Oil & Natural Gas Projects
Exploration and Production Technologies
Cavity-Like Completions in Weak Sands
This project was selected under DOE Solicitation DE-PS26-02BC15377, Technology
Development with Independents Program.
The project goal was to to demonstrate the effectiveness of a new completion
strategy by field trials. The objective is to increase oil production by deliberately
producing sand from a reservoir, creating an underground cavity around the wellbore
that allows oil to flow more easily from the surrounding formation.
Global Petroleum Research Institute (GPRI)
Texas A&M University
College Station, TX
The project conducted laboratory and field tests of a model to predict the performance
of cavity-like completions (CLCs). Texas A&M's Global Petroleum Research
Institute, in a joint venture with a group of oil and gas companies, documented
six field involving CLCs.
A test of the CLC method in Mustang Island, TX, gas wells resulted in incremental
cash of $525,000/month.
CLC offers a new technique to complete wells in friable and unconsolidated sands.
A successfully designed CLC provides significant increases in well performance
at lower costs than alternative completion techniques.
Unconsolidated or friable sand formations pose unique completion problems related
to near-wellbore damage to the formation, resulting in reduced production and
increased costs in wellbore cleanout and recompletion. Sand production is both
a safety issue and expensive to prevent. The objective of the field demonstration
was to show industry that CLC methods offer a more cost-effective method to
produce oil and gas from friable reservoirs and prevent excess sand production.
The concept of a cavity completion is to increase the effective wellbore radius
and reduce skin by removing existing near-wellbore damage. A small volume of
sand removed can reduce a high positive skin factor dramatically. High positive
skin factors that are normally associated with cased and perforated completions
can be reduced dramatically with a properly designed CLC. In several CLCs where
reliable information is available from the field, low skin factors have been
indicated. The promise of CLC technology is to be able to initiate the well
with a low skin factor or to recomplete it and realize increased productivity.
The project modeled four different friable sand scenarios:
- Weakly cemented sediments.
- Thin pay zones overlain by competent cap rock.
- Multiple thin layers.
- Exposure to wear from shutdowns and start-ups.
Factors influencing sand stability included depth (effective stress), particle
size distribution, arching, rapid changes in pressure, and loss of capillary
Researchers designed a CLC method that developed a stable cavity near the
wellbore into which the oil could flow. This entailed flowing the well hard
to get the loose sand out, then producing the well at a slightly lower rate.
To develop and sustain a stable cavity, it is essential for the formation
to have some degree of cementation, which allows for near-wellbore sand to
be produced while maintaining a stable sand face. A controlled clean-up strategy
must be established to assure a stable sand face.
Understanding cavity growth and stabilization requires consideration of the
combined effects of fluid flow, changing in-situ stresses, material failure
and material deformation. Modeling the possible scenarios-and factoring in
these difficulties-provides a "best practices" method for CLCs.
In addition to modeling the completion methods, an economic model for CLC
was prepared. The CLC procedure can be repeated on individual wells as needed.
Analysis was completed for carbonate wellbores as well as friable sands.
A series of case studies of the CLC method was conducted in the Gulf of Mexico.
An overview of these wells was used to design a field demonstration on Mustang
Island, TX. In this successful cavity completion on the Gulf of Mexico shelf,
the sand-free flow rate was increased from 1 million standard cubic feet per
day (MMscf/day) to 4.5 MMscf/day of gas after 10 barrels of sand was produced.
An acoustic sand detector was deployed to continuously monitor sanding. Incremental
revenue from the CLC procedure was $12,800 per day, minus the cost of $5,700
per day, for a profit of $7,000. The Mustang Island well continued to flow
at the increased rates for six months before watering out. The post-cavitation
profit was $3.15 million.
Current Status (November 2005)
The project was completed in April 2004. The industry partners in the project
are continuing to use the CLC technique in Texas. The procedure has been modeled
for use in giant Ekofisk oil field off Norway.
The final technical report is available from NETL by calling 918-699-2000.
Project Start: September 1, 2001
Project End: April 30, 2004
Anticipated DOE Contribution: $130,000
Performer Contribution: $130,000 (50% of total)
NETL - Gary Walker
NETL - Deann Rhea (firstname.lastname@example.org or 918-699-2003)
Texas A&M - D.B. Burnett (email@example.com or 979-854-2274)
Schematic "pathway" leading to rock disaggregation, sand production
Rock failure is a function of shear strength. Sand production is also a function
of tensile strength, stiffness characteristics, drawdown, and total strain.