I need help in either convincing my colleagues that I am right, or help understanding why I am wrong. In question is what happens below a packer when doing an inflow test when the hydrostatic in the annulus below the packer exceeds the hydrostatic inside the drillstring. Consider the example below (and shown in the attachment):
1. The well contains 12.0ppg mud.
2. Open-ended drillpipe is run to 2000mRT, with a packer high up in the string at 200mRT.
3. The drillpipe is displaced to 2000m with 8.5ppg seawater with a surface U-tube pressure of 1194psi.
4. The packer is set.
5. The U-tube pressure on top the drillpipe is bled off and the drillpipe left open to the atmosphere.
6. What happens next?
I maintain that the mud in the annulus below the packer will backflow up the drillpipe until equalized, leaving a void below the packer with a pressure of zero (gauge) below the packer, and on top of the mud.
My colleagues, who are experienced engineers, maintain that the mud on the backside would remain stationary since there is nothing to fill the void below the packer that would be created if the mud U-tubed away. The pressure below the packer would have to be negative, or I suppose zero (but without a void create by equalization). Neither of those outcomes seem possible to me.
Their argument is that this is the way inflow tests have always been done. But I maintain that this could only be done (without backflow) if the packer is set deep enough so that the hydrostatic in the annulus below the packer is less than the hydrostatic inside the drillpipe.
Also, could anyone help me to understand why the pressure inside the void below the packer would only go down to zero (gauge), and not create a vacuum (of negative gauge pressure)?
Thanks for your help.
OK, sorry for not responding sooner, my day job kept getting in the way. (But boy am I glad I have it!)
Anyway, thanks for everyone’s input. I have attached my (the) solution to this particular scenario, which I am confident is correct. I should have posited the question more directly as: “Please confirm that, if a communication path exists or is created between the annulus and an inner string, the external and internal hydrostatic pressures at the point of communication will equalize once allowed to do so.” The inflow test and packer scenario were provided as a practical (or in this case, an impractical) example.
This all seems pretty elementary, but to my surprise it generated quite a bit of debate in the office. A couple of things that strengthen my view:
It is hard to find relevant examples in drilling texts. But in “Well Control for Completions and Interventions” by Howard Crumpton (apparently a Scotsman, with perhaps more credibility than a Sepo like me), in his section on “Opening the Circulation Path”, he comments on p.248:
· There is a risk of the wireline tools being “blown up the hole” if that differential is from annulus to tubing.
· When punching a hole in the tubing, there is a risk of the wireline tools being “blown up the hole” if there is too great a pressure differential from annulus to tubing.
He also works an example very similar to one I present (“Calculate the fluid level (H) in the annulus if the tubing head pressure is bled to 0psi”) on p.258.
My own experience was when testing a gas well with a packer and seal assembly, the string contracted to the point that the seals pulled out of the packer. Of course, the mud in the annulus U-tubed up the tubing string immediately.
Anyway, to address some of the specific comments you have made:
Apologies to the brainier amongst you. I have not considered the effects of temperature or compressibility, nor ballooning or reverse ballooning. All real and appropriate for consideration, but not really critical in understanding this simple concept.
Simon, I am guessing that you make the simplifying assumption that the annulus hydrostatic below the packer at the point of communication with the inner string is less than the internal hydrostatic (thereby resulting in a positive pressure acting on the bottom of the packer). That, I fully agree, is the norm and is the correct way to do inflow tests with a packer. But that misses the premise of my question.
Allan, the equalization will occur even without breaking the string on surface. But if you break the string and open up the heavy side to the atmosphere, it will happen more dramatically (read quickly). [Will bring up whether to use buoyed or air weight in my next post. ;-) ]
Wayne, my old friend, let’s not complicate it. I’m trying to focus on “first principles”.
Chris, the packer was being considered to prevent putting a negative pressure (top to bottom) across the BOP rams/annular. Since then, that plan has been abandoned.
Justin, on that point, I’m not sure if a void would see 0psig or 0psia. I’m guessing closer to 0psig, as presumably there’d be enough entrained gas/air to come out of solution to occupy the void.
Thanks again to all commenters.
My masters thesis last summer was on the behavior of fluids during inflow tests and devising a compter based model to predict the flow rates as a result of this. Without writing another 10,000 words on the subject with citations and reference, here is my simplest explanation…..
In an ideal world without the effects of thermal expansion of fluids, compressibility of fluids and ballooning effect on drill pipe, the pressure would bleed to zero and there would be no flow (as per your left diagram).
Considering ballooning, there will be a dP across the string down to the water / mud interface. This will cause the pipe to squeeze and resulting in a small flow. My investigations found this to be minimal.
Now consider compressibility of the fluid. In this case water (and I assume WBM) is generally not compressible so will result in minimal flow. In the case of base oil and oil based mud the effects is more exaggerated as the base oil component is much more compressible, so you have to determine the compressibility of the base oil and also the compressibility of the oil, water and solid phase of the mud (generally assuming the solid phase to be incompressible except under ultra-HPHT conditions).
Next consider the expansion of the fluids due to temperature and also compressibility as well. As we go deeper in the well pressure and temperature increases. Temperature causes thermal expansion (also reducing density) while the increase pressure works oppositely compressing the fluid and increasing density. These effects are far more profound in oil based fluids, and it is known that temperature is the dominant effect.
The next step is where it gets interesting (yawn)….. before performing the inflow test it is common the circulate the well to get a homogenous mud, which tends to cause the temperature gradient of the fluid in the well to be fairly consistent from top to bottom. As soon as we stop pumping, the well begins to move back toward the geothermal gradient. As a rule of thumb, the top 1/3 of the annulus will cool and the bottom 2/3 will heat and if plotted will form a straight line with a pivot point. As we commence pumping the cold, dense underbalancing fluid, we are displacing hotter fluid up the annulus. The cold dense fluid will try to gain temperature equilibrium drawing heat through the pipe and in turn from the annulus. Initially the top 1/3 will be colder than the annulus, and begin to heat. Once at equilibrium with the annulus it will cool together to geothermal gradient (faster above the mud line). The bottom 2/3 will heat until equilibrium is achieved.
It is this heating and cooling and the effects of expansion and contraction that we witness at surface when we see flow during the inflow test. This tends to manifest itself as an ebbing and flowing again far more profound when using OBM. It is also predictable if you have good data to work with.
I attached some slides on my project.