Titanium Alloy Tubing for HPHT Applications
Authors: Gonzalez Manuel , Maskos Krystian , Hargrave Robert , Kuberry John , Reeves Doyle , Grauman Jim , Skogsberg Jim , Ali Syed 
Published: September 2008

Abstract
Current and future flow streams from HTHP wells usually have to contend with H2S, CO2, and chlorides at high temperatures. At wells with temperatures of over 350°F this hostile environment requires selecting Nickel-based alloys for tubing/casing that are corrosion resistant alloys (CRA). Unfortunately, these Nickel-based or high chrome corrosion resistant alloys (CRA's) can be expensive particularly considering that some HPHT wells are being considered for completion depths of over 30,000 feet. In addition, CRA mill capacities are at their limits with deliveries projected of up to 2 to 3 years. It is imperative for HPHT well projects to be able to move forward by providing alternative alloys that can help to alleviate these issues with Nickel-based CRAs.
Titanium alloys can potentially provide for an alternative opportunity. The price for Titanium alloys has historically been higher than that for Nickel-based alloys. However, Titanium alloys are approximately half as dense as Nickel-base alloys so a pound of Titanium can provide twice as much tubing. Recently, the prices of raw Nickel and Chromium have increased to such a point that Titanium alloys are competitive. Here are the advantages of Titanium alloy: Based on detailed designs, Titanium tubing/casing can be cost competitive with Nickel-based CRA's.Titanium tubing is half the weight of nickel and generates half the axial loads.Titanium is very corrosion resistant and comparable to the high end nickel CRA's, such as Alloy C-276.Titanium is very strong (with some alloys having a strength of 135 ksi minimum yield versus 125 ksi minimum yield for Nickel CRA's).Titanium has half the Young's modulus of Nickel-based alloys and two thirds of the thermal expansion coefficient. It can be latched permanently to the packer and not exceed minimum yield strength during different well conditions.Titanium pipe can be made in large sizes (like 9-5/8" OD) in 40' lengths. Such large OD sizes in cold worked Nickel-based alloys are limited to around 15' lengths.Titanium is isotropic and has uniform strength throughout heavy wall tubulars (very important for connection design)
In addition, with our current well depths of over 25,000', wells cannot be cleaned out (through tubing) by using a snubbing string out of carbon/low alloys steels or the Nickel-based alloys. The weight of these alloys prevents using small tubing (less than 2.875" OD) deeper than 20,000' without exceeding axial load limits when combined with shut-in pressures greater than 10,000 psi (with applicable over pull and safety factors). Titanium will allow the development of a work string/snubbing string that will accomplish the task.
The strategic project detailed in this paper builds on previous work performed in the industry. A specific alloy of titanium called TIMETAL 6246 (UNS R56260) manufactured in tubular form, to specifically target HPHT downhole environments, was chosen for this project. This alloy has been used for more than 20 years in downhole service as logging tools.
There are many HPHT and deepwater fields where there is a need for competition to the high end Nickel-based alloy tubing. Titanium alloys can provide that competition and provide the industry with shorter manufacturing times. Also, the number of these demanding fields appears to be increasing; this could amount to a very large cost savings. In addition, it gives the industry capabilities that we currently do not have in the extreme HPHT horizon.
We developed a complete testing program to qualify the Titanium 6246 alloy for use in Chevron's current sour gas oilfields. The material testing includes exposure to environmental conditions and mechanical testing. Environmentally we have tested the Titanium 6246 alloy for resistance to several hostile sour gas environments, compatibility to packer fluids, effects of galvanic coupling to steel and Nickel-based alloys, methanol effects, compatibility to hydrochloric acid, fracture toughness as it relates to inspection of flaw sizes, and the ability to repassivate (reform corrosion protective films) in brine and production conditions. In addition, mechanically we have tested the Titanium 6246 alloy tubulars for erosion resistance, strength, hydraulic burst, in collapse, with connection make & break tests, and conducted a full connection test on the 4-1/2" Hunting Apex connection following the guidelines of the API 13679 with modification for titanium's specific properties. All connection testing will be conducted in the future as technical service projects and geared to each field's unique requirement.