Kamal M. Morsi
Abu Dhabi National Oil Co.
Abu Dhabi
Continuous chemical inhibition has prevented corrosion downhole, and tests indicate that Inconel 625 cladding will protect the christmas trees on wells producing sour gas from the Thamama C reservoir.
Metallic corrosion is a costly problem. Estimates indicate that corrosion costs the off industry several billion dollars per year. In addition, oil companies spend over $100 million/year on corrosion inhibitors for combating downhole tubular and casing corrosion.
Abu Dhabi National Oil Co. (Adnoc) has successfully completed wells in extremely harsh operating conditions with high temperatures, pressures, and high concentrations of H2S, C02, and brine.
Such environments require special materials for downhole and surface equipment.
The Thamama C reservoir, in an onshore gas field, produces gas containing H2S and C02 in the range of 0.7-8.0 mole % and 4.0 8.0 mole %, respectively. H2S and C02 distribution maps are shown in Figs. 1a and 1b. Gas composition is shown in Table 1.
The reservoir's original pressure was 4,300 psig at 8,500 ft subsea. Bottom hole temperature (BHT) is 260 F.
The Thamama C gas gathering system comprises 19 wells connected to four trunk lines that transport produced gas and associated condensate to a central processing plant (Fig. 1c). Two wells were put on production during July 1982. The remaining 17 gas wells were put on stream in April 1984.
MATERIAL SELECTION
Completion of sour, hot-gas producers is challenging because of the need for:
- Appropriate steel to withstand high pressure and temperature conditions
- Appropriate techniques for corrosion inhibition to ensure long material life with a minimum frequency of workovers.
Thamama C gas composition warrants either corrosion resistant alloys (CRA) or sour service carbon steel with corrosion inhibitors.
CRA tubing such as Incoloy 825 costs up to six times more than standard grade L-80 carbon steel tubing. In comparison, the estimated total cost (capital and operating) of a corrosion inhibition program for standard tubing comes to less than $100,000/well/year. This includes the cost of the chemical injection pump, chemical storage tank, corrosion inhibitor, maintenance of equipment, personnel, and all other costs necessary to operate the system continuously.
Based on material price, reservoir temperature, flow velocity, production rate and laboratory test results for the corrosion inhibitors, it was decided to select L 80 carbon steel tubing material together with a corrosion inhibitor injection system for 16 Thamama C gas producers.
For the remaining three wells, duplex stainless steel (SM 25 Cr 75) tubing was run without provisions for downhole chemical inhibition.
The two common methods of inhibitor injection are batch treatment and continuous chemical injection.
Batch treatment involves circulation and/or squeezing of inhibitor down the tubing or tubing casing annulus. As inhibitor is injected, a protective film coats the tubulars.
If the operator wants to inject inhibitor into the formation, back flow of the inhibitor in association with well effluent provides a good protective film of inhibitor.
Chemical injection gives continuous corrosion protection without interrupting well production because it permits either intermittent or continuous inhibitor injection at the lowest protective rate possible.
In view of these advantages it was decided to complete the wells with continuous chemical injection systems.
After evaluating a number of inhibitor injection systems, the system selected allowed chemical inhibitor injection down the tubing casing annulus and through a wire line retrievable chemical injection valve (CIV) installed in a side pocket mandrel (Fig. 2).
The tubing/casing annulus is filled with 10% corrosion inhibitor mixed with diesel oil.
All items below the side pocket mandrel, which is set as deep as possible, are made of CRA material because this area does not receive any corrosion inhibitor. The volume of corrosion inhibitor, which is continuously injected, is a function of total gas flow for each gas producer. The average inhibitor volume is 1 pint/MMscf.
The injected corrosion inhibitor allows the chemical to form a protective film on the tubing surface, wellhead equipment, flow lines, and trunk lines.
Since being commissioned in 1984, no downhole problems have been encountered in wells completed with either duplex stainless steel or carbon steel tubing with continuous chemical inhibition.
Based on this success, the new Thamama C gas wells being drilled will have L 80 and provision for continuous chemical inhibition.
CHRISTMAS TREES
Type AISI 410 stainless steel (SS), wellhead components rated at 5,000 psig maximum working pressure were installed on the 19 Thamama C gas wells.
Type AISI 410 SS is a martensitic type material extensively used on sour oil and gas wells. It comfortably meets API Type 3 properties, possesses good hardening ability characteristics, and offers a much better general weight loss corrosion resistance compared to low alloy steels. However, Adnoc has experienced a number of failures in wellheads since commissioning of the Thamama C gas wells.
The christmas tree of Well No. 18 has been repaired twice since well completion. The repair work was carried out during well cleaning operations.
Remedial work on Well No. 8 replaced the damaged primary and secondary seals and changed out the incorrectly designed tubing hanger's tie down screws.
The corroded christmas tree on Well No. 5 was replaced after killing the well.
The lower master valve of Well No. 6 was partially leaking, and the upper hydraulic master valve seized in an open position. The well was killed and the faulty christmas tree replaced.
Parts of the corroded christmas tree of Well No. 5 were sent to specialized laboratories for analysis. Results revealed the following:
- The material is AISI 410 SS as per specifications.
- The material was severely attacked by weight loss corrosion.
- Scanning electron microscopy revealed the presence of chlorides, sulfur, and calcium in the microstructure of the corrosion pits.
The chlorides either came from initially produced fluids that had varying concentrations of chloride, or from residual hydrochloric acid (HCI) trapped in crevices after well acidization. The origin of the relatively high sulfur content in the pits could be attributed to H2S in the produced gas. The calcium may have originated from the mud.
The general conclusion indicated that corrosion could be related to HCI attack following an acid job and/or the high concentration of chlorides produced during initial production. The AISI type 410 SS material could have suffered from unacceptable weight loss or localized corrosion (pitting or crevice) under high chlorides, C02, and temperature conditions in the presence of H2S in the produced gas.
In view of these problems, alternative materials were investigated for christmas trees. Clad lining materials such as Inconel 625 provide a highly effective means of producing fluids that are too aggressive for low alloy steel or even stainless steel completions.
The lining materials can withstand H2S, C02, and chlorides. The cladding process basically applies a thin layer of CRA to the surfaces of the low alloy steel component using highly refined welding techniques.
As a trial, one Inconel 625 weld clad christmas tree was installed on Well No. 18 in 1986. Inconel 625 (UNS-N06625) cladding was applied on the base 4130 metal.
No further problems were encountered with the AISI type 410 christmas trees. The cladded christmas tree installed on well No. 18 is performing satisfactorily.
INHIBITOR SELECTION
Recently, a study was conducted in a specialized laboratory to select the most appropriate christmas tree material and suitable corrosion inhibitors for existing and future gas wells to be completed in the same reservoir. The objectives of the study were to:
- Select a suitable corrosion inhibitor for the AISI 410 SS christmas tree and L-80 carbon steel tubing
- Investigate galvanic corrosion in partially clad christmas trees with alloy 625 overlaid on AISI 410 SS or carbon steel (AISI 4130).
As part of this study, a series of autoclave tests was conducted on coupons of AISI 4130 carbon steel and 12% Cr (AISI 410) SS exposed to simulated Thamama C sour gas environment, with and without chemical inhibition.
These tests were conducted on coupons of those materials that were freely corroded and galvanically coupled to alloy 625. The laboratory test results showed:
- One of the corrosion inhibitors was found to be inactive as an inhibitor and was accelerating the corrosion rate of AISI 410 SS.
- Galvanic corrosion occurred when coupling the AISI 410 SS with alloy 625 in both uninhibited and inhibited environments.
- No galvanic corrosion occurred by coupling 625 alloy with low alloy carbon steel (AISI 4130).
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