I’ve been following some of the DWH hearings. Can someone explain to me exactly what they are referring to as a “negative test”?
I will attempt to describe what the term “negative test” means in the context of this rig incident. Most pressure tests performed on well bore seals, cement, pipe, valves, blowout preventers, etc. are either “positive pressure” tests or “negative pressure” tests. The term positive or negative describes in which direction you are testing the device.
Lets use for an example, a cement plug that has been installed inside a well casing to isolate 11,000 psi of reservoir pressure below the cement plug. Once the cement has hardened and set, you may want to perform both a positive test (from above) and a negative test (from below) on the cement plug. Let’s assume the cement plug is located inside the casing at a depth that is 15000 feet below the place where the tests will be performed (the rig). Let’s also assume that the casing is filled with a fluid of a certain density (15.4 ppg) that is exerting 12000 psi against the top of the cement plug.
Our test instructions for this well are to perform a 3000 psi net positive test and a 3000 psi net negative test on the cement plug. Before we begin the tests, the cement plug is being exposed to a 1000 psi [U]positive[/U] pressure differential (12000 psi of well bore fluid hydrostatic pressure from above and 11000 psi formation pressure from below). You could say the cement plug is now under a 1000 psi [U]positive[/U] pressure test because the pressure [U]above[/U] the cement plug (12000 psi) is 1000 psi greater than the pressure below the cement plug (11000 psi). Therefore, to impose a 3000 psi positive pressure test on the cement plug, we would need to pressure up the top of the well bore to 2000 psi. At this point, the total pressure at the top of the cement plug would be 14000 psi, consisting of 2000 psi (pump pressure) plus 12000 psi (fluid column hydrostatic pressure). The pressure at the bottom of the cement plug would be 11000 psi of formation pressure, so the cement plug would be “tested” to a net 3000 psi [U]positive[/U] pressure (3000 psi pressure differential from above).
Now lets perform the 3000 psi negative test (from below) on the cement plug. Remember that the cement plug is exposed to a 1000 psi “positive” pressure differential (from above) before we begin any pressure test. If we want to expose the cement plug to a 3000 negative test (from below), we must reduce the density of the fluid column above the cement plug such that its hydrostatic column will be 8000 psi. If we replaced all of the 15.4 ppg density fluid in the well bore (above the cement plug) with 10.2 ppg density fluid, the resulting condition would be exerting a 3000 psi [U]negative[/U] pressure differential (from below) on the cement plug. (8000 psi hydrostatic column pressure from above, and 11000 psi formation pressure from below). If the cement plug held this 3000 psi “negative” pressure differential (from below), we would declare a successful negative pressure test.
A more simple example of a positive/negative pressure test is when you have your auto air conditioner repaired at the shop. If the technician replaces a part on your unit he will perform a negative pressure test on the unit by applying a vacuum to the system. This is a negative test since the system is being tested under a negative pressure differential (atmosphere pressure outside versus vacuum inside).
If the system hold a vacuum (negative test), the technician will then pressure up your system to 40 psi using freon. If the system does not leak and holds this 40 psi, then it has passed a positive pressure test (40 psi inside versus atmospheric pressure outside).
[QUOTE=cmjeff;34042]I’ve been following some of the DWH hearings. Can someone explain to me exactly what they are referring to as a “negative test”?[/QUOTE]
You close the wellbore and displace the choke or kill line with saltwater and check for pressure and flowback.
Sometimes called underbalancing. If there is a leak in the seal assembly or cement /packers etc…the well will start to flow up the choke/kill, this is a sure indication you have a leak in the well construction.
A positive pressure test against the well isn’t always 100%, since leaks on seals, through debris etc, can be uni-directional…and misleading.