Shooting Fluid Levels
In rod lift applications it is important to know where the producing fluid level is in the casing to tubing annulus, above the pump intake. Knowing where that fluid is, if it is fluctuating or if it is static, tells us how good the fluid conductivity is from formation, thru the perforations and into to the wellbore. Fluid level information can be used to calculate bottom hole pressure, create more, or less, pressure drop (pressure differential) at the point of perforations and can tell us whether the well can be sped up on the surface to increase total fluid production, or slowed down to keep the producing fluid level from dropping below the pump intake, a circumstance called, "pumping off." Low fluid entry wells tend to pump off, for instance, and cannot be produced at very high rates. Fluid entry problems can be caused by poor rock, skin damage outside the perforation windows, or perforation blockage, among other things.
Deep or shallow, rod lift applications require knowing where fluid levels are in the well bore. Down hole pumps can seldom be set more than 15 degrees from vertical so in horizontal wells the fluid conductivity to the well bore must be good enough that fluid levels stay above the radius, in the vertical section of the well, above the pump intake.
It's dark down there and hard to see, so how do we measure where the producing fluid level is in the well bore?
The big boys use downhole transducers to measure real time fluid levels. Us poor folks, however, the ones out of debt and in need of watching costs, use an acoustic liquid level instrument whereby we can send a sound wave down the casing and tubing annulus that bounces back when it hits fluid. As the acoustic signal travels down the annulus it allows us to count tubing collars along the way. The induced sound signal is generally compressed CO2 that is loaded into the "gun," on the right, screwed into the casing, connected to the recording device, left, and shot, or fired downhole with a trigger device on the gun. If there is no natural pressure on the wellbore sometimes 50 PSI of compressed gas is sufficient to create a good sound wave. If however there is pressure on the casing it can sometimes take up to 300 PSI before the sound signal is strong enough to reflect off the fluid level in the tubing to casing annulus. If we are shooting deep wells where the sound wave must travel deep down hole we might even need more PSI to find the producing fluid level. All this stuff is pretty idiot proof but at the same time it's always important to remember that 50 PSI can kill a fella just as dead as 5000 PSI... so you have to have your head out of your ass when shooting fluid levels.
Once we're all hooked up and ready to pull the trigger on the gun to release the sound wave, we turn the chart on and let 'er fly. We can see when the signal hits fluid on the chart, shut the chart mechanism down and this is what we've got to look at...
Each "kick" in the chart represents a tubing collar; the signal in this chart has slammed into fluid at 40 tubing joints. The lease operator has counted the tubing collars to know that. Most tubing meets American Petroleum Institute (API) standards and is roughly, but by no means exactly, 31.5 feet long; In the case then we can say the producing fluid level in this well is roughly 1,260 feet. In this example the downhole pump was set at 2,679 feet so we have 1,419 feet of fluid above the pump intake. We can calculate the hydrostatic weight of the column of fluid above the perforations to determine what our PSI delta is at those perforations and, in this case, can speed the well up significantly without harming the reservoir. To do so we can make the pumping unit go faster, or increase the stroke length on the surface and increase our total fluid production by say, 70 bbls. per day. If the well has an oil water ratio of 35% we can theoretically make 24 more barrels of oil per day and we are very happy to give that a try. After we speed the well up we will take successive fluid level shots until we believe the producing fluid level is static and make adjustments at the surface accordingly.
There endeth the lesson of the day.
The PhD, by the way, stands for post hole digger.