to ensuring that each zone was
treated effectively with hydraulic fracturing.
An important business objective was
to demonstrate the ability to incrementally
reduce drilling and completion costs over

90 THE AMERICAN OIL & GAS REPORTER

time. In addition to reducing overall costs,
it was important to demonstrate predictable
and consistent well performance results
(drilling and completion costs) to facilitate
an accurate economic evaluation of the
program.

WBM Drilling Problems
The numerous drilling problems encountered in the target area when employing conventional drilling techniques
using WBM included:
* Shallow water-sensitive shale formations above the Austin Chalk (namely
the Midway Shale);
* BHA sticking through the Harris
Delta/Woodbine;
* Instability and borehole col lapse/caving issues in the Dexter Shale,
particularly in the finely laminated, water-sensitive Maness Shale (the lower
100 feet of the Dexter Shale sequence
immediately above the Buda), which exhibits strong gas shows and is subject to
the most severe sloughing and washouts
when drilled with WBM;
* Lost circulation above the carbonate
formations, particularly at the bottom of
the Harris Delta/Woodbine and the top
of the Dexter Shale (as WBM weight
was increased to control these intervals,
mud returns inevitably would be lost as
the BHA passed through formation interfaces);
* Lost circulation in the carbonate
formations, especially the Buda (up to
2,000 barrels of WBM in the most severe
cases);
* Higher pore pressures in the volatile
Lower Glen Rose "A" through "G" oil
reservoirs (the Glen Rose exhibits reservoir
pressure between 9.1 and 9.4 PPGE,
which is slightly overpressured in comparison with a normal 8.95 PPGE/0.465
psi/foot gradient in the uphole productive
intervals);
* Difficulties running casing, given
the Dexter Shale's tendency to wash out
and produce large volumes of cavings
(making it difficult to achieve the annular
velocities required to clean cavings and
cuttings out of the hole); and
* Reduced operational flexibility and
resilience, since the formations would
degrade rapidly when exposed to WBM
as a result of mud filtrate pressure penetration, chemical potential differences,
swelling, and hydration (making time exposure a critical factor when drilling with
WBM).
The need for repeatable, predictable
performance drove the operator to consider
using OBM. While the disadvantages of
an OBM include higher cost and increased
handling and disposal requirements, OBM
offered several advantages over WBM
for these wells, including improved thermal
stability, increased lubricity, and better
American Oil and Gas Reporter - October 2015

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