Simulation of Permafrost Thaw Behavior at Prudhoe Bay

Abstract

This paper presents mathematical models to simulate transient heat flow within the wellbore and in the permafrost during drilling, production, antifreeze-back. Calculated temperatures agree well with measurements taken in the wellbore and in the permafrost during field tests at Prudhoe Bay. Production-thaw predictions are presented for a typical Prudhoe Bay well with three different insulation schemes. Introduction The thawing of the permafrost by drilling and production operations at Prudhoe Bay can cause freeze-back and thaw-subsidence problems. Freeze-back exerts pressure on casing strings when a well is shut in after drilling pressure on casing strings when a well is shut in after drilling or production. Thaw subsidence occurs if a large thaw radius is caused by production. The prediction of permafrost performance and the design of well completions permafrost performance and the design of well completions require accurate simulation of thermal behavior in the wellbore and in the permafrost. Wellhead temperatures of producing wells also must be predicted so that surface facilities can be designed properly. Several studies describe thermal models for predicting permafrost thaw. The models developed by Couch predicting permafrost thaw. The models developed by Couch et al. and Merriam et al. can be applied only for crude oil production, not for mixtures of gas, oil, and water. To approximate the actual unsteady conditions, Howell et al. treated the wellbore heat transfer as a series of steady-state conditions. Their thermal model was not intended to predict permafrost thermal behavior for short-term production operations or for rapid changes during drilling. This study presents equations for simulation of transient heat flow within the wellbore and permafrost formulation. The formulation of the problem is permafrost formulation. The formulation of the problem is general enough to handle most aspects of practical interest. To demonstrate the applicability of the mathematical model for thermal predictions in Prudhoe Bay well design, calculated temperature values were correlated with data from four field tests that were conducted at Prudhoe Bay by Atlantic Richfield Co. as operator for itself and Exxon Co., U.S.A. These tests were designed to simulate permafrost thaw during drilling and production. Temperature measurements along the surface casing, in circulating fluid at the wellhead, and in the permafrost around the wells were matched closely with the thermal model. The next section briefly describes the problem. The third section presents mathematical models to simulate flowing-stream heat transfer and permafrost thaw behavior. In the fourth section, comparisons of field data with our thermal-model predictions are shown. Finally, thaw predictions at Prudhoe Bay for three kinds of insulating schemes are presented. Problem Formulation Problem Formulation Permafrost Properties Permafrost Properties The permafrost at Prudhoe Bay is about 1,850 ft deep and is composed of unconsolidated sediments with ice in the pore spaces. Typical lithology in the permafrost, pore spaces. Typical lithology in the permafrost, interpreted from open- and cased-hole logs and core data, is shown in Fig. 1. The thermal properties of each soil type are given in Table 1. Fig. 1 also shows the geothermal temperature, rising from 12 deg. F at the surface to 32 deg. F at 2,000 ft. Below 2,000 ft, the geothermal gradient is 2.1 deg. F/100 ft. The reservoir temperature is assumed to be 200 deg. F at a true vertical depth of 8,900 ft. Freezing points of the permafrost liquids also are shown in Fig. 1. JPT P. 461