American Oil and Gas Reporter - April 2017 - 89

SpecialReport: Offshore & Gulf of Mexico
The producing reservoir section at Cascade consists of
Eocene-Paleocene (Wilcox 1 and Wilcox 2) layered sandstones,
silts and shales. The layered producing section has a large gross
thickness that extends vertically for more than 1,000 feet, but
has low horizontal permeability (<100 milliDarcies) and zero
vertical permeability between the sandstone layers. Other
characteristics include very high static formation pressure (19,500
psi), temperatures to 250 degrees Fahrenheit, and a low gas-tooil ratio (180 cubic feet/barrel). The oil has a saturation pressure of 1,000 psi with 1.5 percent carbon dioxide concentration
in the gaseous phase.
In engineering the completion design for the Cascade wells,
Petrobras considered using at least three frac packs to communicate the various producing zones to the wellbore. The initial lack
of knowledge about the Lower Tertiary reservoir and its production behavior drove the drilling and completion design to mitigate
various potential risks. The overall result was a strong design that
allowed the well to produce at high flow rates with extremely high
drawdown, while maintaining the fracture geometry/conductivity and collecting reservoir information.
The fracturing design was optimized to reduce cost without
compromising the final stimulation objectives of optimum fracture geometry and conductivity. In order to achieve such results
from the first completed well (the Cascade "A," the first producing well in the Lower Tertiary play), a thorough data collection
and rock mechanics analysis was implemented along with stateof-the-art fracturing design.
The well test analysis showed expected skin values for the
stimulated producing wells. The pressure transient analysis (PTA)
of the various well tests was quite complex as a result of the simultaneous production of several sandstone layers spread over
hundreds of feet. Extensive data collection was implemented to
characterize the reservoir and answer questions about aquifer communication, compartmentalization, flow patterns and reservoir
continuity.
Production management has identified the requirement of
reaching very low flowing bottom-hole pressures (BHPs) by applying booster pumps at the seafloor. The low flowing BHPs impose very high stress loads on all completion components. To accommodate all well design requirements, the production casing
anticipated large bore completion equipment (8.5-inch internal
diameter). As a result, several drilling sections were under-reamed
and cased with heavy-weight pipe to withstand all anticipated stress
loads during the project's 25-year expected production life.

and the producing downthrown Northeast block. A very complex
and intense secondary system of faults can be recognized in both
sides of the main fault.
A four-way closure also has been mapped in the downthrown
block, where all three producing wells are located. The up-thrown
block is water-saturated, as shown by an appraisal well (Figure
1). The top of the producing reservoir is at a depth of 25,000 feet.
It is part of the thick Wilcox formation and is subdivided in four
stratigraphic zones. At Cascade, only the Wilcox 1 and Wilcox
2 are completed at the three producing well locations, except in
the second ("B") well, in which tar was encountered in the Wilcox
2 zone and prevented a completion.
The fine- to very fine-grained producing Wilcox sandstone was
deposited by distal low-density turbidite currents and has 10-25
percent porosity and an average permeability of 15 mD. The total net pay ranges between 300 and 500 feet, and the gross oil column is 1,500 feet thick. There are variable oil/water contacts and
several fault-defined producing block compartments. Oil is
highly sub-saturated and has variable API values from 18 to 26
degrees, but the produced oil has a gravity of 24 degrees and a
viscosity of 8-19 centipoise.
Completion Design
Completing wells at depths to 27,000 feet with a large gross
reservoir thickness using conventional stacked frac packs is very
challenging and time-consuming because of several ultrahigh-pressure perforating operations and additional trips to deploy hardware and execute the fracturing. Recognizing that reducing completion time is critical to Lower Tertiary well economics, a goal
from the outset of the Cascade/Chinook project was to develop
a strategy that would reduce completion time without sacrificing well productivity, ensure the safety of personnel and the environment, and maintain well control during all operations.
Several completion scenarios were examined to reduce completion time, including stacked frac packs, open-hole completions,
standalone screens, natural completions, cased and fractured with
no sand control, and horizontal multilateral completions. Each
scenario was analyzed with respect to production deliverability,
cost and risk. Ultimately, a single-trip multizone (STMZ) frac pack
FIGURE 1
Cascade Field Top Wilcox 1 Zone Structure Map
With Well Locations

Accounting For Uncertainties
The Cascade A well's design included enough flexibility to
account for initial uncertainties related to lithology, pore pressure,
fracture gradients, stimulation operations requirements, flow rates,
production management, drilling rig limitations and well control
requirements. Several lessons have been learned on Cascade that
not only have eliminated many of the early uncertainties, but the
new knowledge also is being applied to make well design enhancements feasible and reduce well construction costs in the future.
The Cascade Field is located in a remote area beyond the Sigsbee Escarpment. The trapping structure (Figure 1) is the result
of ascending autochthonous underlying salt that deformed the thick
Lower Tertiary turbiditic Wilcox package and produced a rounded salt-cored anticline, faulted northwest-to-southeast with two
well-defined main blocks: the up-thrown Southwest fault block

Legend
Exploration/Appraisal Well
Development Wells

APRIL 2017 89

American Oil and Gas Reporter - April 2017

Table of Contents for the Digital Edition of American Oil and Gas Reporter - April 2017