April 07, 2015
The continued search for new oil and gas fields has resulted in deeper wells, often with more aggressive downhole environments. Nickel-containing alloys play small but vital roles in the exploration and production of new fields. One such role is the use of nickel-rich alloys in wireline.
In recent years, considerable effort and investment have been directed towards developing alternative sources of energy in order to reduce the world’s dependence on fossil fuels (oil, natural gas and coal). Renewable and pollution-free energy sources, particularly solar and wind, have experienced increased research, investment and application. Other energy sources, such as tidal, run-of-river and bio-mass, are being developed and may play a larger role in the future.
However, until these alternative sources are fully developed and brought on-line on a significant scale, the world will remain dependent on fossil fuels.
When oil and gas wells are drilled, the geologists and drill rig personnel need to know the nature and characteristics of the formations that they are encountering downhole. To help them obtain this vital information, drilling is periodically halted and logging tools packed with sophisticated, and very expensive analytical equipment are lowered into the well. These tools carry out chemical and physical measurements on the various downhole strata and capture the data for evaluation.
The tools are lowered into a well attached to a wireline. Wireline comes in two basic forms: “slick” and “electric” line. Slick lines are solid, load-bearing metallic wires which are commonly available in diameters of 1.83–4.06mm. Electric wirelines consist of insulated electrical signal wires surrounded by braided metallic wires for protection that make up the essential load-bearing cable which supports the weight of the measuring tool.
In addition to suspending measuring tools, slick lines are used to place essential production equipment downhole, such as packers and valves. When production is started from a productive formation zone, slick line is used to suspend “bomb hangers” which, when detonated, drive projectiles through the well casing pipe. The holes thus produced allow hydrocarbons to flow into, and up, the well. Slick line is also used during “fishing” operations in order to retrieve equipment that is no longer needed downhole or to remove broken components that may be blocking the well bore.
Strength, corrosion resistance, flexibility
Both forms of wireline must, of course, be very strong in order to support both the weight of the tool and the weight of the long length of wire that is necessary to lower the tool to the required depth. Wells are now routinely drilled kilometres deep, and higher strength alloys such as 2205 (UNS S32205) can replace lower strength Type 316 (S31600).
Wirelines must, of course, possess very high tensile strengths (breaking loads), but must not suffer excessive elongation (stretching) when loaded. As wells are drilled to greater depths, higher downhole temperatures and pressures are encountered. In addition, downhole chemical environments are often very hostile to the wireline materials. Aqueous chloride conditions are frequently encountered, often accompanied by high levels of carbon dioxide and hydrogen sulphide gases, which make the aqueous phase acidic. These harsh operating conditions require wirelines made from nickel-containing alloys highly resistant to general corrosion, pitting corrosion and stress-corrosion cracking, such as Alloy 926 (N08926), Alloy 28 (N08028), Alloy 31 (N08031), Alloy 936 (N08936), Alloy 27-7Mo (S31277) and Alloy MP35N (R30035).
High nickel-containing alloys meet these challenges and will be the alloys of choice for wirelines for as long as drilling for hydrocarbons continues. And that looks set to continue for many years to come.
Wireline nickel alloys
Additions of nickel enable stainless steels and higher alloys to attain the necessary superior mechanical properties and corrosion resistance levels. Type 316 stainless steel (10-14% nickel) is usually offered as the basic grade of wireline, for wells with moderate conditions. For more demanding conditions, alloys with progressively higher nickel content are required.