Calibrating Numerical Modelling (particle flow code) Techniques with Petrological Data to Obtain Rock Strength Parameters for Otway Basin and NW Shelf Seals
Honours Degree, 2003
University of Adelaide
Numerical simulation, using Particle Flow Code 2 Dimension (PFC2D) was performed to predict geomechanical properties of sealing rocks from the Otway Basin and North West Shelf. Using this technique, the effect of clay abundance in specimen on seal strength was investigated. Uniaxial compressive strength (UCS) was modelled using PFC2D and was compared with results from experimentally determined UCS values.
In order to model samples, petrological data needed to be collected to be used as input to the PFC2D model. Scanning Electron Microscopy (SEM) images, XRD results and thin sections were used to calibrate the numerical modelling. From thin sections and SEM images, the mean grainsize of sample were estimated (between 100-300 m) and the framework of the specimen were described. Different clay types were identified; these included kaolin, smectite and illite. The XRD analysis showed that quartz mineral is the main composition of the grains and there are also clay particles. This information and the compositional percentage of quartz and clay were used to create the representative synthetic core models. Multiple samples from individual seals were petrographically analysed and those with the best seal characteristics were chosen for modelling.
Due to the its time-consuming nature, the creation of the model was simplified by clumping set of balls together as a representative of quartz grains and clay particles. Contact bonds, stiffness of grains and density were the parameters used to differentiate between quartz grains and clay particles in the modelling.
The numerical modelling results for the UCS values for Muderong Shale (10.3 MPa) is consistent with the known UCS value (7-11 MPa). The other samples from Laira Formation (Banyula 1 and Kalangadoo 1) showed UCS values which are 45-60 % lower than the laboratory UCS values. This may due to several pitfalls in calibrating numerical simulation including possible error in estimating the clay abundance. The modelled UCS value for Belfast Mudstone is 10.9 Mpa.
The study has shown that PFC2D could be use to predict the strength of seals using petrological data to calibrate model parameters. It is possible to use microparameters to obtain macro-mechanical rock parameters. This result is valuable in evaluating the geomechanical properties of top seals. They can be used to predict seal integrity by calculating the probability of seal breach through fracturing.