The search for oil has driven companies to explore and produce reservoirs located in deep waters. Sometimes these reservoirs produce corrosive fluids that lead to the use of expensive alloys. The high production costs sometimes can be minimized with the use of Extended Reach Wells (ERW), horizontal wells and the proper selection of corrosion resistant alloys (CRA). Horizontal wells have been used to allow higher production rates. CRA's are used either because it is absolutely necessary due to the corrosive environment or just to guarantee a longer time between work-over if the production fluids show moderate or even low corrosiveness. The economics of these expensive wells require high production rates. Therefore, acid jobs are often applied to remove reservoir damages produced during drilling or completion operations. Acid jobs are very aggressive to the tubing and down-hole equipment, so corrosion inhibitors are always used. This paper describes laboratory tests performed to verify the effectiveness of some corrosion inhibitors suggested for use with mud-acids to be injected in horizontal wells equipped with CRA tubing. For the conditions tested, no corrosion inhibitor showed acceptable performance for all materials tested if HC1 was used as the base for the mud-acid generation. Better results, however, were observed if the corrosion inhibitors were used to mitigate corrosion of mud-acids that were produced from organic acids.
Oil production in Brazil comes mostly from deep waters offshore wells. A number of new fields have been discovered and are being put into production. They are expected to contribute to Brazil's production target of 2 million barrels a day by 2005. These large fields are located in water depths between 2000 to 3000 meters, which requires the continuous development of complex deepwater technology. In order to increase well productivity, most of the wells to be drilled are planed to be horizontal. The horizontal sections can extend up to 1000 meters long, and the total departure from vertical can be larger than 3000 meters. The production tubing design for these wells sometimes requires the use of several different CRA alloys in order to address specific mechanical and corrosion requirements. In the horizontal part of the well, for instance, gravel packers screens are made out of 316 stainless steel wire or meshes and 13% Cr core tubing. The 316SS mesh prevents sand production and the 13% Cr core tubing is strong enough to prevent the collapse of the formation. The remaining part of the production string often shows components made out of 13Cr, 13Cr-5Ni-2Mo and/or Duplex 22Cr. Very specific reasons can even lead to the use of low alloy steels, (N80), production tubing. Therefore, in the same well one could expect to have different down-hole equipment made out of N80, 13Cr, 13Cr- 5Ni-2Mo, 22Cr-7Ni-3Mo and 316 stainless steel.
In order to keep or increase productivity mud-acid jobs are often used to remove formation damage produced by drilling and completion operations. This acid operation can be done with mud-acids generated from organic or inorganic acids. In Campos Basin, sandstone reservoirs consist mostly of SiO2 with almost no carbonates. Mud-acid fluids are used in these reservoirs. The most efficient acid formulation for this area are 13.5wt% HC1 + 1.5 wt% HF. Due to the minute carbonate concentration in the formation and to the long horizontal well length, only HF is consumed in the formation. In addition, the spent acid return can last for 7 days, causing severe localized corrosion to most of CRA tubular.
A number of papers have been written regarding corrosion inhibitors for carbon steels and corrosion resistant alloys in mud-
