Hydraulic Fracturing Design Optimization Using a Fieldwide Calibrated Geomechanics Model

Abstract Hydraulic fracturing optimisation for tight sandstone requires a reliable geomechanical model in the reservoirs and bounding formations to achieve a desired hydraulic fracture geometry (height, width and length) and optimum production recovery post fracturing. The objective of this study was to validate and calibrate the horizontal stress profile in various intervals of the target formation. This paper presents a few cases from Asia Pacific region tight reservoirs where field structure is composed of multiple reservoir sandstones with interbedded shales. Sandstones with low porosity and permeability were targeted for hydraulic fracturing. An integrated approach was proposed to use existing wells data including acoustic and wellbore image logs, as well as miniFrac and Step-rate tests to calibrate the fieldwide pore pressure and in-situ stress models. The model was applied to optimise hydraulic fracturing design and treatment schedule. In pre-fracturing stage, geomechanical model was developed for target intervals using offset wells data including regional 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 nearby wells were used. Poroelastic minimum horizontal stress in the sandstone intervals was calibrated with closure pressure data from nearby wells and formations while bounding shale stress was calibrated with regional leak-off pressures. Number of deviated wells sub-parallel to maximum horizontal stress azimuth were drilled for reservoir stimulation using hydraulic fracturing. Full-wave Acoustic logs were acquired and used to validate the preliminary elastic properties of target formations which were then used for poroelastic stress calculations and fracture toughness estimation for hydraulic fracturing design. Analysis of real-time mini fall-off tests revealed significant level of reservoir pressure depletion in the target sandstones. Depletion induced poroelastic stresses were calculated while closure pressure interpreted from formation breakdown and miniFrac tests was used to calibrate the minimum horizontal tress profile. Re-calibrated model with actual reservoir pressure and closure pressure data revealed considerable stress contrast between reservoir sandstones and bounding shales, which provides sufficient bounding stress for fracture containment within the target sandstones. Model was used to design the main hydraulic fracturing including optimisation of size, volume and concentration of injected proppants and volume of fracturing fluid. Integrated Geomechanical modelling with advance acoustic logging and fracturing design enabled to achieve a successful hydraulic fracturing stimulation by exceeding the planned production rate. To date, more than five well stimulation campaigns have deployed the proposed workflow and successfully completed the fracturing operations with outstanding production results. Post fracturing production showed initial rate of tens of barrel oil per day higher than expected production rate from stimulated reservoir volume. Furthermore, 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.