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This paper presents the results of a case study performed in Well BT-1 in the Blacktip field; an offshore field located in northern Australia. An analytical model based on mud logging data was used to estimate a formation's strength in Well BT-1. The drilled well is vertical and the input data to the model included the rate of penetration, rotary speed, weight on bit and torque values of the bit obtained from mud logging data. The final model was completed in several steps: estimation of the bit constants from either lab or field data, backward calculation of rock strength using a drilling performance model, smoothing of the estimated log to obtain the apparent rock strength log and classifying the formations into zones with similar strength values. The results of a log-based approach carried out in Well BT-1 were used to do the modelling. The results presented here indicate that the drilling performance model is able to predict the formation strength reasonably well. Based on the results, care should be taken when the model is used for the intervals where the bit is worn and dull, since within these intervals the model tends to over-estimate the rock strength. [ABSTRACT FROM AUTHOR]

There have been many studies on reservoir compaction where different mechanisms being suggested as the reasons behind the changes of porosity, permeability, and integrity of reservoirs during production. The theory of poroelasticity is often used to evaluate the likelihood of reservoir compaction, however, it has failed to explain the time dependent viscous behavior of unconsolidated formations. In this study, attempts are made to have a closer look into the compaction mechanism of deep-water sandstone reservoirs by bringing a case study from a gas field in Australia. The results obtained indicated that depending on the type of clays presented in the rock matrix, confining pressure and the loading rate, unconsolidated/poorly consolidated sandstone may exhibit an elasto-plastic or viscoplastic behavior during compaction. It was also found that the deformations initiated during compaction is a time-dependent process and can be a function of clay type in the sandstone matrix. It appears that kaolinite pushes the sandstone to exhibit an elastoplastic behavior while sandstones with a huge amount (more than 45%) of illite/smectite show a viscous (time dependent) response to hydrostatic loading. It was also revealed that the viscous behavior is induced once the confining pressure reaches a certain threshold (above 10 MPa). It seems that as the loading rate increases from 0.5 MPa/min to 1 MPa/min, a viscoplastic deformation dominates but an elasto-plastic response is revealed once the loading rate reduces to 0.25 MPa/min. It was concluded that the type of clays and their contribution in the rock matrix should be analysed very carefully to determine the geomechanical responses of deep-water reservoirs under different in-situ stress conditions. [Display omitted] • Time-Dependent compaction of deep-water reservoirs was studied. • Theory of poroelasticity is not able to explain the behavior unconsolidated sandstone. • Creep strain of sands during compaction is a function of clay type. • Unconsolidated sands may exhibit a brittle behavior under uniaxial loads. • Viscos behavior is induced in smectite/illite dominated sandstones. [ABSTRACT FROM AUTHOR]