New Improvements on Managed Pressure Drilling

Abstract One of the advantages of Managed Pressure Drilling (MPD) is to determine formation pressures and fracture gradients while drilling. However, in order to determine these pressures accurately, the rheological model of the drilling fluid and pressure losses accruing in annulus should be determined precisely. In addition to the pressure losses in annulus, pressure losses in pipes should be determined accurately in order to determine pump sizing requirements for a successful MPD operation. Un-weighted n-paraffin based drilling fluid system is analyzed in this study. HPHT rotational viscometer is used to determine how rheology of this invert emulsion system changes under down hole conditions. The fluids are tested in the temperature range of 40 – 280 °F and the pressure range of 500 – 12,000 psig. Three rheological models, Bingham Plastic, Power Law and Yield Power Law, widely accepted by the drilling industry, are used to determine rheological characteristics of the drilling fluid and compared with the experiments at various pressures and temperatures. It is found out that, at high shear rates (i.e. > 100 rpm) all models predict shear stresses accurately. However, at low shear rates only shear stresses calculated using Yield Power Law model agrees with the shear stresses measured by the HPHT rotational viscometer data. Pressure losses predictions in pipe and annulus are determined using rheological parameters measured under surface conditions and then compared with pressure losses calculated using Yield Power Law model at actual downhole pressure and temperature conditions. Effects of using surface based rheological parameters and different models on estimating pressure losses in pipe and annulus are shown. Introduction Basically, MPD is a system where wellbore pressure management is obtained by adjusting the pressures along the wellbore using a choke valve at the return line in the annulus. This modern drilling process is preferred over conventional over balance drilling as well as underbalanced drilling in areas where pore pressure and fracture gradients are very close (i.e. deep and ultra deep offshore drilling) or pore pressures are very low. MPD is indicated in situations where conventional drilling techniques are not feasible or non economical1–4. While MPD provides total wellbore pressure management and may also allow real time determination of pore pressure and fracture gradient, the accurate determination of pressure losses in the annulus is essential for the success of the operation. In addition, in the planning phase of an MPD operation, pressure losses inside the drill string should be determined accurately in order to obtain required operating pressures, pump sizing etc. Usual industry practice is to either use Bingham Plastic or Power Law model to define the shear rate – shear stress relation of the drilling fluid considering surface measurements. Similarly, the density of the drilling fluid system is determined at surface and used together with surface conditions' rheological parameters to calculate estimated pressure losses in the well. This methodology will not induce significant errors while drilling shallow onshore reservoirs with water based drilling fluids. However, in deep and ultra deep offshore applications not only the downhole conditions but also the type of fluid being used is different than regular onshore drilling.