Technology for Improving Life of Thermal Recovery Well Casing

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[a] Institute of Drilling technology， Shengli Petroleum Administration Bureau， Dongying， Shandong， China.

* Corresponding author.

Received 20 February 2013； accepted 18 March 2013

Abstract

In steam injection process， casing is heated by steam， the change of casing temperature produces thermal stresses in the casing， the casing deforms when stresses exceed the yield point of its material. Casing failure is becoming increasingly prominent in thermal recovery wells， which severely restricts the development effect of such reservoirs， improving casing life of thermal recovery well has become a urgent problem to be solved. Through on-site survey and analysis， reasons for casing damage were determined as follows： strength change by high temperature， sand flow over of oil formation， poor cementing， unfavorable heat insulation and bad material for casing. In order to improve casing life， the supporting measures are introduced， the measures include pre-stress cementing technology， using casing head， thermal stress compensator， high-performance insulation tubing， high temperature cement slurry system， FRT110H special casing， and early sand control completion technology. Field application of these measures has gotten better effect in Shengli oilfield in recent years， the damage rate of thermal recovery well has decreased obviously， and this can provide reference for the efficient development of similar reservoirs at home and abroad.

Key words： Casing failure； Thermal recovery wells； Special casing； Cementing； Sand control completion

Tang， Z. J.， Zhou， Y. J.， & Jia， J. H. （2013）. Technology For Improving Life of Thermal Recovery Well Casing. Advances in Petroleum Exploration and Development， 5（1）， -0. Available from： URL： http：///index.php/aped/article/view/j.aped.1925543820130501.1136

DOI： http：///10.3968/j.aped.1925543820130501.1136

INTRODUCTION

Thermal recovery well technologies， such as cyclic steam stimulation （CSS） and steam assisted gravity drainage （SAGD）， have been widely used in the production of heavy oil reservoir. Investigation indicates that a large portion of these casing string and connection failures can be attributed to the severe loading conditions of these applications （Kaiser et al.， 2005）. A common feature for thermal recovery wells is the cyclic thermal loading with high peak temperatures that may result in high thermally-induced stresses， which typically exceed the elastic limit of the material and cause the casing and connection deformed plastically （Kaiser， 2005）. In addition， curvature loading resulting from casing buckling and formation shear movement is also a critical load condition that inducing thermal recovery well casing and casing connection failure. Therefore， ensuring adequate structural integrity and seal ability of the connections over the full service life of a thermal recovery well is a significant challenge. In this work， the casing failure mechanism was investigated and the integrated technical measures were introduced， application results show that the proposed technique can effectively improve the thermal recovery well casing life and have good popularizing application value.

1. REASONS FOR CASING FAILURE IN THERMAL RECOVERY WELLS

1.1 High Temperature and Severe Temperature Changes

During steam injection， casing expands when it is heated. Axial stress inside the casing is compressive when the two terminals of the casing are fixed. When the axial stress exceeds the yield point， it can not be effectively released. Thus， the axial stress changes to side stress and therefore casing failure occurs due to the production of plastic deformation or permanent deformation of the casing. When steam injection stops and the temperature declines， this will cause the casing stretched （Gao， 2002； Maruyama & Inoue， 1989）. Therefore， the compressive stress changes to tensile stress. When it exceeds the yield point， casing failure occurs due to the excessive tensile stress at the casing joints or casing main body. Casing is exposed to temperature change by heating and cooling for several times per year during the extra heavy oil production due to 3-4 cycles of huff & puff operation for a single well.

1.2 Sand Flow over of Oil Formation

Formation sanding-out can cause rock skeleton structure distorted. Therefore， the overlying formation loses the support to some degree in space or the supporting force decreases， the original stress is unbalanced and the formation deforms vertically caused by forces from such as collapsing and compacting. In this case， when the cementation between the casing and overlying formation and between the casing and underlying formation is good， the deformation would transfers to the casing where a big amount of vertical stress would exert on the casing （Lin et al.， 2007； Li et al.， 2009）. When the cementation is not good， a relative slip movement happens， resulting in a big friction in the casing. It is found that in sanding-out formation， there are two types of casing deformations， namely， an elliptical deformation and squeezing deformation （Zhang et al.， 2008； Bruno， 2002）. Therefore there are relatively two types of casing failures， namely， beam column buckling bending exerted by vertical stress in an approximate long shank and cylindrical flexion.

1.3 Casing Material

If there are micro pores or slits in casing， its thread does not meet the engineering requirements， or shearing strength and tensile strength are lower than the standard value， casing failure will happen during the long term steam injection after well completion.

1.4 Poor Cementing Quality

During steam injection， shearing stress between casing and cement sheath will be generated because their linear expansion coefficient is different （Chen et al.， 2009）. When the maximum bonding force between casing and cement sheath is smaller than the shearing stress， casing will be separated from cement sheath. So it will be easily damaged under thermal stress.

The wide application of this casing head on site reveals that it can meet the production requirements by effectively controlling the casing elongation from heating， so it can contribute to the pre-stress cementing operation.

2.3 Using Thermal Stress Compensator

Thermal stress compensator installed at a proper position on the casing string allows for a certain axial elastic deformation which mitigates the thermal stress and restrains the internal stress within the yield limits. The main function as follows： （1） A function of axial stretching （Expansion distance 150 mm-160 mm）. （2） Solution of local casing damage. （3） Useful complement to Pre-stress completion technology. There are two thermal compensator used in Shengli oil field： Pin type thermal compensator and intelligent thermal stress compensator.

2.3.1 Pin Type Thermal Stress Compensator

Pin type thermal stress compensator was sealed by HTHP seal bellows welded to upper and lower beckets placed at the stretching resistance pedestal seat on the central line. To prevent axial displacement of the center tube and limit joint when trip in， a snap head flat key is applied. The HTHP seal bellows were in a state of nature， when the outer tube moved upward， it would be stretched， compressed conversely. Production casing would be slightly stretched by thermal stress， which is the main cause of casing failure. Thermal stress compensator placed above the reservoir can compensate the slight stretch of the production casing， so as to prevent casing failure （Yu & Bao， 2005）. When placed at the top of the reservoir where the production casing failure occurs most frequently， it can serves as wellhead protector. The schematic diagram of pin type thermal compensator is showed as Figure 3 and the main technical specifications were showed in Table 2.

2.3.2 Intelligent Thermal Stress Compensator

The intelligent thermal stress compensator adopts temperature sensitive material as the startup device， and it is in the state of dead lock in normal temperature， preventing the string from stretching out axially. When the temperature value reaches the set value， the compensator is in the open state to set free the string， keeping the strings at both ends of the compensator in free extension. The purpose of reducing stress concentration and protecting the string is reached by compensating the extension of the string. The schematic diagram of intelligent thermal stress compensator is showed as Figure 4 and the main technical specifications were showed in Table 2.