Abstract

Version 1.0 of a computer model for thermal recovery wells was used to examine the effects of some common design parameters. This version, valid for the surface-to-reservoir portion of vertical wells, extends present design techniques by including casing and formation interaction instead of assuming that formations rigidly constrain the casing in the vertical direction. The model requires only stratigraphic information and common wellhead dam to predict casing and formation temperature distributions from two-phase flow and heat transfer algorithms. Those temperature distributions are passed to purpose-specific nonlinear finite element software, along with relevant materials properties, for calculation of thermally-generated strains and stresses in the casing and surrounding formations. The model has yielded interesting results, some of them being:

  • substantial influence of annulus pressure on casing temperature;

  • stress concentrations where casing penetrates formation interfaces;

  • substantial interaction between clustered wells.

The model promises to be an extremely powerful design tool after it is expanded to include well deviation and the effects of geotechnical activity in the reservoir.

Introduction

Much of Canada's, remaining hydrocarbon supply is asphalt-like bitumen which can be recovered economically only by heating it in the subsurface reservoirs. The bitumen becomes less viscous as its temperature increases, allowing it to flow to wells from which it can be pumped to surface.

Steam injection is the most popular method of heating the bitumen in the reservoir. The steam is carried to the reservoir down wells lined with steel casing. The casing has been cemented w the surrounding formations, so high steam temperatures generated large thermal expansion stresses.

Many casing failures have been attributed to thermal expansion of the casing, and conviction is growing that some casing failure are caused by thermally-induced deformations in the surrounding formations. Total failure numbers are known only by regulatory agencies, but operating com- panie's interest in improved design methods indicates that casing failures are numerous enough to be an economic and/or safety problem.

Casing failures occur because design "safety factors" necessarily are small even though loads are imperfectly understood. Steam injection requires wellhead pressures between 10 MPa and 17 MPa, with corresponding wet steam temperatures between 310 ºC and 350 ºC, because appreciable qualities of steam normally can be injected only by fracturing the reservoir to create fluid paths. Medium strength L80 casing usually is specified to avoid stress corrosion cracking. The large thermal expansion stresses load such casing beyond its yield strength the first time it is exposed to steam temperatures, and recent laboratory testing(1) suggests that stress relaxation effects may subject this casing to cyclic plasticity.

Imperfect understanding of casing loads stems from the operating environment's severity and its complexity. Directly measuring casing strains has not been successful; most instrumentation cannot survive exposure to the steam temperatures. The occurrence of casing failures clearly indicates that analytical determination of loads has not been entirely successful either, probably because all thermal recovery well designers have made simplifying assumptions.

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