Leak off test « Thread Started on Jul 5, 2009, 2:09pm »
Dear all, during a leak off test we found that LOT pressure is less than that estimated in the program with differance of 1 ppg, so could we say that the pore pressure is also less than the predicted in the program by the same amount (1ppg)?
Re: Leak off test « Reply #1 on Jul 6, 2009, 2:02am »
Not easy to say without seeing the LOT graph. Is it indicative of cement channeling ? Was the casing pressure tested prior to performing LOT ? If yes, how did it go ? What sort of formation is the shoe cemented in ? Sounds like it leaked of alright but through what ?...cement channel ? Certainly not the formation provided the estimate is correct. Suggest you go over the method used to derive the estimate first.
Give us more information if you can and if I can't help then perhaps someone who knows better might just be able to do so.
Joined: May 2009 Gender: Male Posts: 80 Location: MENA
Re: Leak off test « Reply #2 on Jul 6, 2009, 3:27pm »
I would think you might have missed the casing point and performed the LOT in a sand not a shale... just thinking out loud with no informations as pup joint mentioned above...
Re: Leak off test « Reply #3 on Oct 28, 2009, 2:10pm »
I think LOT is more indicative of the fracture initiation pressure and not directly related to the PP. How deep is the shoe you were testing? What is the temperature. Was the MW calculated based on surface wt or downhole weight?
Re: Leak off test « Reply #4 on Oct 28, 2009, 3:05pm »
LOT shall give you the pressure for initiation of the fracture. It is not related to the pore pressure in any way. No matter if actually the formation is fractured or has big permeability the frac is not related to pore pressure. Imagine a situation when the overburden is supported by the rock matrix only. Or the overburden is supported by the fluids only.
Re: Leak off test « Reply #5 on Oct 29, 2009, 12:21am »
Loki,
I beg to differ. In order for pressurized test fluid to work its way into - for example: a pressurized shale - it has to counter act formation pressure acting in the opposite direction first, before it can get into and prize open the shale matrix. Visualize the test pressure fluid trying to push into the interstices betwixt the matrix whilst at the same time formation fluids in the same interstices are acting in the opposite direction into the well-bore due to matrix overburden.
However the point you made will apply if the shale is normally pressured and a LOT in the case of very weak formation. There is a difference between LOT and FIT.
Mal,
there is a difference between PP, SPP and Choke pressure namely pressure losses due to friction along surface lines from the pumps to the standpipe pressure gauge and choke line losses respectively. These are even more pronounced when mud is not properly conditioned ie: MW out =MW in prior to performing the test, especially in weak formation.
Re: Leak off test « Reply #6 on Oct 29, 2009, 11:51am »
Pupjoint, that supposes the test fluid is going into pores not brakes the matrix. In this case the superficial tension of the fluids can be quite high depending on many factors. My point was you cannot make a rule to determine the pore pressure from the frac pressure. But if you know such a rule, please share it with us. Many thanks in advance!
Pupjoint, that supposes the test fluid is going into pores not brakes the matrix.
No, it supposed that test fluid goes into the pore spaces and / or pressurizes pore fluid by acting on it to an extent that the net effect of increased pressure from pump stroke increments minus pore pressure, eventually breaks the matrix ( this is the peak of the LOT pressure graph) further pump strokes will them force test fluid into the formation and cause a drop in pressure ie: leak off.
My point was you cannot make a rule to determine the pore pressure from the frac pressure.
Point taken and agreed. This is not practical at the rig site but can be approximated from other data theoretically like sonic, density logs etc.
But if you know such a rule, please share it with us. Many thanks in advance!
No I don't know of such a rule either. Your point does apply though if the formation is pure rock with no fluids and as I mentioned earlier if test fluid ideally balances normal pore pressure ie: MW has no overbalance factor or trip margin etc.
My point is that the pressure for initiation of the fracture in the case of a fluid bearing formation is not 100% dependent rock frame matrix stress but is the resultant nett effect of rock matrix strength, pore pressure of fluid, and test fluid pressures acting against both of these. To state a rule that Fracture pressure " is not related to the pore pressure in any way" is not always true. If I get time I will try to find a youtube video clip with an ariel view of subsidence following a blowout.
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