American Oil and Gas Reporter - April 2017 - 90

system was selected.
Previously, STMZ technology never had been used in the deep,
HP/HT conditions encountered in Lower Tertiary wells. Consequently, a large bore version was developed specifically for Cascade/Chinook. The larger-diameter system enables reduced
well flow restrictions, but also requires large-bore production casing capable of withstanding extreme pressure loads. STMZ techFIGURE 2
Open-Hole Wilcox Formation Log Section
(Cascade "A" Well)
SS

Wilcox 1

MD
ft
25,500

Fault

26,000
Calcitic SS
Wilcox 2

O/W
Contact

Silty SS

Wet SS

26,500

Shale

Wilcox 3

27,000

15.00

67.50
Gamma Ray

120 0.10

Resistivity

100 0.45

1.95

0.15
Neutron
2.45
Density

-0.15 18,000
2.95

19,500 21,000
BHP Curve

TABLE 1
Average Formation Properties
Variable

Estimated Value

Formation Depth
Water Depth

15,100 ft TVDss
8,143 ft

Formation Thickness

Wilcox 1 - 400 ft TVD
Wilcox 2 - 600 ft TVD (to water contact)

Porosity

16-20%

Permeability, k

Wilcox 1: 10 to 110 md kAvg ≈ 25 md
Wilcox 2: 2 to 30 md, kAvg < 10 md

Reservoir Pressure

19,300 psi at 25,500 ft TVD - 0.75 psi/ft

Fluid loss height, Hl

Assume all sand is fluid loss H

Fluid loss coefficient, C

Wilcox 1: 0.004 ft/√min
Wilcox 2: 0.003 ft/√min

Young's Modulus, E
Poisson's Ratio

In situ stress

2 to 3✕105 psi - Sand & Shale
6 to 8✕105 psi - Thin "Hard" Streaks
0.25 to 0.27
Sand - 0.77 psi/ft
Maximum stress difference between
sand/shale layers, 100 psi
(i.e., < 0.01 psi/foot of depth TVD)

Formation Temperature

260 °F

Deviation over Perfs

17 deg

90 THE AMERICAN OIL & GAS REPORTER

nology has provided high flow rate completions and highly efficient fracturing while reducing operational time. Given its high
success rate, the operator continues to believe this type of system is the optimal way to economically complete Lower Tertiary wells.
Completion Procedures
In the Cascade/Chinook STMZ completion operations, all intervals in a well were perforated with a single tubing-conveyed
perforating run. Next, all downhole completion hardware (such
as screens, sleeves and packers) were assembled and run in the
well in a single trip and the packers were hydraulically set.
Using an inner work string with shifting collets, a frac sleeve
at the top of each interval and a pressure monitoring sleeve at the
bottom of the interval were shifted open before the interval was
frac packed. Before moving to the next interval, the sleeves of
the frac packed interval were closed, providing isolation between
the wellbore and reservoir. The zones were frac packed sequentially from the bottom interval to the top.
The completion design requires a reliable production string
to allow efficient operation of the well for the project's expected 25-year life. The diameter size of the production tubing allows
a flexible range of flow rates. In addition, corrosion-resistant alloys were used in all flow-wetted components.
The upper completion was designed to achieve multiple goals,
including pressure data collection, flow assurance and regulatory requirements, while eliminating the need for downhole
through-tubing wireline or slickline operations. The upper completion included a hydraulically actuated circulation device, a hydraulically set 10,000-psi production packer, a 25,000-psi dual
pressure/temperature gauge, a two-point downhole chemical injection system for asphaltene and scale inhibitors, and an ultradeep-set surface controlled safety valve with a redundant hydraulic
system operated by two separate control lines.
The completion design identified two areas where trapped pressures were present during the isolation assembly string and the
upper completion string. Specific operation procedures were implemented to mitigate the issue by reducing the trapped pressure
left between the packers. Tubing stress analysis was performed
to identify safe production limits in order to preserve the integrity of the production string components.
Fracturing Program
The Cascade A well was completed with three propped fracture treatments in the two uppermost Wilcox zones. Typical formation properties are included in Table 1, with an open-hole log
section illustrated in Figure 2. The major questions addressed during the preliminary design included fracture treatments capable
of treating the 1,200-foot thick interval, the desired fracture halflength and conductivity, the optimum perforation length and locations, and the desired pump rate needed to fracture the deep,
high-pressure formation.
One of the key enhancements made to the STMZ system was
incorporating multiple pressure/temperature gauges along the perforation interval to better understand fracture propagation during post-frac analysis. Data from the gauges were invaluable in
demonstrating fluid flow across the perforations during frac jobs.
Figure 3 shows pumping data from a Cascade frac job demonstrating even cooling across the perforation interval. This indicates that the entire interval was evenly treated and that the fracture initiated vertically at the wellbore.
High closure stress (>22,000 psi) in the Cascade Field was
expected to exceed the overburden stress, potentially creating
multiple fractures near the wellbore, or worse, horizontal frac-



American Oil and Gas Reporter - April 2017

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