Intermediate Casing Collapse Induced by Casing Wear in High-Temperature and High-Pressure Wells
- Zheng Shen (Texas A&M University) | Frederick Eugene Beck (Brigham Oil & Gas LP)
- Document ID
- Society of Petroleum Engineers
- SPE International Production and Operations Conference & Exhibition, 14-16 May, Doha, Qatar
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 4.2.3 Materials and Corrosion, 1.14.1 Casing Design, 1.6 Drilling Operations, 1.6.6 Directional Drilling, 2 Well Completion, 1.11 Drilling Fluids and Materials, 1.14 Casing and Cementing, 1.10 Drilling Equipment
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Casing integrity is extremely important to downhole zonal isolation and preventing well instability. The reduction of casing strength not only occurs in directional drilling, but is also observed in vertical drilling with a slight deviation angle. Deteriorated casing in most hydrocarbon wells is reported from the onset of casing wear by the presence of friction force during the rotation of drillpipe. The friction on the casing wall causes the reduction of casing strength. Furthermore, the combination of corrosive drilling fluids with the rotation of drillpipe could dramatically degrade the casing strength. Although casing burst and collapse strength have been emphasized by many researchers, little research has presented the mechanical response of the worn casing. The studies that do exist on casing wear are not relevant for field applications because they do not consider the effects of high temperature and the surrounding formation. Therefore, it is urgent to obtain a proper stress profile of worn casing in order to reveal the true downhole information.
Based on the boundary superposition principle, we propose an analytical solution for the worn casing model that accounts for the contribution of thermal stress. We focus on the stress evolution in worn casing from the effects of high temperature and the confining formation. The predicted results show that the higher thermal loads largely increase the stress concentration of the worn casing, subsequently weakening the casing strength. The finite element solutions indicate that the radial stress in worn casing is not impacted as much as the hoop stress. The remaining part of the worn casing is subject to compression failure, along with an increase of the burst pressure or the elevated temperature.
There is a need for determining stress profiles in and around worn casing in cased hole in terms of casing integrity. Casing wear is considered a serious well instability problem, and it causes casing wall reduction because the rotating of drillpipe during the drilling process creates significant contact forces. Casing wall reduction can cause stress concentration at worn locations and results in new burst and collapse resistance of the remaining casing. In the presence of corrosive fluids in the well, the loss of casing thickness will be accelerated because of the chemical reaction destroying the surface of casing.
Casing wear is not only a problem related to directional or extended wells; it also occurs in vertical wells because the contact pressure generated on the inner surface of the casing becomes much harder to control during the penetration into deeper formation. Casing burst and collapse pressures depend upon the thickness of the casing wall; calculation of the burst and collapse pressure is given in Appendix A.
To extract hydrocarbons from deeper formation, a growing interest is to understand the stress distribution after casing wear. Worn casing is under the high risk of collapse in high-pressure, high-temperature (HPHT) wells due to large variations of temperature and pressure. High temperature can bring significant pressure increase in sealed annuli, causing further failures of the casing string or the production liner (Oudeman and Kerem 2004). As shown in the Fig. 1, the upper cased zone is heated by the produced fluids from the lower zones since T2 is larger than T1.
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