Mathematical Temperature Simulators for Drilling Deepwater HTHP Wells: Comparisons, Applications and Limitations
Authors: Stiles David , Trigg Mark J. 
Published: December 2007

The widespread application and acceptance of mathematical simulators to model wellbore temperatures during drilling operations has grown in recent years. The increasing complexity of today's wells makes these physics-based tools invaluable for such tasks as designing successful cementing operations, selecting elastomers for blowout preventers, and selecting proper downhole tools. Limited previous work has validated some of these models against measured well temperatures but no comparison among the results, applications, and limitations of the various models has been published.

Part one of this paper presents a comparison of cementing temperature results from three models widely used by the industry today. The input parameters for this study were based upon a typical deepwater well in West Africa. While each of the simulators utilizes similar heat transfer models, the output from the models was found to differ significantly under certain well conditions. In some of the cases studied, differences among the simulated cementing temperatures were significant enough to warrant performance of risk analysis and implementation of risk mitigation measures.

Part two of this paper presents the circulating temperature model and temperature surveillance program utilized to drill and test a deepwater high temperature high pressure (HTHP) well. The predictions from the model were used to evaluate design considerations such as the selection of blowout preventer (BOP) elastomers and qualification of downhole drilling and testing tools. The model aided in completion fluid selection to optimize hydrate prevention and influence flowing wellhead and surface temperatures. A comparison of modeled parameters and measured field data is included.

In addition to the model results, the functionalities of each of the temperature simulators and how those functionalities may influence the results are discussed. A discussion on the applicability and limitation of the models to the deepwater conditions studied is included as well as recommendations for managing and mitigating the associated risk.

Introduction

Mathematical temperature modeling has been used for various drilling applications for more than 40 years1,2. For well cementing, a predicted bottomhole circulating temperature (BHCT) correlation developed and published by the American Petroleum Institute3 (API) has been widely used for wells drilled on land or in shallow offshore waters with few reported incidents. The correlation used to calculate API BHCT is based upon a very limited data set from measurements on 66 wells with depths ranging from 7,750 to 24,840 ft. Because the wells in the data set were all nominally vertical and located on land or relatively shallow water, the correlation is not relevant for predicting BHCT in deepwater or deviated wells. For wells where the API correlation is not suitable, physics-based mathematical models must be used.

Computer models specific to well cementing have been available for more than 20 years and have been shown to have reasonable accuracy in conventional wells4, deviated wells5, HTHP wells6, and deepwater wells7,8,9. Until recently, output from these simulators had been considered adequate for most cementing applications.

The advent of recent drilling projects in West Africa presented a combined deepwater and HTHP environment that had not previously been encountered. Mathematical temperature simulators were used for estimating BHCT under these deepwater HTHP conditions. Use of those BHCT estimates resulted in cement being left in pipe due to premature setting on two occasions. The possibility these cementing failures were caused by inaccurate BHCT predictions raised the need to investigate and compare the results from three commercially available temperature simulators under similar conditions.