The magnitude and direction of in-situ horizontal stresses are crucial parameters for a successful hydraulic fracture design and operation that are often conducted in tight unconventional reservoirs. The Poro-Elastic Horizontal Strain method where the minimum horizontal stress magnitude is calibrated to closure pressure from extended leak off test or leak off test is commonly used to estimate the horizontal stress profiles. However, in exploration areas it is common that calibration data is very limited or missing, hence horizontal stress calibration from other sources like multi-mode sonic measurements becomes useful.
This paper presents a recent application of using multi-mode sonic log acquired in a deep vertical exploration well in Saudi Arabia to determine in-situ stress profile for the design of hydraulic fracturing job in a tight gas reservoir. The azimuth of fast shear was used to determine maximum horizontal stress direction, whilst, horizontal stress gradients were determined using Velocity Dispersion Gradient (VDG) method at five intervals where the acoustic anisotropy is predominantly stress-induced. Subsequently, a Mechanical Earth Model was constructed in which the horizontal stress estimated using Poro-Elastic method was calibrated to horizontal stress gradient from VDG method at the five intervals. The minimum in-situ horizontal stress profile was later confirmed by the closure pressure from MiniFrac test and the hydraulic fracture height indicated by the temperature log. The difference in magnitudes between minimum in-situ horizontal stress and closure pressure at the perforation zone was 1.2%, and the hydraulic fracture height agreed with the minimum in-situ horizontal stress profile.
Considering that the multi-mode sonic logs are commonly acquired to derive compressional and shear slowness, the VDG method can then be used to improve the accuracy of horizontal stress profile without any additional rig cost to operators.