Circulating Temperatures Existing Prior To Cementing Casing In Prudhoe Bay Wells
Authors: Nelson William C. 
Published: December 2007

Due to an uncertainty in the prediction of subsurface temperatures needed to design cement programs in arctic regions, a numerical model has bee programs in arctic regions, a numerical model has bee developed for simulating the heat transfer effects of drilling in regions containing permafrost. The model assumes quasi-steady-state radial heat flow in the wellbore and two-dimensional transient heat flow in the soil surrounding the well. Mud circulation temperature data from two Alaskan North Slope wells have been compared with drilling simulation results to verify the accuracy of the model.

The drilling model has been applied to the problem of computing circulating temperatures problem of computing circulating temperatures existing in the wellbore just prior to cementing the 9-5/8-in. casing in place on Prudhoe Bay development wells. Ten variables have been proposed that could have a possible effect on these down-hole circulating temperatures and the results of a sensitivity analysis involving these variables are discussed in this paper. Many of the significant variable effects discussed have not been applied previously to arctic environments. A summary of the parameter study results is presented in the form of three nomographs that permit a rapid calculation of the bottom-hole circulating temperature inside the 9-5/8-in. casing prior to cementing. prior to cementing

Introduction

The development of the Prudhoe Bay field in Alaska has necessitated several extensive studies by the operating oil companies into the effect of permafrost on drilling and completion practices. permafrost on drilling and completion practices. Although much of this work has involved the mechanics of the permafrost soil in addressing such problems as thaw subsidence and freezeback pressures, the thermal aspect of the permafrost as a heat sink also has led to a number of other engineering problems. Recently, attention has been focused on assessing the influence of permafrost on subsurface drilling temperatures. The accurate prediction of down-hole circulating and formation temperatures is essential in the proper design of the cementing programs involving the 13-3/8-in. casing, the 9-5/8-in. casing, and the 7-in. liner used in the development wells on the eastern half of the Prudhoe Bay field. Estimates of the bottom-hole circulating temperature and the maximum annular temperature just prior to cementing the casing in the hole are necessary in determining the specifications of the cement slurry. Failure to accurately predict these temperatures can result in a flash set if the slurry is underretarded and unsatisfactory compressive strength if the slurry is overretarded.

Beginning with Farris in 1941, numerous investigators have studied the convective heat transfer present during mud circulation, and several correlations and analytical methods have been developed for predicting down-hole circulating temperatures. However, these correlations assume nonpermafrost soil conditions and, thus, are not applicable for engineering use in arctic regions such as the North Slope. In this paper, the effect of several drilling variables on subsurface circulating temperatures is discussed for wells on the eastern half of the Prudhoe Bay field. A numerical model has been developed to simulate the convective heat transfer associated with drilling and mud circulation, and the results of a sensitivity analysis using this model are discussed for 10 potentially significant drilling variables. Although the information presented in this paper pertains solely to the presented in this paper pertains solely to the estimation of down-hole circulating temperatures inside the 9-5/8-in. casing for North Slope wells, the sensitivity analysis results should provide a qualitative means of evaluating the effects of several drilling parameters on down-hole temperatures for any arctic region containing a sizable amount of permafrost.

NUMERICAL MODEL DEVELOPMENT

The use of numerical methods to solve wellbore heat transfer problems is well documented in the literature. A number of papers also have dealt with heat transfer in a phase-changing medium.