A) Pore-pressure-gradient
B) Fracture-pressure-gradient
C) Downhole-mud-weight
1. Do we have specific variation pattern with depth for A, B, & C?
2. It is well known that the fracture gradient represents the maximum mud weight that a well can hold without mud losses and without much fracture-growth (tensile failure).
Is there an improved method that can precisely estimate the fracture gradient - other than by means of PIT (FIT / LOF / XLOT) and the following methods??
Hubbert and Willis Method
(Can Biot coefficient be equal to unity?
Whether the approximation on the effective minimum horizontal stress to be one third of effective over-burden stress is justified?)
Mathews and Kelley's Method
(How to estimate the value of matrix stress or effective stress coefficient?)
Eaton's Method
(Poisson's ratio for fractured formations?)
Daine's Method
(How to deduce the constant associated with the estimation of horizontal tectonic stress?)
From well-bore tensile failure
(How to deduce the steady-state thermal stress?)
Dear Suresh
1. Although they generally increase with depth, there are exceptions. Encountering overpressured (caused by disequilibrium compaction, thermal cracking etc.) or underpressured (caused by cooling of pore fluids during uplifts, production of formation fluid) formations during drilling is common.
2. There is no prediction method that can be replaced with the actual well test results as they are far less accurate and although Eaton and Daine's method can provide reasonable results, their empirical constants needs to be calibrated with well test results so that they can be used in other section of the target formation.
As for the Daine's method constant you need a few actual measurements and then you can find the constant value by using a linear regression in Horizontal tectonic stress vs. (vertical stress - pore pressure) scatter plot.
I hope it helps
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