A rock mechanical model developed for a Coal Seam Well
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Drilling operation in order to produce from Coalbed methane (CBM) is prone to various geomechanics related problems not only within the coal seam but also across the overburden layers. Wellbore instability in the form of shear failure (breakout) and washout in one hand and mud loss and fracturing in other hand are examples of failures which a wellbore may experience if a proper mud weight is not used for drilling. In order to conduct such an analysis the input data required includes mechanical properties of formations as well as the magnitude and direction of in-situ stresses and pore pressure. It is well known that mechanical properties of formations are related to their physical characteristics. For example, the formation Young’s Modulus or strength is expected to be higher in formations with larger sonic velocities or lesser porosities. Petrophysical logs reflect various rock physical properties from which continuous curves of rock mechanical properties could be estimated using several correlations developed in similar fields. Similarly, continuous logs of in-situ stresses (i.e. vertical as well as minimum and maximum horizontal stresses) could be estimated, for example from poroelastic formulae, in conjunction with rock physical properties. The estimated logs could be calibrated against lab tests on cores and field test data. For example, performing triaxial tests in the lab on cores obtained at different depths, the elastic and strength properties such as Young’s Modulus, Poisson’s ratio and uniaxial compressive strength (UCS) could be measured and this is used to correct the corresponding estimated logs. Similarly, the minimum horizontal stress log could be calibrated against any existing leak-off-test data whereas pore pressure curve can be calibrated if any MDT data is available.The direction of horizontal stress can be estimated from the image logs, for example FMI. The combination of continuous curves of formation mechanical properties and magnitude of in-situ stresses together with stress directions is referred to as rock mechanical model (RMM). The RMM is constructed for a drilled well and then it is used for prediction of events in a new planned well in a nearby area. The RMM includes the input data for any geomechanics study such as wellbore instability analysis, fracturing design or sanding prediction. In this study the RMM was constructed for data corresponding to Well Ridgwood 2 drilled in Surat basin in Queensland, Australia. The results indicate how the mechanical properties are changing across the coal seam comparing to other intervals and that the stress magnitudes experience significant changes accordingly. The results are used to predict the fraccability of the CBM for stimulation purposes using a hydraulic fracturing operation. Other applications of the constructed RMM will be discussed and the results interpreted.
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