A model is described that is capable of simulating in detail the time variation of formation pressures measured while drilling, in situations where supercharging is significant. Simulation results illustrate the variation of supercharging pressures with formation permeability, drilling-fluid-filtration properties, and drilling-fluid hydraulics. The model is used to explore how drilling operations influence the levels of supercharging when drilling two formations, widely separated along the well trajectory, and of significantly different permeabilities. The forward-simulation capability presented is believed to be a useful aid to the planning, understanding, and interpretation of formation-pressure measurements while drilling.
The new formation-pressure-while-drilling tools will bring great benefits for improving well placement, navigating narrow mud-weight windows, and generally avoiding hazards (Pop et al. 2005). To best use these new tools, it is essential to understand the effects of drilling on formation pressure, and to exploit this understanding to plan jobs so as to ensure that fit-for-purpose data are obtained. One reason this is necessary is that supercharging [elevation of near-wellbore formation pressure because of drilling-fluid-filtrate leakoff (Pelissier-Combescure et al. 1979; Phelps et al. 1984)] can be a problem for formation-pressure-while-drilling measurements. The problem occurs because active mud circulation limits filter-cake growth, so leakoff rates are larger than when the mud is static. Also, because the formation-pressure-while-drilling tools may test the formation shortly after it has first been drilled, there can be comparatively little time for elevated pressures to relax.
So, when do we need to worry about supercharging? The usual answer is "in low-permeability formations." But the drilling-fluid-filtration properties play a controlling role too, and the sequence and timing of drilling operations are also important. Furthermore, the use to which the measurements will ultimately be put dictates whether any particular level of supercharging is acceptable.
Hence, to answer the question properly, it is necessary to calculate supercharging pressures, taking account of the influential factors and details of the particular situation, and to assess the results in light of the intended use of the measurements. This paper demonstrates a simulation model with these capabilities.
We first outline the basic ideas behind the models used to simulate drilling-fluid-filtrate leakoff, formation pressure, and supercharging. Some example results illustrate how supercharging varies with formation permeability and drilling-fluid-filtration characteristics. The simulation results are used to motivate a simple approximation, which is used to map out the ranges of formation permeability and the filtrate-leakoff rate over which significant supercharging is to be expected. A further set of simulations is then used to show how the sequence of drilling operations affects the relative levels of supercharging in two formations of different permeabilities, separated by approximately 1 day of drilling time. "What-if" simulations allow operational sequences to be found that give low supercharging in both zones.