E XPERIENCE IN THE DEVELOPMENT AND USE OF ALUMINUM ALLOY CASING FOR CONDITIONS OF HIGH CONTENT OF ACID GASES IN FORMATION FLUIDS

. When selecting a casing material for fields with a high content of H 2 S and CO 2 , it is recommended to use specialized corrosion-resistant tubulars with high content of chrome of the Sanicro 29 type. The high cost of the material can be critical for the project economy. A promising approach for these problems elimination could be the application of aluminium alloy casing pipes. They are remain inert to corrosion even if the formation environment is fully saturated with H 2 S and/or CO 2 . They are also lightweight, have high strength-to-weight ratio, and thus decrease the existing tensions in the string and reduce well construction costs.


Description of the oilfield for application test
For Bayandyskoye oilfield, belonging to "LUKOIL-Komi" LLC, within the framework of pilot works the 1953T1 aluminum alloy casing was designed, manufactured and successfully applied as a production sting.
Operating All casing pipes came with highly sealed Tenaris Blue threaded connections. Casing equipment (stage cementing collars, packers, valves) was also made with similar high-strength corrosion-resistant steel grades.

Selection of aluminum alloy and corrosion testing
The casing material selection was based on the results of corrosion tests in compliance with NACE TM0177, ASTM G47, GOST 9.019, ASTM G31, at different pressure and temperature of 88°C [1 -5]. Considering rated strength, corrosion resistance and based on results of stress-strain state analysis of casing string, a highstrength aluminum alloy 1953Т1 was chosen as a material for aluminium alloy casing. This material underwent complex testing in corrosion aggressive environment similar to well conditions [6].
Taking into account the different stages of the well construction process, corrosion tests were carried out in four environments: − a mixture of oil, formation fluid and gas components (hydrogen sulfide and carbon dioxide), corresponds to the corrosive properties of the produced oil; − the same, but without the oil component, corresponds to the formation fluid at the well completion stage. − solution based on HCl, imitating the acid treatment of the bottomhole zone. − alkaline environment that simulates the conditions of cementing wells. Tests in these environments include, above all, the standard types of corrosion tests: − general corrosion; − contact corrosion; − corrosion cracking; − special tests of samples that simulate the conditions of acid treatment and well cementing; − autoclave tests.

Tests in reservoir gas-saturated fluidenvironments 1 and 2
The mineral component of the test fluid was a water with a total mineralization level of ~ 10%, including ~ 3% chloride.
Gas saturation was carried out with a mixture of gases: 13% H2S + 1% CO2 + the rest N2 supplied by contractors in one bottle at ~ 100 atm.
Autoclave tests were carried out for 30 days at a temperature of 88° C and pressures of 50 atm and 150 atm.
The 12 samples of aluminum alloy 1953T1 were placed into each autoclave (Fig. 1).

Fig. 1 General view of the samples before testing
After testing the surface of the samples for general corrosion was covered with small ulcers along the direction of polishing the samples. Visual examination of samples for contact corrosion revealed slight corrosion damage to aluminum alloy. Samples for corrosion cracking showed no susceptibility to corrosion cracking. Samples at all three levels of load did not break, no cracks were found in the working area of the samples (Fig. 2). In solutions of hydrochloric acid at concentrations from 5 to 20% and exposure from 5 to 120 minutes, intense gas formation was observed.

Fig.3 Samples after testing the interaction with cement slurry
Since after 10 days of testing, the nature of the inner surface of the pipe has not changed at all, it can be assumed that the cement slurry used in combination with the action of produced water does not affect the corrosion properties of 1953T1 alloy.

Acid treatment challenge
Additional research has been fulfilled to demonstrate that sulfamic acid (NH₂SO₂OH), known as a noncorrosive solution for aluminum alloy, is effective in treatment of carbonate formations, thus making it a good substitute for HCl acid.
There was a necessity to check whether treatment with 15% sulfamic acid instead of 14% hydrogen chloride, which had been in use at the field, would result in increase of rock permeability. In order to achieve this, the Core and Formation Fluid Research Center of "PechorNIPIneft" LLC (Arkhangelsk) did some laboratory tests of acid solutions effects on productive formations of Bayandyskoe field.
For this test, several prepared core samples were taken from the productive Zadonsky horizon of Upper Devon deposits (4087-4185 m depth) from the earlier drilled exploratory wells. Lithologically, the samples were porous and cavernous limestone, microbial-detrital with secondary clotted, crumby and clastic structure (Fig. 4, 5).  The preliminary research included an express analysis of efficiency of sulfamic acid: dry weighed samples were immersed in 15% solution of sulfamic acid and in 14% solution of hydrogen chloride, after which a comparison was made of weight loss and solution pH change over time. The obtained data were used to assess rock dilution rate and change of this parameter over time.
As a result of sulfamic acid treatment of samples with gas permeability over 2×10 -3 µm 2 have a substantial growth of permeability factor to 490×10 -3 -840×10 -3 µm 2 . Rock dilution rate with sulfamic acid is 2 -2.5 times slower than is the case with hydrogen chloride, yet the period of intensive dilution is longer with sulfamic acid (12-20 minutes for hydrogen chloride, 30-40 minutes for sulfamic acid).
Research has shown efficiency of employing sulfamic acid to treat rock in productive section of Bayandyskoe field.  (Fig.6); − Pipe body collapse test under ultimate external pressure (Fig.7); − Threaded connection collapse test under ultimate external pressure.  The bench trials of pipe body and threaded connection of full-size specimens of LOT178х14 pipes made of 1953Т1 alloy showed the following: -Strength and tightness of threaded connections were confirmed for an internal pressure equal to 1.5× operational pressure, and combined effect of 40 MPa internal pressure and 1800 kN tensile load (1.5× operational load); -It was confirmed that under ultimate internal pressure the tightness and strength of connection survived until the pipe body failure; -Threaded connection did not reduce collapsing external pressure of the pipe.

Strength tests
The results of tests were summarized in Table 1.

Field trials
All pipes have been tested for internal pressure before delivery to the rig site. Compared with 4219 meters 7" steel casing, weight in air of aluminum casing decreased from 1316 KN to 608 KN.
The casing string was successfully run in a hole to target depth, cemented and perforated. After sulfamic acid stimulation, the well worked for about three years with the expected debits. The reason for stopping production was the watering of products. Currently, it is planned to install a cut-off bridge and the transition to production from the overlying interval.

Conclusion
The designed aluminum alloy casing fully met requirements for strength and corrosion resistance, which are set for the pipes to be used in aggressive environments with high content of H2S and СО2.