In order to mitigate predrill uncertainties and provide the well team with the best information to drill proactively, monitoring of borehole operations in realtime is vital on wells drilled today. A realtime monitoring service (eg Pore Pressure while Drilling – PPwD) gives the team the best chance to drill the well safely and efficiently to planned targets. Particularly important aspects of realtime monitoring are estimation and prediction of pore- and fracture pressures.
Most analysis of drilling data, such as petrophysical, drilling parameters, gas and geology, relies solely on depth-based data, which of course are only available while drilling ahead, i.e. wellbore depth is increasing. The depth-based analysis model has relatively low resolution with data points at 0.5 foot or 0.2 meter intervals. If calibrated correctly the models are valuable and accurate, providing the well team with useful insight about present and possibly future events.
However, there are many operations during the drilling of a well that do not generate depth-based data but that do generate time-based data, such as pipe connections, circulating off-bottom, wiper trips, flow-checks etc. When analyzed, such data provide invaluable information to permit further calibration of the models. Realtime pore pressure models that only contain depth-based information are potentially flawed because the many calibration points that occur while the bit is off bottom are not captured, and therefore not usable, in the model. Building a time-based model in parallel to the depth-based model allows for a far more robust picture of downhole conditions.
A good example is in the use of gas data. Depth-based gas data can be a useful pore pressure indicator of pore pressure. However, complete interpretation of the gas information can only be made by considering time-based data, which allows for the gas behavior to be a quantitative indicator of mud weight overbalance to formation pressure by enabling a far better understanding of the relationships between the mud weight at surface, the static mud weight downhole (ESD), Effective Circulating Density (ECD), mud flow rates and pipe movement. Hole conditions, drilling parameter trends, and abnormalities are other primary calibrators for a pore pressure model. However, changes in these parameters would all plot at the same depth unless the analysis software can also handle the time-based components.
We will use real examples to explain the importance and benefits of constructing a time-based model that is used alongside the depth-based model. It will also demonstrate how discrepancies between depth-based models and time-based models can arise.
The result of using all available data, in both depth and time domain, is a more robust, integrated model on which to base the pore pressure estimation and prediction and ensure the well team gets the best possible information.