Impact of Casing Eccentricity on Cement Sheath

Kui Liu,Deli Gao,Arash Taleghani

doi:10.3390/en11102557

Kui Liu, Deli Gao + Show 1 more

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https://doi.org/10.3390/en11102557

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Abstract

Sustained casing pressure (SCP) in shale gas wells caused by cement sheath failure can have serious impacts on safe and efficient gas production. Considering the fact that horizontal wells are widely used for production from shale, the cementing quality and casing centricity is barely ensured in these wells. Among other indications, the casing eccentricity is identified very often in wells with SCP problems in the Sichuan field in China. Hence, the objective of this study is to analyze the effect of the casing eccentricity on the integrity of the cement sheath. To better understand stress distribution in eccentric cement sheaths, an analytical model is proposed in this paper. By comparing the results of this model with the one’s with centric casing, the impacts of the casing eccentricity on the integrity of the cement sheath is analyzed. During fracturing treatments, the casing eccentricity has a little effect on stress distribution in the cement sheath if the well is well cemented and bonded to the formation rock. However, on the contrary, the casing eccentricity may have serious effects on stress distribution if the cementing is done poorly. The debonding of casing–cement–formation interfaces can significantly increase the circumferential stress in the cement sheath. At the thin side of the cement sheath, the circumferential stress could be 2.5 times higher than the thick side. The offset magnitude of the casing eccentricity has little effect on the radial stress in the cement sheath but it can significantly increase the shear stress. We found that the risk of cement failure may be reduced by making the casing string more centralized, or increasing the thickness of the casing. The results provide insights for design practices which may lead to better integrity in shale gas wells.

Highlights

The development of multistage hydraulic fracturing and horizontal drilling has made the economic exploitation of shale oil and shale gas from low permeability shale formations possible
According to the original parameters for the casing and cement sheath used in the field, the equivalent stress of the cement sheath in five wells is 16.26 MPa, 16.88 MPa, 17.64 MPa, 17.38 MPa and 20.86 MPa, respectively, which is much higher than permissible equivalent stress value, i.e., k
The cement sheath integrity failures detected in these five wells in the field are consistent with the calculated results

Summary

Introduction

The development of multistage hydraulic fracturing and horizontal drilling has made the economic exploitation of shale oil and shale gas from low permeability shale formations possible. Shale gas production first became commercial in the United States and is about to develop around the world. A reliable cementing job is necessary to prevent potential environmental impacts due to production and stimulation of these wells. The presence of high sustained annulus pressure in shale gas wells has created some operational and safety challenges for operators, especially when fracturing treatments are implemented through casing rather than tubing. More attention is required to address integrity concerns in horizontal wells, especially when the fluid pressure inside the casing is considerably high. The cement shrinkage can lead to circumferential fractures which can be propagated by the accumulation of gas with high pressure [2].

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