Production Casing Performance In a Thermal Field
Authors: Lepper G.B. 
Published: March 2010

High temperature steaming of wells can cause plastic yielding of the production casing. However, when properly designed and installed these casing strings will last the full life of a thermal project. Inferior designs or installations can cause significant well problems ranging from wellhead growth and settling to buckled or parted casing.

Since 1979, Shell Canada limited has been operating a thermal project in the Peace River/Cadotte Lake area. Steam injection temperatures have been as high as 335 ° C, but only two of the 286 steamed wells have incurred thermal induced casing failures. Three additional casing failures were caused by corrosion. Some 10 to 15 other wells have experienced wellhead settling (casing contraction). Several wellheads have gone down 25 to 50 cm, requiring wellhead and piping modifications but casing integrity has been maintained.

This paper discusses the casing and cement designs used historically, field observations and explanations for the casing "shrinking" phenomena and the field testing of K-55 grade casing. Shell Canada Limited's current design utilizes K-55 premium connection casing with class G thermal 1,880 kg/m3 and 1,400 kg/m3 weight cements.

Introduction

Shell Canada Limited has operated a steam injection project at Peace River since 1979. The production zone is an unconsolidated sandstone of the Lower Cretaceous Gething formation. This bitumen reservoir is 550 m from surface with an average pay thickness of 25 m. The bitumen gravity is 8 - 10 ° API with a viscosity at reservoir conditions of 200 Pa.s. The bitumen saturation decreases with depth in the zone. At bitumen saturations below 65%, the water phase is mobile and allows steam injection below fracture pressures. Most, but not all the wells drilled have a mobile bottom water layer.

The primary recovery process has been a pattern steam drive. Dedicated injection wells were utilized, however all wells were initially steamed and produced to establish bitumen mobility and communication between wells. In 1993, four horizontal wells were drilled to evaluate the Enhanced Steam Assisted Gravity Drainage Process (ESAGD). Since then, 25 horizontal wells have been drilled for ESAGD or cyclic steaming bitumen recovery. All of these wells have used K-55 casing with premium connections.

Casing designs have varied from NM-80 LT&C, L-80 VAM, L-80 OBTC (oversize buttress threaded and coupled), and most recently K-55 NS-CC premium connection.

Between 1979 and 1997, there were five casing failures at the Peace River complex. One was a buckled casing string, another was a parted connection and the remaining three were due to corrosion. Ten to 15 wells have had casing "contraction," causing the wellhead to settle 10 to 50 cm. The casing/wellhead movement is not a failure but can impose additional operating expenses and difficulty. In general, the casing performance has been satisfactory, as the above failures can be explained and continued reliability from the existing casing strings is anticipated.

Casing Design

An understanding of the stresses imposed on the casing during thermal operations is needed to properly design the casing and evaluate its performance.