Hydraulic fracturing optimisation for tight sandstone requires a reliable geomechanical model in the reservoirs and bounding formations to achieve an optimum production after fracturing. This paper presents a case study of Upper Cibulakan tight sandstone reservoirs in an oil field located in Offshore Northwest Java, Indonesia. Field structure is composed of multiple reservoir sandstones with interlayer shales. Two sandstone units with gross thicknesses up to 60 feet, effective porosity of 15% and permeability of 8 mD were targeted for hydraulic fracturing. An integrated approach is proposed to use available offset wells data, real-time acoustic logs, calibrated geomechanical model, and miniFrac and Step-rate tests to optimise hydraulic fracturing parameters and treatment schedule.
In pre-fracturing stage, geomechanical model was developed for target intervals using offset wells data including fracture closure pressures from past miniFrac tests. To estimate the reservoir and bounding formations Young’ modulus and Poisson's ratio, compressional and dipole shear wave slowness logs as well as bulk density logs from offset wells were used. Poroelastic minimum horizontal stress in the sandstone intervals was calibrated with closure pressure data while bounding shale stress was calibrated with regional leak-off pressures. The final stress model of offset wells was verified with the borehole condition and drilling experiences.
Target well for hydraulic fracturing was drilled with a 12¼° wellbore, 45 degrees deviated and oriented sub-parallel to maximum horizontal stress azimuth (north south). Processed acoustic logs were used to revise the pre-frac rock mechanical properties which verified the low ranges of static Young's modulus. Analysis of mini fall-off tests revealed important information about reservoir pressure depletion of ~250 psi which was not captured by offset wells pore pressure data. Pore pressure profile across the reservoirs was modified and depletion induced poroelastic stresses were estimated. Stress profile calibrated with actual closure pressure data from miniFrac test integrated with actual reservoir pressure revealed the stress contrast of up to ~350 psi between reservoir sandstones and bounding shales, which is favorable for fracture containment. Calibrated Geomechanics model was used to update the treatment schedule for main hydraulic fracturing including optimisation of size, volume and concentration of injected proppants and volume of fracturing fluid.
Integrated Geomechanics modelling with acoustic logging and fracturing design enabled to achieve a successful hydraulic fracturing stimulation by exceeding the planned production rate. Post fracturing production test showed initial rate of approximately 50-barrel oil per day (bbl/d) higher than expected production rate from stimulated reservoir volume. Calibrated geomechanics model provided valuable inputs for proppant size and conductivity optimisation to reduce the effects of proppant embedment as well as proper estimation of injected proppant volume based on robust minimum horizontal stress profile to minimize the risk of unwanted vertical fracture propagation to other zones such as water.