Question about LOT calculations

**Phildosaurus** · 04-20-2015, 10:15 PM

In studying this process, I've had a few questions and am a bit unsatisfied with the answers that professor friend of mine gave, so I was hoping that someone here might elaborate a bit. I understand than in basically all leak-off tests, the amount of hole drilled below the shoe is very short and we can therefore state that we're testing the formation strength "at the shoe." I understand that the difference in the bottom of the hole and casing seat are negligible, but if we were to have a very long rathole or just an interest in extreme precision, wouldn't we actually be testing the formation strength at the bottom of our openhole? This is my suspicion, as hydrostatic pressure would be the greatest at the bottom of the fluid column, so I'd assume breakdown happens at the point of highest pressure. In the capacity that we're discussing this, assume we know nothing else about the formation and that it is homogeneous from bottom of the shoe to the bottom of the open hole. Can anyone explain to me why my friend would state that we're testing formation strength "at the shoe"? Thanks in advance

**joseluismondragon** · 04-21-2015, 08:56 PM

The main purpose of the leak off tests is to evaluate the maximum pressure wich the casing shoe and the formation just below it can withstand and consequently the maximum mud weight wich can be used in the hole section. This test is performed at that depth becouse the fracture pressure increase with depth (fracture gradient), so the weakest hole section is just bellow the shoe. This test is very important to ensure that your casing design has been properly planned, and that the shoe can handle kick pressures. I recomend you to read abot Kick tolerance this can help you to understand LOT.

**Phildosaurus** · 04-23-2015, 04:46 AM

Thanks for the response Jose. I just wasn't ready to make the leap that rock strength difference due to overburden would ALWAYS be greater than difference in hydrostatic pressure along the rathole, but it seems that we have to assume the formation is weakest AT the shoe.