Choosing the right material for control lines in HP/HT applications

The maturing of some of the more easily accessible plays and a culture of continuous technological improvement are empowering the exploration and production of oil and gas from increasingly more challenging locations.

As the industry drills deeper and wells are extended further than ever before, pressures and temperatures increase. For example, equipment made to perform well in more benign conditions is unlikely to be suitable for harsher environments.

It’s critical to ensure that materials used for any project are fit for their intended purpose. Failure to do so can increase the risk of integrity problems, costly unscheduled repair and maintenance downtime, and the potential for safety and environmental breaches.

Control lines and chemical injection lines are essential pieces of infrastructure within the wellbore. Control lines are small diameter, continually pressurized, hydraulic lines used to operate downhole equipment. Because most systems linked to control lines operate on a fail-safe basis, any leak or failure in a line can lead to the closedown of the well. Chemical injection systems provide accurate chemical management of the well to optimize flow assurance and reduce costly well intervention.

High pressure and high temperature (HP/HT) wells above 150°C (300°F), or 69 Mpa (10,000 psi) place great demands on the materials used for control and chemical injection lines. Corrosive fluids and high-pressure hydraulics, combined with an aggressive external environment, can impact crucial downhole safety valves or hydraulic production packers, for example.

Increasing the temperature of control line materials reduces their mechanical properties, forcing a redesign of wall thicknesses to maintain pressure ratings. Modern higher pressure-rated applications such as 20,000 psi systems can also place significant demands on material performance. For these types of applications, it’s important to choose a material with high mechanical strength such as SAF 2507â, a super duplex stainless steel, which is stronger than high alloy austenitic grades and is able to withstand pressure collapse.

SAF 2507â continues to be an excellent choice for control line materials in many HP/HT wells around the world. However, for chemical injection lines or where ‘sour’ hydrogen sulfide (H2S) is present, a high nickel-based superalloy that can combine both strength with superior stress cracking corrosion resistance, such as alloy 625, can be an effective solution.

The high-nickel content of alloy 625 will help solve many corrosion challenges, but this impacts material cost. To counter this, and bridge the gap between stainless steels and higher-cost nickel alloys, Sandvik recently introduced the unique super austenitic stainless steel alloy Sanicro® 35 (alloy 35Mo). With high mechanical yield strength and excellent structural stability, the new grade offers superior corrosion resistance and outperforms conventional 6Mo grades, with only moderate nickel content.

Because no two wells are the same, it’s important to select materials that are fit for their intended purpose. Our experts are here to help you select the materials with the correct chemical composition to meet your demanding application needs.

Learn more about Sandvik’s range of materials for HP/HT control and injection lines here.

Leandro Finzetto, General Manager for Oil & Gas US Production