The first geothermal energy plant in Iran has been developed in northwestern Sabalan for direct heat extraction and indirect electricity generation. For these reasons three geothermal wells have been drilled up to 3000 m depth. The temperature of the reservoir determined by well log is about 240°C. Geological investigation as well as production tests showed that the permeability of reservoir is medium to low. One of the practical solutions for increasing permeability of reservoir is hydraulic fracturing. To design a successful fracturing job, we have to characterize material constants and in-situ conditions. In this study, we have used the drilling and geomechanical data to constrain the in-situ stresses in one of the geothermal wells, NWS3. The magnitude of vertical stress (óV) was calculated from lithology log, the gradient of which is 0.031 MPa/m. Data of Leak-Off test and poroelastic theory were used to calculate minimum horizontal stress (óh) which is 0.011 MPa/m. The magnitude of maximum horizontal stress (óH) was constrained using frictional limit to stress that yield a gradient of 0.014 MPa/m.
Since 1975 some studies have been carried out to indicate potential of geothermal resources in Iran. Primarily studies were included aeromagnetic and gravimetry operation and preparing geological and geochemical maps of probable area over more than 8300 km2. After these studies, some regions were introduced as main geothermal resources: Sabalan, Damavand, Khoy-Maku and five other small parts in Sahand. After a long standby, in 1990 previous documents were reviewed and resistivity operations were conducted by KML (Kingston Morison Ltd.). Then, resistivity anomalies, geological evidences and hydrothermal alteration compared and finally Meskin-shahr introduced as the first geothermal potential. According to completion investigations in wells indicated that temperature of reservoir is about 240°C. Three exploration (production) wells up to 3200, 3176 and 2260 m and two injection wells up to 650 m have been drilled and tested. Permeability controlling factors and results of flow tests in production wells showed that permeability of reservoir is medium to low [1]. Presence of permeability is one of the important factors in using captured heat. Generally, the higher the permeability, the higher hot water can be extracted. Base on some calculations and estimations, it is essential to increase permeability by artificial methods. To increase permeability, we suggest hydraulic fracturing to create fracture in reservoir. During hydraulic fracturing, high pressure fluid injects into desired packed depth and the surrounding rocks are cracked. To carry out successful fracturing jobs, we have to determine in-situ stresses and geomechanical constants such as: Poisson's ratio. In-situ stresses play very important role in fracture geometry and fracture initiation, propagation and closure of created fractures. In this study, we try to determine in-situ stresses in Meshkin-Shahr geothermal field using both geomechanical and well drilling data.
