Validation of Advanced Hydraulic Modeling using PWD Data

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.

The Effect of Drillstring Rotation on Equivalent Circulation Density: Modeling and Analysis of Field Measurements

“…A series of Pressure-while-Drilling (PWD) measurements (Charlez et al 1998) obtained from North Sea wells agree closely with the results of earlier studies (Delwiche et al 1992;Isambourg et al 1998). Another field study (Green et al 1999) was conducted in several wells in the Niakuk field in the North Slope, Alaska.…”

“…Unlike the results from the majority of the laboratory studies, most of the field measurements (Delwiche et al 1992;Ward and Andreassen 1998;Isambourg et al 1998;Charlez et al 1998;Green et al 1999;Hemphill et al 2007;Hemphill et al 2008) have shown a significant increase in pressure loss as the pipe rotation increases. The discrepancy between lab observations and field measurements can be attributed to several factors, such as drillpipe wobbling or instability, the irregular geometry of the wellbore, tool joint effect, or a combination of these factors.…”

Annular Frictional Pressure Losses During Drilling—Predicting the Effect of Drillstring Rotation

“…Unlike the results from the majority of the laboratory studies, most of the field measurements [22][23][24][25][26][27][28][29][30] have shown a significant increase in pressure loss as the pipe rota tion increases. The discrepancy between lab observations and field measurements can be attributed to several factors, such as operat ing with multiple dimension scales, drillpipe wobbling or instabil ity, the irregular geometry of the wellbore, tool joint effect, far too simple fluid systems in the laboratory or a combination of these factors.…”

A Simplified Method for Prediction of ECD Increase with Drillpipe Rotation

“…Studies have ranged from field measurements to purely theoretical treatments of the problem. In separate studies conducted at drilling sites in the North Sea (Charlez 1998;Isambourg 1999), the effects of drillpipe rotation were studied while circulating and rotating at various rates inside casing (without cuttings), and the results were analyzed in terms of changes in ECD as calculated from downhole pressure tool data. In an early study of slimhole applications, the effects of drillpipe rotation on changes in ECD on narrow-annuli wells in Gabon (Delwiche 1992) were measured.…”