**7.4 Discussion on the results**

The calculation of the in-situ state of stress from the measured strains obtained from overcoring measurement in sandstone is based on the theory presented by Amadei [2]. Stress 201X programme allows the user to ascertain stresses and rock properties from raw data output from a CSIRO HI cell and plot overcore (**Figure 31**) and biaxial tests (**Figure 33**). The programme is for genuine ES&S CSIRO HI cells having preset values for alpha and beta angles for the strain gauges. Stress 201X

**Figure 32.** *Biaxial test.*


#### **Table 14.**

*Change in strain from cyclic biaxial chamber loading to 7 MPa.*

**Figure 33.** *Plot of micro strain versus pressure.*


#### **Table 15.**

*Elastic properties calculated from biaxial test.*

code was originally developed for CSIR-type triaxial strain cell with a maximum number of 12 strain rosettes with up to 4 strain gauges per rosette. The input values for this program (**Table 16**) are from the averaged circumferential and axial strains for overcore (**Table 13**) and biaxial test results (**Table 15**). The final calculated stress values from Stress 201X program are given in **Table 17**.




#### **Table 16.**

*Input parameters for software—Stress 201X.*


**Table 17.**

*Stress values from the ovecoring tests.*

#### **7.5 Comparison of the different stress results**

The final results of magnitude of the minor and major stress components in the horizontal plane obtained from hydraulic fracturing method show in good agreement with the corresponding stress components obtained from overcoring (**Table 18**).

Two tests were conducted in apparently uniform sandstone by using hydraulic oil as the fracturing fluid. Impression packer images revealed the induced cracks in the reopening pressure. The average horizontal orientation of the cracks obtained at this site shows reasonable agreement with the orientation of the major secondary principal stress component in the horizontal plane obtained from overcoring.


**Table 18.**

*Comparison of results from the two methods.*
