The state of stress in SW Iran and implications for hydraulic fracturing of a naturally fractured carbonate reservoir

Abstract

The first part of this study provides an interpretation of the present-day stress state and mechanical parameters for the giant Bangestan reservoir in SW Iran. A total of 28 drilling induced tensile fractures (DITF) and 172 breakouts were mapped in seven wells in the Bangestan reservoir using image logs. The average maximum principal horizontal stress (SH) direction for each well is given a quality ranking according to the World Stress Map (WSM) criteria. The mean orientation of SH finds to be N44°(±9.95°). A correlation is seen between this data and stress orientations from earthquake focal mechanism solution. This suggests that stresses are related to tectonic forces generated by Arabia–Eurasia collision. Estimates of vertical principal stress (Sv) gradients vary between 23.3 and 24.7 kPa/m (1.03–1.09 psi/ft), based on empirical methods and density log data, respectively. Minimum horizontal stress (Sh) gradient estimates vary from 15.2 to 17.4 kPa/m (0.67–0.77 psi/ft) using extended leak-off test (XLOT) and step-rate test results, respectively. Uncertainties remain in estimating SH from acid fracturing in the deep naturally fractured carbonate reservoirs, even though numerous papers have been written on the subject. Herein, an approach called Deformation/Diffusion/Thermal (DDT) is suggested to estimate SH in the Bangestan Reservoir. The method combines poroelastic theories for the circular underground cavities, bilinear theory for fluid flow through the preexisting induced fracture and thermal stress. Using this method, SH gradient is calculated to be 18.9 kPa/m (0.83 psi/ft). For comparison, horizontal stress magnitudes and mechanical parameters were computed from sonic wave velocity using poroelastic-tectonic stress equations over the full reservoir layer thickness. All data have been calibrated with field tests and lab results. The measured SH was in good agreement with the previous analytical solution. The study suggests that the contemporary state of stress in the reservoir is characterized by a normal faulting (NF) stress regime. Next, current and prospective hydraulic fracturing operations in the field are discussed. Based on the NF stress regime, a vertical plane fracture is expected and the wells drilled in the direction of SH are likely to intersect the most natural fractures.