In both Gas Interference and Fluid Pound the pump chamber is partially full of gas and partially full of liquids (oil/water), so what is the difference between the two and how do you distinguish between the two when looking at dyno cards? That is what I will explain here. First blog post of my life: I'm a little nervous!
Fluid Pound (F#) is a condition in which there is not enough liquid available to fill the pump chamber, meaning the top of the fluid level is down at the pump intake (at the Seating Nipple) (see Figure 1.A). As a result, on the up-stroke the pump sucks in all of the liquid that is available and then it starts to suck in gas from the casing annulus. F# only occurs when the well is fully "Pumped Off" (the top of the fluid level is at the pump intake), which means the well is producing at maximum capacity
Figure 1: A) a pumped off well. B) a well with a high gassy column of fluid above the pump.
Gas Interference (GI) is a condition that only occurs when a well makes gas (as most wells do) and that gas is not properly separated out before before entering the pump intake. Gas in the pump chamber is problematic due to its high compressibility.
Liquids are essentially incompressible, so as the pump plunger moves up and down in the stroke the pressure in the pump chamber changes rapidly with any small movement of the plunger and the Traveling Valve and Standing Valve quickly open and close as a result. When gas is in the pump the pump plunger has to move a long distance in order to create a sufficient change in the pressure of the pump chamber for these valves to properly open and close. This means a large portion of the downhole stroke is wasted compressing and expanding gas instead of displacing fluids. In its worst condition gas interference can manifest as "Gas Lock" where neither valve in the pump opens during the stroke and the pump just expands and compresses gas the whole stroke.
So you might do a rod design simulation that shows you can produce 300 barrels-per-day and find out that if your gas separator design is inadequate that you might only be making 150-200 BPD. Gas interference is the kryptonite of rod pumping wells, especially with the modern gassy horizontal wells we are bringing online these days.
The best way to deal with gas interference is to place the pump intake 60+ feet below the bottom perf, but sometimes this is not feasible (and not even possible on horizontal wells). If the pump has to be above the bottom perf you need a good gas separator design. There are various gas separator designs that are sold out there in the industry with some being better than others, but ultimately there is no perfect gas separator that can handle any amount of production: if you were to create one you would become uber rich as the industry demand is there. If the well's production rate is low (<100 BPD), you should be able to separate out all off the gas, but sometimes that isn't even possible if the gas is emulsified in the oil and is not easily separated.
So lets say you have a dynamometer card from a POC that shows incomplete liquid fillage: how do you distinguish if the incomplete liquid fillage is due to the well being fully pumped off and pounding fluid or if it is due to gas interference as a result of poor gas separation???
The simple way to answer this is to take a Fluid Level Shot: if the top of the fluid level is at the pump intake then it is Fluid Pound, and if the top of the fluid level is above the pump intake it is gas interference. But what if you don't have a fluid level gun? Are there any other ways to distinguish between the two?
I recently checked a well for a customer and this topic came up. The well had just been worked over and they were struggling to get much of any production from the well. The well had a casing leak and so a packer was installed above the perforations to try and produce the formation while holding back the casing leak water. The well is very old, 2500' deep and producing from the San Andreas formation in an area that is water flooded and there is little gas production, but my customer was wondering if the well was hopelessly Gas Locked due to the fact the Packer was installed and any/all produced gas would have to go through the pump (since it can't go up the backside with the packer being installed). He thought this was the reason it wasn't producing right.
I did a dyno test on the well and the cards I captured are shown in Figure 2 (notice it has a light tag every stroke seen by the negative load at the bottom of the stroke; they put it on tag to help break the gas lock). Since a fluid level shot can't be used to determine why the pump fillage is so low I had to use the shape of the card to try and interpret what is happening. Well, what do you think??? Before you keep reading look at the card and answer this: is this pump Pounding Fluid or Gas Locked???
Figure 2: dyno cards from well producing under a Packer. F# or Gas Locked?
Two aspects of the card tell me the well is fully pumped off and Pounding Fluid: First, the fluid load on the plunger is high (higher than the Fo Max line, which is where it should be if the well was fully pumped off). Second, notice the right angle of the card in how the fluid load is picked up and released on the up/down-stroke. At the start of the up-stroke the fluid load is picked up instantly, and on the down-stroke (presumably when it runs into the fluid) the fluid load is released near instantly. Both sides of the card do not show the more gradual pick-up and release of the fluid load that is associated with gas interference.
Look at Figure 3. This is a case of severe gas interference. Notice the upper-left corner of the pump card is heavily rounded and is not a right angle: this is due to gas expansion. Also, as the fluid load is released on the down-stroke you can see the characteristic slow release of the fluid load that is associated with gas interference. This well is choking on gas, yet it isn't technically gas locked (...but the liquid fillage is so low many people in the industry would refer to it as gas locked). This well has 1200' of Gas Free Liquid Above the Pump (GFLAP) but it isn't producing at all near its capacity.
Figure 3: severe gas interference with about 30% liquid fillage. Notice gas expansion on the up-stroke and gas compression on the down-stroke.
In the case of true Gas Lock, the pump valves stay closed the whole stroke and never open and as the pump plunger moves up and down the gas in the pump chamber is just expanded, compressed, expanded, compressed ...but no new fluids are moving into the pump. This is true Gas Lock. Gas Lock is a term commonly thrown around in the oilfield and people use it too often; I have had many pumpers tell me a pump is gas locked even though 99% of the time it is just gas interference.
I don't see true Gas Lock often. When I do find it, it usually means there is no gas separator (or a terrible gas separator design) and the pump plunger is spaced very far off bottom and the rods need to be dropped to space the plunger closer (...which increases the pump's compression ratio and helps the Traveling Valve to open earlier in the down-stroke and minimizes the inefficiencies).
To see what a truly gas locked pump looks like, see Figure 4. This card looked so bad I wasn't sure if it was gas locked or something was not plotting correctly on my dyno cards. So after catching enough cards, I turned the well off for 10-min and then turned it back on and I got the cards shown in Figure 5.
Figure 4: gas locked pump.
Figure 5: after the well had been down 10-min: normal gas interference cards.
So you can see from Fig. 3-5, that when there is gas interference you will get rounding of the upper left corners of the pump card at the start of the up-stroke. This is Gas Expansion. As the plunger starts to move upward on the up-stroke the gas starts expanding to fill the void left by the plunger, and since gas is compressible the pressure in the pump chamber does not drop instantaneously as the plunger moves up (which is what happens in Fluid Pound cards and that is why the upper-left of these pump cards have a right-angle shape).
Notice how the slope of the Gas Expansion (at start of the up-stroke) and Gas Compression (on the down-stroke when the fluid load is being released off the plunger) depend on how much gas is in the pump. Both Figure 3 & 4 have bad gas expansion, but the gas locked card shows much more gas expansion (and gas compression) as the amount of gas in the pump (and its affects) are more severe. By turning the well off for 10-min and then back on, I was able to break the gas locked condition and some liquids entered the pump chamber on the succeeding stroke, and that is why the gas expansion/compression are far less severe.
In summary, if you are trying to identify between gas interference and fluid pound and you don't have the possibility to use a fluid level shot to distinguish the two, then try to use the shape of the pump cards. Look at how quickly the fluid load is picked up at the start of the up-stroke and how rapidly the fluid load is released on the down-stroke. The shape of the pump cards tells you how rapidly the pressure in the pump chamber is changing as the plunger moves up and down (...more on this in another blog post).
I will be periodically putting up blog posts. Please send any feedback you have or topics you would like an explanation on. If you like the information, I am creating an e-course that will teach all of this information to you in an entertaining video format and I hope to have it live by mid-2020. Cheers!