Validation of Advanced Hydraulic Modeling using PWD Data
Authors: Charlez Ph. A. , Easton M. , Morrice G. , Tardy P. 
Published: October 2008

Abstract
Using Pressure-While-Drilling (PWD - measurement of downhole annulus pressure and temperature), an extensive campaign was undertaken on recent North Sea ERD wells to compare measured ECDs and static mud densities with hydraulic model predictions. In addition, the effects of drill pipe rotation and reciprocation have been analysed. The hydraulic model used to calculate downhole pressures predicts fluid downhole density and rheology according to surface properties, pressure and temperature input. Results show that using a Herschel-Bulckley law, ECDs are accurately predicted both in laminar and turbulent regimes. The causes of serious mud losses on two wells have been identified and operational procedures were changed to successfully reduce drilling risks.

Introduction
Complex wells have made, the control of downhole pressure whilst drilling more and more important. For instance, in HPIHT wells, the margin between pore and fracturing pressures can be very small, sometimes less than 0.1 SG. In such cases, the precise knowledge of downhole equivalent static density and ECD (Equivalent Circulating Density) are therefore of a strategic importance. In current Drilling Engineering, static density is adjusted at the surface and ECDs are calculated using hydraulic models, This practice sometimes leads to uncertainties larger than the required precision. Surface density which is generally measured (using a classical mud balance) in the mud pits or at the mud return is very sensitive to pressure and temperature. Depending on the case downhole density can be smaller or larger than surface density. The main input of ECD models are the well geometry (hole, string including tool joints, BHA, bit), mud rheology (calibrated on classical FANN measurements - mainly Bingham, Ostwald or Herschel-Bulckley) and flow rate. These models generally assume that the drill pipe is centred and they rarely take into account pipe vertical movements (reciprocation) and rotation. There is therefore no doubt that measurement of downhole Pressure1,2 and downhole temperature while drilling should allow to adapt real time the surface mud properties to obtain the required downhole conditions. It can also help in the development and improvement of hydraulics models by providing calibration points. Apart from the validation of static and ECD models a real time acquisition of downhole pressure and temperature can also provide valuable information about operational problems such as:–hole cleaning and optimisation of tripping procedures,–detection of abnormal thick cakes (reduced hole),–detection and control of mud losses,–better optimisation of bit hydraulics,–better estimation of LOT and FIT,–better control of downhole mud properties (mud sagging).

The Dunbar field
The Dunbar field is located in the northern part of the North Sea (Vicking Graben - Fig. la). The three main targets (below 3500mTVD) are in the middle and bottom Jurassic (respectively Brent and Stafjord reservoirs) and in the Triassic (Lunde reservoir). Depending on the location they can be oil bearing, gas bearing or both. Typical well design, mud weight strategy, and leak of test values are presented in Fig. lb. After batch setting a conductor pipe (26") 80 meters below the sea bed, drilling is initiated in 23 1/2" with a 1.08SG water base mud. The 18 5/8" casing shoe (where a LOT in the range of l.30SG to l.35SG is classically obtained) is set in the boltom of these recent (mainly sandy) sediments.