Thermal Sealants Improve Cementing in SAGD Areas
Authors: Witt C. , Tahmourpour F. 
Published: March 2010

With the high demand for oil and gas, operators are becoming increasingly interested in unconventional sources of hydrocarbon. One of the major sources in the world is heavy oil, which has been defined by the API as oil that has API gravity of less than 22.3 °API at 15.6 °C (60 °F) (extra heavy oil, or bitumen, has 10 °API or less). Because heavy oil does not flow on its own in the wellbore, a mechanism must be used to recover the oil by making it mobile. Among the different experimental and operational methods, steam-assisted gravity drainage (SAGD) has been the method of choice for the past few decades. This process uses steam to heat the immobile oil and reduce its viscosity so it can be extracted, usually by using pump jacks. These heavy oil wellbores need to be zonally isolated to help ensure that the reserves are produced properly without environmental or production optimization challenges.

In Alberta and Saskatchewan, many wells are drilled and cemented in designated heavy oil fields where SAGD is applied to stimulate production. Most of these wells are successfully cemented, however, some wells experience zonal isolation failures that result in steam breakout or steam loss into non-targeted zones.

While the problem may be related to the primary cement job, it may also have been created by post-cementing operations and/or conditions. Sometimes the problem may be temporarily solved by conventional methods, however, the ultimate remedy may have to be applied eventually. The long-term integrity of the cement sheath behind the casing should be planned with drilling, completion, production/injection and abandonment stages considered. A properly designed and engineered cement slurry system can help save a wellbore/field from additional unplanned expenses. The process is shown below:Cement slurries are designed under defined wellbore conditions and exposed to field conditions in the laboratory.The mechanical behaviour of the cement sheath under downhole conditions is studied using three-dimensional, finite element analysis software.The endurance of the cement sheath under applied conditions is predicted and optimized.Operation and placement of the selected slurry is studied to help ensure optimum execution.

Stresses exerted on the cement sheath from wellbore operations during construction, injection or production that could damage the cement sheath are integral to the study. These conditions are analyzed to help engineers understand the post-cementing operational effects on the sealant system. This paper presents the design criteria for finding the fit-for-purpose sealant systems that can successfully protect wellbores under harsh environmental conditions of SAGD and cyclic steam stimulation (CSS) operations.

Introduction

The long-term integrity of a cement sheath throughout a well's life is the ultimate factor for determining whether the sealant will withstand the planned operations, production and injection that are planned for the well. In the case of heavy oil operations, the primary design considerations for a long-lasting sealant are the temperature and pressure regimes. The stresses caused by the extreme changes are exerted on casing, cement and the rock.