Over the past few years, the Strain Relaxation (SR) method (or the an elastic strain recovery) has been used for predicting the orientation of induced hydraulic fractures in oil wells. Reliability of the method requires a sensitive measuring device and cores free of natural cracks.
A SR device that uses DC displacement transducers was developed and tested in laboratory and field conditions.
An extended version of the SR method is suggested for laboratory use to predict directions and magnitudes of the in-situ stresses. In the laboratory, effects of pore pressure and natural fractures on SR data are investigated. It is observed that jacketed cores that have been subjected to hydrostatic pressure, expand in all directions on the release of the pressure, and that unjacketed porous cores subjected to the same pressure porous cores subjected to the same pressure contracted in all directions during their relaxation period. Agreement between the laboratory azimuth period. Agreement between the laboratory azimuth predicted by both type of tests is observed. predicted by both type of tests is observed. The calculated fracture direction suggested by SR data collected through testing cores which contain natural fractures is likely to be perpendicular to the maximum horizontal in-situ stress, thus the calculated direction will be 90 deg away from the true direction. On-site, the SR method is applied in three different wells. Results from two wells are in a fair agreement with predictions of other fracture orientation detection techniques.
General
A few years ago, Teufel introduced the strain relaxation method (or the an elastic strain recovery method) to the oil industry as a technique for predicting directions of principal in-situ stresses predicting directions of principal in-situ stresses in deep wells. Knowledge of these directions is significant since they are related to the direction of the induced hydraulic fracture. The importance of knowing the direction of the induced fracture is well established in the oil industry.
The SR method as outlined by Teufel is based on the assumption that a rock core of certain properties, retrieved from its downhole confined properties, retrieved from its downhole confined conditions, will relax (creep) in all directions. The magnitude of recovered strain in any direction is proportional to the magnitude of stress in that proportional to the magnitude of stress in that direction. Thus, most recovered strain is aligned with the direction of the maximum in-situ stress or the direction of induced hydraulic fracture.
Recently, the method has been also used to calculate the magnitudes of in-situ stresses, after deriving the mathematical relationships from the theory of visco-elasticity. Reliability of the results is highly dependent upon the accuracy of strain measurements and core properties.
In general, the following are the idealistic core properties demanded by the method to produce reliable results:
Core must be homogeneous and linearly viscoelastic.
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