Abstract

Thermal recovery is a very common and effective method for producing heavy oil. Casing failure is a very serious problem in thermal recovery wells. In some oilfields, more than 95% of the casing for thermal recovery wells failed due to thermal stresses. In the steam injection process, the casing is heated by steam. The change in casing temperature produces thermal stresses. The casing deforms when stresses exceed the yield point of its material. If using conventional cementing technology for thermal recovery wells, the casing will deform as a result of thermal stresses as a result of the steam injection process. Conventional technologies used to protect thermal recovery well casing from failure are theoretically ineffective. Casing cementing with internal pre-pressurization for thermal recovery wells refers to the exertion of inner pressure on payzone production casing to make payzone production casing expand in the cement solidifying period. Calculations show that casing with internal pre-pressurization technology achieves a greater safety coefficient during the whole production cycle and results in no plastic deformation. This technology will be an important method to prolong thermal recovery well casing life.

Introduction

Steam injection is a common and effective operation to recover heavy oil. It has been adopted by many oil fields in China, including the Liaohe Oil Field and the Shengli Oil Field. It comes in two forms: steam huff and puff and steam drive. These two methods follow the same processes. First, run insulated tubing to the oil layer and fix it to the casing with a packer. Then, inject nitrogen into the annular space between the tubing and the casing. Finally, inject saturated steam into the oil layer through insulated tubing using injection equipment. The operating mechanisms are different, however. With the first method, injected saturated steam will transfer heat to the formation, thinning out the crude oil and raising the reservoir pressure. After the well is sealed for a short period, it will begin production. With the latter method, saturated steam will drive crude oil into the bottom of the production wells. During the steam injection period, thermal stress is produced by the temperature increasing in the casing string downhole, which may lead to the plastic yielding. The casing may deform due to thermal stress, uneven external pressure and heterogeneity in the casing itself(1–7). After production begins, the downhole temperature gradually decreases, elastic stress in the casing string declines and the plastic deformation remains. The result is that axial tensile stress often makes the casing thread off.

Current cementing technologies for protecting casing from failure for thermal recovery wells, such as pre-tension stress, slipjoint casing and preheating the whole well, are proven to be not so effective in solving deformation failure of the casing caused by overheating(8). Thus, the authors of this paper developed a safe and operable method to protect thermal recovery well casing from failure, using, in part, pre-heating cementing technology for thermal recovery wells(9), as well as pre-pressurized cementing for thermal recovery wells(10). These technologies keep the production casing in an elastic state for the entire thermal recovery operation. The procedures for casing cementing with internal pre-pressurization for thermal recovery wells are as follows: when finished pumping the cement slurry, run the drillstring with packer at its lower end to the top of the payzone casing, set the packer, then add additional pressure from the surface to the payzone production casing through the drillstring, tripping

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