Rupture disk tolerances got you down?

The majority of toe valves run today are single open toe valves. These valves are designed to open at a specific pressure and when this pressure is exceeded (hydrostatic+applied) the valve opens. These valves have proven to be extremely effective and reliable. The downside is that the operator must exceed their pressure test value to open the valve. The second downside deals with the tolerances of the rupture disks.

+/- 3%

Most rupture disks used today have an accuracy of +/-3%. This means that if you have a rupture disk that is set to 15,000 psi, it will shear between 14,550 and 15,450 psi. This doesn’t sound like a huge gap in tolerances, however this is the tolerance for the total pressure seen by the valve which is applied + hydrostatic. In the well application the hydrostatic remains constant and the tolerance from the rupture disk affects only the applied section of the pressure. This is even further complicated by temperature affects on the rupture disk nominal values. Let’s look at an example that shows how this tolerance can cause issues for a single open toe valve.

Example

Let’s say, for example, you have a well with 6,000 psi of hydrostatic and you want to pressure test the casing at 8,500 psi applied. The goal will be to open the toe valve without exceeding the 10,000 psi rating of the surface equipment. 

For a single open toe valve like the HydraSTART STV you could choose a 15,000 psi rupture disk which would range from 14,550 psi to 15,450 psi absolute or 8,550 psi to 9,450 psi applied. That 8,550 is close to the casing pressure test and if anything is off in the calculation including well temperature or exact hydrostatic the valve could open during the pressure test. If, in this scenario, you chose to go with at 15,500 psi disk, they come in 500 psi increments, then you could be bumping up against the 10,000 psi limit on surface if the disk went high. Which could cause the toe valve not to open.       

All of this is just an example to show how casing test pressures, surface equipment max pressures, and rupture disk tolerances sometimes make it difficult to set up a single open rupture disk toe valve (like the HydraSTART STV) to fit every situation. In many cases the toe valve will have a quality design and construction, but the issues described above will cause it to open prematurely or not at all.

HydraSTART Multi-Cycle Toe Valve   

The HydraSTART MTV, from Rubicon Oilfield International, helps to alleviate this situation. In the scenario above with the MTV you would set the rupture disk to open at 11,000 psi absolute. As you were ramping up your pressure to 8,500 psi applied (14,500 absolute) the rupture disk would burst. When the rupture disk bursts the cycling module in the MTV is activated. After the pressure test has been performed you will bleed off the applied pressure. When you pressure up the second time the well pressure can now act against the piston shifting the valve open at a pressure below that of the casing pressure test.

The MTV operates outside the window of the rupture disk tolerances and eliminates the issues described above. This reduces the chances of having to do further intervention. If you would like to learn more about toe valves or the HydraSTART portfolio, please contact me at matthew_crump@hotmail.com. I'm committed to sharing my completions knowledge through posts like these, if you'd like to know more please hit the follow button above. Thanks!