Method and apparatus for in-situ borehole stress determination

A method for determining the stress condition of a subterranean formation while drilling a wellbore, wherein the stress is determined in a plane normal to said wellbore at a selected wellbore depth, said method comprising the following steps:

• (a) lowering a downhole jack to said selected wellbore depth, said jack having first, second and third platens oriented about 60 degrees apart and wherein said platens are independently extendable to bear against the wall of said wellbore;
• (b) measuring the pressure exerted by drilling fluid on said wellbore wall;
• (c) extending said first platen to contact and bear against said wellbore wall;
• (d) measuring the pressure exerted on said wellbore wall by said first platen;
• (e) increasing the pressure exerted on said wellbore wall by extending said first platen to fracture said wall:
• (f) recording a first parameter comprising the breakdown pressure of said wellbore wall as the sum of said drilling fluid pressure measured in step (b) and the pressure exerted by said first platen measured in step (d);
• (g) retracting said first platen from contact with said wall when said wall fractures;
• (h) repeating steps (c) through (g) for said second platen and said third platen, wherein second and third parameters are recorded corresponding respectively to the breakdown pressure responsive to extension of said second and third platens;
• and
• (i) calculating the least principal stress and maximum principal stress operating in said normal plane based on using said first, second and third parameters in standard equations for two dimensional axial transformations.

Apparatus for in-situ determination of a stress condition of a subterranean formation traversed by a borehole, wherein the stress acts in a plane normal to said borehole at a depth corresponding to the depth of said formation, said apparatus comprising:

• (a) a generally cylindrical downhole jack;
• (b) said jack including first, second and third independently extendable platens for bearing against the wall of said borehole to fracture said formation, and wherein said platens are oriented about 60 degrees apart about the axis of said jack;
• (c) means for using said jack to initiate a plurality of independent fractures in said wall and for obtaining a plurality of actual breakdown pressure measurements for said formation corresponding to said plurality of fractures;
• and
• (d) wherein the minimum principal stress and the maximum principal stress in said normal plane are calculated using said plurality of actual breakdown pressure measurements in standard equations for two-dimensional axial transformations.