EVALUATION OF METHODS OF DETERMINING IN-SITU STRESS FROM OBSERVATIONS ON BOREHOLE CORES.
Reliable methods of predicting the orientation of massive hydraulic fractures are of great importance to oil and gas industries as well as to hot dry rock (HDR) activities. In order to predict the azimuth of hydraulic fracture, it is necessary to know the direction of the minimum horizontal compressive stress, because a hydraulic fracture propagates perpendicular to this stress direction.
The minifrac technique has been extensively used to measure the vertical distribution of the minimum horizontal in situ stress. However, its implementation is often hindered by technical and economic considerations. Consequently, the determination of the stress field on the basis of laboratory measurements in oriented core samples has been investigated. New developments and the results of 3 methods are presented: an elastic strain recovery (ASR), differential strain curve analysis (DCSA), core discing analysis.
Through the study of cores with these methods, good indications were obtained on in situ stresses.
Field tests were carried out on 3 cores retrieved from 2 wells. With well A the north south direction of the major stress which is roughly indicated by the 3 methods is in agreement with the direction of the small axis of well ovalization. This direction also fits one of the figures ususlly considered for the regional maximum stress.
With well B ASR as well as DSCA indicate the same orientation for the major deformation. This orientation is perpendicular to the major principal stress as deduced from the minifrac result. Consequently these methods, although indicating the orientations of the principal stresses, cannot be used, in this case, to classify the minor and the major stresses. Moreover 1 of the principal deformations measured by ASR is negative. These observations clearly show, than taking into account stresses is not always sufficient for interpreting ASR and DSCA deformations. The effect of natural fractures should also be considered. In this case a well known natural fracture direction oriented north 140 degrees east could explain the deformation recorded perpendicular to this direction.
Discing criteria provide a value for the ratio of the average in situ stress orthogonal to the core axis to the in situ stress parallel the core axis while, in the case of oriented cores, the study of shape of discs gives the direction of the smaller lateral in situ stress. The mount of information contained in the discs is not yet fully understood and further research is needed. The major limitation of the method is that core discing is not very frequent, particularly because weak rocks such as chalk or poorly cemented sandstones do not produce discing when cored but developed instead distributed microcracks which contribute to rather poor core recoveries. It is felt that a better understanding of discing conditions can lead to a better estimation of the in situ stress tensor and to a better knowledge of the coring process.
THREE TECHNIQUES WILL BE EXAMINED WHICH CAN, IN PRINCIPLE, ALLOW INFORMATION TO BE DERIVED ABOUT THE STATE OF STRESS IN A BOREHOLE FROM WHICH CORES HAVE BEEN RECOVERED. THE METHODS TO BE INVESTIGATED ARE:
- THE RELAXATION BEHAVIOUR OF THE CORES
- ANALYSIS OF MICROFRACTURING IN THE CORES
- ANALYSIS OF 'DISCING' IN THE CORES.
THE PROCESS OF RELAXATION WILL BE FOLLOWED BY PLACING THE CORES IN A SPECIALLY DESIGNED CELL WHICH ALLOWS MEASUREMENT OF DEFORMATION ALONG FOUR INDEPENDENT AXES AS A FUNCTION OF TIME (24-48 HOURS). AN OIL BATH WILL CONTROL SPECIMEN TEMPERATURE AND AVOID LOSS OF PORE FLUIDS FROM THE CORE.
MICROFACTURING WILL BE ANALYSED BY RECOMPRESSING THE CORES AND DETERMINING THE STATE OF STRESS NECESSARY TO RE-CLOSE ALL THE MICROFRACTURES. THE QUESTION WILL THEN BE ADDRESSED AS TO WHAT EXTENT THIS RE-IMPOSED STRESS REFLECTS THE ORIGINAL IN-SITU STRESS.
THE STUDY WILL ALSO ANALYSE THE STRESS CONDITIONS UNDER WHICH 'DISCING' OF THE CORE CAN OCCUR AND THE AMOUNT OF INFORMATION THAT CAN THUS BE DERIVED. THE RELATIVE VALUE OF THESE TECHNIQUES WILL THEN BE ANALYSED.