Techniques for Determining In-Situ Stress Direction and Magnitudes
Authors: Bloch M. , Siqueira C.A.M. , Ferreira F.H. , Conceicao J.C.J. 
Published: December 2007

Knowledge of the in-situ stress field is highly beneficial when designing the drilling direction of horizontal and deviated wells or programming a massive hydraulic fracturing. According to the available technology it is well accepted that reliable in-situ stress orientation and magnitude determination must be based upon the utilization of several techniques, instead of only one.

Field examples of Petrobras effort in determining in-situ stresses with the An elastic Strain Recovery Test (ASRT), breakout analysis and microfracturing is shown in this paper. Strain relief techniques like the ASRT are very economical since they do not require to occupy the drill hole for testing, although obtaining oriented cores may not be in the drilling original schedule. While reliable data can be obtained with the ASRT for the stress orientation, the same is not true for the stress magnitudes. Breakout analysis has proved to be a very powerful indicator of the horizontal stress direction since it is caused exactly by the in-situ stress contrast. The issue here is to ascertain that the detected borehole elongation has enough evidences of been caused by breakout. Hydraulic microfracturing, when including poroelastic effects, seems to be the best option for determining the minimum horizontal in-situ stress magnitude.

Introduction

Measuring in-situ stress is still a challenging task to the oil industry in 1997. Due to the lack of a fully reliable method capable to work on a large variety of rock types, stress field and harsh deep downhole conditions, operators and service companies usually apply different methods to compare their result and compute a statistical average for the in-situ stress direction and magnitude. They can be divided into core-based methods, borehole-based techniques, near-wellbore techniques and regional geologic indicators. The list of techniques for determining in-situ stress is quite long:

An elastic strain recovery test (ASRT);

Axial point load test;

Borehole breakout analysis;

Borehole deformation;

Borehole imaging;

Circumferential velocity anisotropy (CVA);

Differential strain curve analysis (DSCA);

Direct observation of overcored open-hole microfractures;

Directional gamma ray logging;

Drilling induced fractures in core;

Earth tilt surveys;

Leak-off inversion technique;

Microfracture pressure analysis;

Microseismic logging;

Overcoring of archived core;

Petrographic examination of microcracks;

World stress mapping.

The an elastic strain recovery technique is based on the core strain relief after coring. Individual sand grains become stressed during burial and lithification of the sedimentary materials, resulting in compression and distortion of the grains. The stored energy within the grain may vary in different directions, depending on the amount of stress that was applied in each orientation. When a rock stratum is cored, the sand grain attempt to expand elastically as soon as original stresses are relieved, but they are held back by cement bonds. Many of these cement bonds will eventually be broken, forming a microcrack population preferentially aligned with the stress field. Microcracks will have then observable effects on petrophysical properties in a homogeneous isotropic material. Combination of core-based techniques with breakouts may be the only way to obtain specific stress estimates when rock conditions constrain the use of drill hole testing methods.