A change in the chemical makeup of cleanup fluid in deepwater horizontal wells offshore Cabinda resulted in fewer trips, reduced formation damage, the elimination of intervention costs and better wells for the operator.

A new technique uses a chelant-based system that serves as gravel-pack carrier and cleanup fluid for the filter cake in wells, according to American Association of Drilling Engineers paper AADE-06-DF-HO-20 presented at the 2006 AADE Drilling Fluids Technical Conference and authored by C.S. Svodboda, M.R. Luyster and A. Patel with M-I SWACO and S.A. Ali of Chevron Corp.

Drilling program

In a typical drilling program for the field offshore Cabinda, the company batch drills the vertical holes and makes the turn to near-horizontal and drills to total depth, normally another 1,400 ft to 1,800 ft (427 m to 549 m), with an 81¼2-in. hole. During the drilling of the horizontal well, the company switches out to a clean, reversible reservoir drilling fluid using calcium carbonate to minimize whole mud and filtrate invasion into the surrounding rock.

It also uses the reversible, oil-based reservoir drilling fluid in Bohai Bay, China, and in the Gulf of Mexico.

With drilling completed, the company runs a pre-drilled 7-in. liner with an external casing packer and a pressure-actuated cementing (PAC) valve. The company then prepares the horizontal section for gravel packing, using a hydroxyethyl cellulose (HEC) viscosified push pill to displace the reversible reservoir drilling fluid system. It follows that with a slicked potassium chloride brine to pump the push pill back into the vertical casing above the shoe.
The brine primes the horizontal section with the base fluid that will be used to carry the gravel pack. After pumping sacrificial clean-up brine, it re-enters with filtered potassium chloride brine that meets clarity standards.

Following a circulation-rate test, the company uses the brine to run in 12/18 synthetic proppant as the gravel pack. After setting the gravel pack, the company pumps a viscous pill to sweep the annulus and then turns pumping operations over to the rig.

Former technique

Before using the chelant-enhanced solution, the company used no acetic or hydrochloric acid-based systems to clean up the residual filter cake and dissolve the calcium carbonate bridging solids. It spotted the acid following the completion, which required another trip in and out of the well bore. In some cases, the company used coiled tubing.

The company tried another technique. It ran a stinger on a work string, but the stinger only extended up to 35 ft (11 m) into the heel of the horizontal section once it stung through the packer. That made it unlikely that the acid would reach the toe of the well, particularly since early dissolution of the filter cake at the heel would promote wormholing.

Something better

The operator needed something better. It wanted efficient dispersion or dissolution of the reversible reservoir drilling fluid filter cake. It wanted the capability to delay the cleanup without eroding or removing the residual filter cake until it achieved mechanical isolation of the horizontal section, and it wanted the ability to use a compound for cleanup that also could be used as the gravel-packing carrying fluid.

Laboratory tests showed a chelant-enhanced system could meet those requirements.
The use of reversible invert reservoir drilling fluid, by manipulation of surfactant, will convert a water-in-oil emulsion to an oil-in-water emulsion. By changing the drilling fluid from
an oil-wet state to a water-wet state the reversible fluid decreases formation damage and simplifies wellbore cleanup at significantly lower cost. But, there’s a risk. The use of acids during cleanup creates an instant reaction in the horizontal section that can lead to the premature loss of fluids to the producing formation.

New method

Chelating agents (poly carboxylic amino acids) block reactions with metal ions. In this case, the agent would first slow reactions with any iron present in the fluid system, and then it would slow reactions with metal-ion calcium in the calcium carbonate filter cake formed with the use of the calcium carbonate bridging agent during completion.

Among advantages of using a chelating agent in a breaker system are:
• Relatively slow reaction rate;
• Minimal corrosion;
• Mitigation of emulsions and system upsets; and
• Compatibility with formation fluids.

The companies took their findings and to the laboratory for testing and to validate calculations of the erosion of the reversible reservoir drilling fluid filtercake using the chelant-based cleanup system, particularly the ability to mitigate that erosion and delay the dispersion of the filtercake until the completion of the gravel-packing operation.

At the Cabinda site, the team would need about a 4-hour delay in the reaction to give it time to set the gravel pack and pull the work string.

If the dissolution of the filter cake was delayed past that point, the operator could close the flapper valve at the heel of the horizontal section and leave the cleanup fluid in contact with the filtercake in the horizontal hole until it dissolved and dispersed the filtercake.

It looked as if they had a working system.

Field operations

The acidity of the solution was a key element in the successful working of the cleanup system. Unfortunately, large tanks for blending are scarce in the Cabinda area, so the companies batch-mixed the first two deepwater completions with the new technique. For each of those completions, it loaded out the final blend in plastic-lined tanks. With those tanks, the chelant-based cleanup system could be stored without deterioration for several weeks if the operator faced delays in rig availability.

That batch-mixing system required only one tank at the well site, but that tank had to hold enough mix for as much as 7,500 gallons of 9.2 lb/gallon chelant-based cleanup fluid. Since available tanks had a capacity of only 5,000 gallons, the group had to concentrate the solution and mix it with 9.7 lb/gallon potassium chloride brine on location to get the required density for the gravel-pack placement.

The group also had to carefully monitor the acidity of the mix to maintain a pH of 5. If it had to raise the pH, it added potassium hydroxide.

It also needed a hydroxyethyl cellulose viscosified (HEC) solution as a slicking agent to “slick” the gravel-packing fluid. The group tested concentrations of 20 and 40 lb/1,000 gallons. Both reduced friction pressure.

Application

In the first two completions, the group spotted the chelant-based system after placing the gravel pack. In the next three wells, the testing team placed the system during the gravel packing operation, pulled the work string and closed the flapper valve to isolate the horizontal section and let the mixture soak and dissolve the filtercake.

The application resulted in only anticipated fluid losses of four to 6 bbl an hour.
More striking, according to the paper’s authors, “To date, no post-intervention cleanup was required. This cleanup strategy has been successfully applied in five openhole gravel-pack completions and initial production has met expectations.”

Conclusions

During the development, testing and application of the new system, the group members learned some valuable lessons. Obviously, one lesson was that a chelant-based cleanup system works with a reversible reservoir drilling fluid system.

They also found a dipotassium ethylenediaminetetraacetic acid (EDTA) chelant with an amine surfactant will mix with the HEC slicking agent to disperse filtercake.

They found that they needed a pH between 5 and 6 to get a synergistic action between the EDTA and the amine surfactant. That gave them the reversal from oil-wet solids to a water-wet state in the filtercake, promoted the chelation of the metal calcium ion in calcium carbonate and the slowing of the reactions with iron ions.

They also learned that the plastic-lined tanks could prolong the life of the chelate-based fluid. In one sample test, there were no relative changes in the blending targets after 24 days, except for degradation of the rheology, and the addition of the HEC solved that hitch.

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