Managed Pressure Drilling (MPD) is an alternative to over balanced and under balanced drilling in conditions where pore pressures and fracture gradients are so close to each other (depleted reservoirs, deep and ultra deep offshore reservoirs) that it is not possible to drill significant depths without setting a casing. While MPD enables to drill longer footages without setting a casing, it requires precise estimation of equivalent circulating density (ECD) during drilling and static bottom hole pressure (SBHP) during non-drilling times.
General practice in drilling industry is to use rheological and volumetric properties of drilling fluids measured at surface to estimate ECD and SBHP. Consequently, ECD and SBHP measured using MWD and LWD tools in the field do not match the theoretical calculations.
This study shows the importance of introducing the effect of downhole conditions to hydraulic equations in order to estimate ECD's and SBHP's accurately. Paraffin based synthetic drilling fluid is used for this purpose. Effect of pressure and temperature on density of fluid is determined using PVT cell experiments. An equation relating density of the fluid to pressure and temperature is determined using linear and nonlinear regression techniques. Rheological characterization of the fluid was obtained on a Fann 75 HPHT rotational viscometer. Bingham Plastic model was used to define shear stress - shear rate relation of the fluid in all pressures and temperatures. Effect of pressure and temperature on plastic viscosity and yield point are determined using linear and nonlinearregression techniques, similar to the ones used in PVT analysis. Both onshore and offshore cases are investigated and effect of incorporating downhole effects to density and rheological parameters on ECD are analyzed.
As a result of the depletion of most of the known reservoirs around the globe, companies are looking to search for oil and gas in more challenging areas such as deep and ultra deep ffshore locations. In addition to that, high oil prices motivate industry to produce the last scoop of oil from mature oil fields where pressure is depleted. Conventional over balance drilling technique creates major draw back to drill in ultra deep and depleted reservoirs. In ultra deep offshore locations, pore pressure and fracture pressure gradients are very close to each other and with conventional drilling, it is hard (sometimes impossible) to drill a hole up to the target depth1. In the case of depleted reservoirs, pore pressure is so low that, it is not possible to drill without damaging the formation.
These challenges create a need for a new technology to drill in such hostile environments. Managed Pressure Drilling allows drilling of longer intervals by drilling overbalance while maintaining near constant bottom hole pressure using combination of drilling fluid density, ECD and casing back pressure in a closed system2,3. While MPD will enable to drill longer sections and the use light drilling fluids, it requires better wellbore pressure management. Only with that, it will be possible to decide on which type of drilling fluid to use and how deep it can be used.
