Analysis of Temperature Measurements during Drilling
Authors: Karstad Eirik , Aadnoy Bernt S. 
Published: December 2007

The temperature effects during drilling are investigated in this paper. A simple model is presented to analyze temperature profiles during drilling. Comparisons between the model's predictions and field data indicate that the model is quite accurate within its range of application. The model is valid for short time frames, and therefore applicable for the drilling phase of the well.

Introduction

During the past 8 years several severe incidents have occurred during high pressure-high temperature (HPHT) drilling in Norway The worst case was an underground blowout that took one year to control. Interpretation of well signals during drilling of the bottom section was identified as one of the key problem areas, resulting in a number of research activities in the years to follow.

Aadnoy performed a larger study about the temperature effects during HPHT drilling. The conclusion was that heating of the mud caused large variations in the effective bottom hole pressure. In other words, the pressure at the bottom was different from the pressure given by surface measured mud density. This effect was also strongly flow rate dependent. The variation in effective bottomhole pressure is mainly caused by expansion or contraction of the drilling fluid in the well. This volume change gives the appearance of a gain or a loss in the active mud pit. Gill describes the field observation well, and define the phenomenon as mud ballooning. To further understand the volumetric mud behavior, Aadnoy and Bjorkevoll et al. investigated changes in open hole annulus volume, changes in casing volume, and changes in mud volume due to variations in temperatures and pressure inside the entire borehole. The conclusion was that mud volume variation had a contribution in the order of 90% of the total volume variation, and was therefore the dominating parameter.

During the last couple of years measurement-while-drilling systems have been used to measure downhole pressures and temperatures. Field studies from the North Sea are given by Ward and Andreassen and Easton et al. The immediate conclusion when the first bottom hole data arrived was that the variations in temperatures and pressures were much larger than expected. The measured data are consistent with the results from the study reported by Aadnoy.

This paper will summarize results of several research projects conducted. Temperature effects were early identified as a major cause of variation in effective mud density and variation in mud volume. The paper defines the thermodynamic model for the borehole, the drillstring and the formation. This is actually an exact analytic model of a counter-current heat exchanger, applied on the physical conditions that exist in HPHT wells. This paper will aim at deriving exact temperature information from the well during drilling. That is: for the cases of drilling, and while circulating the well. Both forward and reverse circulation models are given. This temperature information is essential to modelling related to effective mud density, forward predictions and reservoir and borehole stability analysis. Applications to these subject areas will be reserved for later publications. Finally, the paper presents a field case demonstrating the applications of the model. The model derived is believed unique, and it is an important tool to interpret mud ballooning effects, to more exactly determine the actual bottom hole pressure, and to determine the transient temperature profile that exist along the borehole.

P. 381^