A key goal of the Corrosion and Scale at Extreme Temperature and Pressure project was the creation of next-generation methods and technology for use in extreme HP/HT scale and corrosion science for new offshore oil and gas production. There is a lack of data and models for corrosion and scale at extreme temperatures and pressures encountered in ultradeepwater reservoirs. The project aimed at shedding new light on these areas, thereby increasing economic security and decreasing risk in offshore production. The project was funded through the Research Partnership to Secure Energy for America and the U.S. Department of Energy.

Through this project, new methodologies for testing at temperatures and pressures ranging from normal production through extreme production conditions (up to 24,000 psi and 250 C [482 F]) were established. The project, led by Tomson Technologies, made new discoveries in inorganic scale types, developed new standards, created more accurate scale and corrosion predictive models and is poised for additional scientific discoveries through continued research.

Project summary
Tomson Technologies conducted the theoretical and experimental extreme HP/HT research at realistic ultradeepwater temperature, pressure and salinity conditions to validate parameters for HP/HT modeling and to assess the risk of scale and corrosion. Realistic, reproducible and rapid methodologies were developed to study scale, corrosion and inhibition at HP/HT, strictly anoxic conditions (less than 1 part per billion [ppb] of oxygen) and realistic field brine compositions.

A methodology to apply vertical scanning interferometry (VSI) as a tool to automatically inspect the severity of general and pitting corrosion and deposit formation was developed. VSI requires little sample preparation, uses nondestructive white light and has vertical resolution of about 0.1 nanometers. In collaboration with Bruker, methods to use VSI for measuring both localized and uniform corrosion also were developed. The key deliverables include solubility of various scale species of interest at HP/HT, realistic brine composition and strictly anoxic conditions.

A large database for material selection was created for a wide variety of steel alloy types with experimental data from linear polarization resistance; weight loss; cyclic polarization; electrochemical impedance spectroscopy; and detailed analysis, including scanning electron microscope, X-ray diffraction, VSI and other methods. Various scale and corrosion inhibitors were evaluated for their performance and temperature stability up to extreme HP/HT conditions. Kinetics and minimum inhibitor
concentration dosage for various scale species of interest under realistic field conditions were evaluated.

Developed extreme HP/HT test methodologies, equipment
To fully assess corrosion at extreme HP/HT conditions, new methods using imaging techniques such as VSI were combined with HP/HT electrochemistry probe data from autoclave experiments, inductively coupled plasma solution measurements and weight loss data. A new methodology to use VSI as a tool to rapidly and accurately analyze general and pitting corrosion was created. The project designed, built and tested both static and dynamic extreme HP/HT equipment used to study scale and corrosion up to 24,000 psi, 250 C and 300,000 mg/l total dissolved solids (TDS).

This project’s advisory panel, consisting of 18 oil and gas production companies and service companies, recommended a series of alloys and brines for detailed research. The project used these fluids and alloys to further simulate realistic scale and corrosion effects in ultradeepwater production. The project experimentally obtained scale solubility measurements between the range of 100 C (212 F) to 250 C and 10,000 psi to 24,000 psi with salt concentrations that represent wells found in the ultradeepwater Gulf of Mexico.

Systems of interest such as calcium carbonate, barium sulfate, iron carbonate, iron sulfide and iron oxide scales and inhibitors with various brine compositions were studied. Until this work, scale and corrosion tendencies of these systems at extreme HP/HT conditions were unknown, and these accurate measurements helped yield more accurate prediction modeling.

New methodology: strictly anoxic testing
The current industry standard for removing dissolved oxygen from solution is to sparge. However, when studying oxidation-sensitive scale and corrosion effects, improvements were needed to reduce the dissolved oxygen content. The project designed and developed a laboratory system to produce strictly anoxic test solutions without altering the chemical makeup of the fluid. The system was able to create these solutions on the fly in real time to feed flowthrough scale and corrosion experiments. Strictly anoxic conditions were used to simulate the downhole reservoir environment with extremely low dissolved oxygen concentration and were applied throughout this work. In the presence of only a few tens of mg/l of dissolved oxygen, the reaction products can be quite different from what occurs in strictly anoxic conditions, as in production wells with virtually zero dissolved oxygen.

New observation: ankerite
Siderite and magnetite are often used as model compounds in corrosion studies, but Tomson Technologies was able to demonstrate that in the presence of field brine with a distribution of divalent metal ions, the most prevalent phase to form can be ankerite. For example, ankerite formed as the corrosion product layer instead of magnetite at 200 C with simulated field brine under anoxic conditions. In addition, the dramatic effect of dissolved oxygen on siderite particle formation and appearance also was observed.

Experiments were conducted using a custom Hastelloy C-276 dynamic flow-through apparatus built during Phase I for solubility studies of various minerals including siderite, troilite and magnetite under various extreme HP/HT and TDS levels. Data collected in this research expand the solubility database of these minerals into the extreme HP/HT range, which has not previously been shown.

High-temperature scale inhibitors
The industry struggles with finding high-temperature-resistant scale inhibitors. Through this research, better candidates have been identified. Nucleation kinetics and inhibition of various scales such as siderite, iron oxides and barite under extreme HP/HT conditions were investigated. Seven commercial-scale inhibitors including polymeric inhibitors and phosphonates were tested for scale inhibition. Most of these inhibitors did not show effective inhibition for siderite at or above 100 C. However, neutral species carboxymethyl inulin was found to perform significantly better than others under these conditions. At 250 C, only sulfonated polycarboxylic acid (SPCA) showed inhibition or stabilization for iron oxides between 1 mg/l and 10 mg/l active inhibitor concentrations.

Another development seen during this project was that overdosing SPCA resulted in an iron-SPCA pseudo-scale complex, which removed inhibitor from solution and created a secondary flow assurance issue with precipitation of this pseudo-scale complex. Proper dosing is key in controlling scale under any circumstance, and this was shown to be especially true at HP/HT conditions.

Scale has major impacts on corrosion
Studying the interplay of corrosion and scale, the project found that calcite’s scale tendency has a strong effect on the products and extent of corrosion on metal samples. To simulate calcite in equilibrium with the reservoir at downhole conditions, the saturation index for calcite was set to zero. Corrosion was then studied with C1018 steel coupons under these scaling tendencies. Tomson Technologies found that corrosion at 200 C was not severe (0.10 mm/year) and that the corrosion products were ankerite and siderite. Pitting corrosion was studied by VSI and showed no pitting in the test coupon. However, at a calcite saturation index of 1.15 at the same temperature (200 C), corrosion was severe, and the corrosion products were magnetite and ankerite, not siderite.