Best practices limit fluid effects on CRA tubulars

Oct. 16, 2006
Proper procedures in the design of clear brine completion fluids combined with proper handling of the fluids can mitigate annular environmentally assisted cracking (AEAC) in wells completed with corrosion-resistant alloys (CRAs), according to TETRA Technologies Inc.

Proper procedures in the design of clear brine completion fluids combined with proper handling of the fluids can mitigate annular environmentally assisted cracking (AEAC) in wells completed with corrosion-resistant alloys (CRAs), according to TETRA Technologies Inc.

TETRA says that it and JFE Steel Corp. spent more than 3 years researching corrosive factors associated with AEAC.

Based on this research, the company has written software programs and guidelines for selecting the fluid and determining the best practices for ensuring proper quality controls during transportation and handling of the completion fluids from the plant to the wellbore.

AEAC research program

The companies conducted more than 3,700 tests simulating actual conditions found in a well. The tests examined various combinations of highly stressed CRAs with numerous clear-brine-fluid blends in a broad range of fluid densities and temperatures (100-400° F.).

The testing program also looked at the effect of corrosion inhibitor packages and common contaminants such as those that might leak through faulty connections or packers or be introduced by careless transportation or handling practices.

From this data, TETRA developed a software program called MatchWell Fluid Compatibility Selector to assist operators in choosing the safest and most cost effective fluids for the specific metallurgy in a well.

Field contamination

To minimize potential for AEAC events, TETRA says it is equally important to retain the requisite chemical parameters of the clear brine fluid from the time it leaves the manufacturing facility until the fluid enters the well.

Its research improved the understanding of the relationship between contaminating substances in the fluid and AEAC events. The testing showed many opportunities for contamination and chemical compromise that could cause the fluid to deviate from the original specifications set by the operator (Fig. 1).

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In fact, Tetra says that field contamination could contribute as much to AEAC failures as selecting a fluid that is incompatible with the tubing.

Transportation, handling

Quality control during transportation and throughout the handling of the fluid in the field is important, and TETRA says the practices that transporters, resellers, and end-users follow often are inconsistent and can compromise the fluid chemistry. The company’s testing showed that even small amounts of contaminants can contribute to initiating AEAC.

For this reason, TETRA developed a set of best practice guidelines and procedures to help fluid suppliers, transporting contractors, and operators to ensure proper quality control of the fluids. It says maintaining active quality assurance and control throughout the transportation cycle ensures that fluids pumped into a well will have the same specifications as ordered or designed.

The company determined that the chemical effectiveness of the fluid can be compromised when fluids are conveyed in lines or placed in trucks, workboats, or rig tanks that previously held a fluid incompatible with the metallurgy used in the well completion.

TETRA, therefore, designed a best practices program that provides specific guidelines and procedures for inspecting and cleaning trucks, tanks, lines, and vessels to ascertain that the chemical integrity of the clear brine fluids has been maintained.

For example, because the use of thiocyanate-containing corrosion inhibitors is common in fluids used in non-CRA applications, the company says it is reasonable to assume that any vessel or storage tank may be exposed routinely to a reclaimed fluid containing the thiocyanate ion or other contaminants. As a consequence, small but significant amounts of residual thiocyanate or related sulfur compounds can be inadvertently introduced into a clear brine fluid.

These contaminants can be damaging even for wells with lower pressures and temperatures than would normally be associated with high pressure, high-temperature (HPHT) applications, the company says.

It adds that thiocyanate and other contaminants pose a potential risk to tubing failure, but such contamination is avoidable through proper inspection and cleaning when necessary.

TETRA’s procedures specify maximum limits of allowable thiocyanate and sulfur and other potentially contaminating substances, below which the properties and the performance of the fluid will not be compromised for the intended CRA application.

Its new best-practices program ensures that chemical integrity of the fluid is maintained throughout the transportation cycle, TETRA says.

TETRA has developed a method for sampling the fluid prior to each transfer for quality-assurance analysis (Fig. 2).

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It will file the results of these analyses to provide a quality-control trail, documenting the proper maintenance of the fluid during each step of the fluid’s trip from the plant to the well.

Fluid at the well

Tetra says improper handling of the fluid at the well is equally important as handling during the transportation cycle. It, therefore, includes in its best-practices program a guide for rig personnel that covers proper procedures to receive, store, and handle all fluids so as to minimize AEAC.

The procedures address inspection and cleaning of rig tanks and lines and involve additional sampling and analysis by TETRA technicians to attest to the chemical integrity of the clear brine fluid.

The program can alert rig personnel to the potential for contamination through otherwise normal rig procedures and preclude other potential problems.

For example, Tetra says a common practice of some operating and drilling companies is to use corrosion inhibitors in brine fluids as a means of protecting storage tanks from corrosion. In some cases, the practice results in contaminating the fluids and possibly causing stress corrosion cracking in the well.

The program also provides safe procedures to deal with an incomplete drilling mud displacement, which Tetra says is a common occurrence in many well completions.

It warns that the effects of an incomplete displacement can be particularly pernicious in wells drilled with oil-based mud.

TETRA says that with the muds having entrained additives, residual mud in the hole becomes a potential source of sulfur that can contribute to sulfide stress cracking, a form of AEAC.

Final steps

TETRA’s program also recommends taking a final sample from the well annulus before putting it on production.

If this fluid is found to be contaminated by materials such as thiocyanate, iron, or other detrimental substances, the operator can take remedial steps to restore the fluid to a proper chemical balance to minimize potential future AEAC problems during production.