Method of determining in-situ stresses

Details Method of determining in-situ stresses Method of determining in-situ stresses

1999-07-06

2001-01-02

Tsay, Frank (Department: 3672)

Wells

Processes

With indicating, testing, measuring or locating

C073S784000

Utility Patent

active

06167964

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of determining an in-situ stress of an earth formation, the formation being subjected to an in-situ stress state with a first, a second and a third principal stress. The three principal stresses are generally referred to as the first, the second and the third in-situ stress.
BACKGROUND OF THE INVENTION
In the technology of hydrocarbon production from an earth formation it is often required to know the magnitudes and directions of the in-situ stresses in the formation, or at least to have an indication thereof. Such knowledge is needed, for example, for the purpose of achieving wellbore stability, conducting hydraulic fracturing of the formation, geological modelling or preventing sand production. The direction of the in-situ stresses can be determined in several manners such as differential strain analysis, various acoustic techniques, or so-called minifrac tests. In this respect it is to be understood that one of the in-situ stresses is generally in vertical direction and its magnitude is determined from the weight of the overburden. Therefore, in general only the two horizontal in-situ stresses are subject of investigation with respect to direction and magnitude. It has been tried to determine the magnitudes of the horizontal in-situ stresses by measuring strains and using constitutive properties of the rock to determine the stresses. However, the constitutive properties of the rock are generally not accurately known.
It is therefore an object of the invention to determine more accurately the magnitude of one or more of the in-situ stresses in the earth formation.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a method of determining an in-situ stress of an earth formation subjected to first, second and a third in-situ stresses, wherein a borehole has been drilled into the formation, the borehole containing a borehole fluid inducing a selected pressure to the borehole wall so that in a region of the formation the first in-situ stress is replaced by another stress depending on said selected pressure induced to the borehole wall, the method comprising the steps of:
selecting a sample which has been removed from said region, the sample having first, second and third reference directions which coincide with the respective directions of the first, second and third in-situ stresses prior to removal of the sample from the formation; and
conducting a plurality of tests on the sample whereby in each test the sample is subjected to selected stresses in the reference directions so as to determine a damage envelope of the sample and to determine from the damage envelope at least one of the second and third in-situ stresses, wherein the magnitude of the selected stress in the first reference direction is substantially equal to the magnitude of said another stress.


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B. J. Pestman: “An Acoustic Emission of Damage and Stress—Memory Effects in Sandstone”, Mining Sciences & Geomechanics Abstracts, vol. 33, No. 6, Sep. 1996 (1996-09), pp. 585-593.

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