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Impact of Casing Eccentricity on Cement Sheath

Figure 3. Sketch of a casing–cement–formation system in a horizontal well.

Impact of Casing Eccentricity on Cement Sheath

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.

Authors:

Kui Liu1, Deli Gao1, and Arash Dahi Taleghani2

1College of Petroleum Engineering, China University of Petroleum (Beijing), Beijing 102200, China; 2Department of Energy and Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA.

Received: 2 August 2018; Accepted: 3 September 2018

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.

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 now about to develop around the world. However, 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. Hence, more attention is required to address integrity concerns in horizontal wells, especially when the fluid pressure inside the casing is considerably high.

The analysis of the mechanical response of the cement sheath shows that the integrity of the cement sheath is affected by the mechanical properties of the cement, geometrical parameters and the mechanical properties of the formation rock [1]. The cement shrinkage can lead to circumferential fractures which can be propagated by the accumulation of gas with high pressure [2].

Finite Element Analysis (FEA) is widely used to simulate the emergence and propagation of fractures along the wellbore axial and circumferential directions [3,4,5]. Chu et al. analyzed the cement failure or formation of micro-annulus cracks caused by changes in fluid pressure and temperature [6]. Casing expansion caused by high internal pressure may also lead to radial cracks in the cement sheath [7,8] or debonding [9]. During hydraulic fracturing in shale gas wells, incomplete or poor cementing can cause shear failure and broaching, especially along vertical sections of the wellbore [10,11]. These works are based on assuming that the casing is centered in the borehole; however, it is quite challenging to make sure that casing is completely centered in the well, particularly in horizontal wells [12].

Since casing strings are usually eccentric in wells, a thorough analysis is required to identify the susceptibility of the cement sheath to failure. Andrade et al. (2014) performed an experimental study on how, where and when temperature variations affecting the isolation capability of the cement sheath in cases with casing eccentricity [13]. Then, they used the FEA to evaluate the effect of casing stand-off and possible initial defects on stress and integrity of cement sheaths [14].

However, to date, there is no specific study to look at the stress of a cement sheath when the casing is eccentric in the well. In this paper, an analytical model based on the bipolar coordinate system is established to study the stress in a cement sheath with casing eccentricity. Furthermore, by considering high fluid pressure fluctuations during hydraulic fracturing treatments, the effect of casing inner pressure, the casing eccentricity, and the properties of casing and cement on the isolation capability of cement sheath is analyzed.

Field Observations

Sustained Casing Pressure in Shale Gas Wells

Natural gas may often migrate to the surface through the annulus or the channels among the cement sheath. Especially in the horizontal shale gas wells which are gone through hydraulic fracturing, most of the wells show sustained casing pressure (SCP) to some degrees. Based on the field data from shale gas wells in China, the change in the number of the wells with sustained annulus pressure during the whole life of the shale gas wells are shown in Figure 1a [15].

Figure 1. The sustained casing pressure in shale gas wells: (a) the percentage of wells with SCP during the whole life time of wells; (b) the percentage of the different SCP in wells.

It can be seen that the percentage of wells with SCP has significantly increased from 19.04% before hydraulic fracturing to 63.70% after the hydraulic fracturing treatment. While 19.14% of wells show SCP after one month, 5.07% more wells show SCP after two months. The pressure in the annulus of shale gas wells in this area are also measured, and the results are shown in Figure 1b [15].

Casing Eccentricity in Shale Gas Wells

The isolation scanner (Schlumberger technology) is used in the shale gas wells of this study to evaluate the wellbore integrity. The results show that casing eccentricity occurs very often in these wells, as shown in Figure 2, where the red dashed line indicates the center axis of the well and the black dashed line represents the center axis of the casing. It is extremely hard to make sure that the casing is laid absolutely centered in the field.

Figure 2. Isolation Scanner measurements show the quality of cementing work along a horizontal section of a well.

Figure 2. Isolation Scanner measurements show the quality of cementing work along a horizontal section of a well.

From Figure 2, we can see that most of the length of the casing is eccentric in this well, which leads to eccentricity of the cement sheath. As the result, the thickness of the cement sheath varies along its circumference at different cross-sections. In Part 1, the magnitude of the casing eccentricity is small so it can be treated as located at the center of the well, while in Part 2 and Part 3, the casing eccentricity increases significantly.

The stress in the cement sheath may be affected by the casing eccentricity, and we try to understand the effect of the casing eccentricity on the stress distribution in the cement sheath to come with a solution to decrease SCP in similar wells in future.

Mechanic Model

Multi-stage hydraulic fracturing technology is widely used in shale gas wells, hence the casing–cement–formation system undergoes periodic changes of casing internal pressure (fluid pressure in casing) between 20 MPa and 90 MPa during fracturing treatments. Meanwhile, the casing–cement–formation system is also experiencing the crustal stress environment. As the axial length is much larger than the diameter of the wellbore, the mechanical model of the casing–cement–formation can be treated as a plane strain model.

There are two major factors which cause casing eccentricity in shale gas wells: (1) the difficulties in assuring that the well is drilled perfectly straight in the horizontal section; (2) the casing bending between two centralizers due to the gravity and the axial stress (Figure 3a). The sketches of loads acting on the casing–cement–formation system for centric and eccentric casings are shown in Figure 3b,c. For future reference, the casing–cement interface is named the first interface and the cement–formation interface is named the second interface. The crustal stress is σo and casing internal pressure is shown by Pi.

Figure 3. Sketch of a casing–cement–formation system in a horizontal well.

Figure 3. Sketch of a casing–cement–formation system in a horizontal well.

From Figure 2, we can see that most of the length of the casing is eccentric in this well, which leads to eccentricity of the cement sheath. As the result, the thickness of the cement sheath varies along its circumference at different cross-sections. In Part 1, the magnitude of the casing eccentricity is small so it can be treated as located at the center of the well, while in Part 2 and Part 3, the casing eccentricity increases significantly.

The stress in the cement sheath may be affected by the casing eccentricity, and we try to understand the effect of the casing eccentricity on the stress distribution in the cement sheath to come with a solution to decrease SCP in similar wells in future.

Mechanic Model

Multi-stage hydraulic fracturing technology is widely used in shale gas wells, hence the casing–cement–formation system undergoes periodic changes of casing internal pressure (fluid pressure in casing) between 20 MPa and 90 MPa during fracturing treatments. Meanwhile, the casing–cement–formation system is also experiencing the crustal stress environment. As the axial length is much larger than the diameter of the wellbore, the mechanical model of the casing–cement–formation can be treated as a plane strain model.

There are two major factors which cause casing eccentricity in shale gas wells: (1) the difficulties in assuring that the well is drilled perfectly straight in the horizontal section; (2) the casing bending between two centralizers due to the gravity and the axial stress (Figure 3a). The sketches of loads acting on the casing–cement–formation system for centric and eccentric casings are shown in Figure 3b,c. For future reference, the casing–cement interface is named the first interface and the cement–formation interface is named the second interface. The crustal stress is σo and casing internal pressure is shown by Pi.

Stress on Interfaces

As shown in Figure 3b, the casing is centric and the casing–cement–formation system consists of three rings, including the casing string, cement sheath and formation rock. The thickness of the cement sheath is uniform around the casing. The boundary conditions at the interface are stress and displacement continuities. The Lamé formula in elasticity can give the stress in each ring for the plane strain geometry. The polar coordinate system is used to simplify the expressions. By employing the Lamé formula [16], the radial stress on interfaces can be achieved according to the stress and displacement continuities at the interfaces, as shown in

By employing the Lamé formula [16], the radial stress on interfaces can be achieved according to the stress and displacement continuities at the interfaces, as shown in

The details of deriving these equations are shown in our previous paper [17]. The k indices are used for simplification purposes [18].

Stress in Cement Sheath

As shown in Figure 3c, the casing eccentricity causes uneven thickness of the cement sheath. The radial stress S1′ on the first interface and S2′ on the second interface which are induced by the crustal stress and casing internal pressure are the external loads. The stress in the cement sheath with casing eccentricity can be much different from the stress in wells with no casing eccentricity. The conventional polar coordinate system is very useful to calculate the stress of a cement sheath when casing is centered in wells, but it is not very useful for calculating stresses in the case of eccentricity. Therefore, the bipolar coordinate system [19] is employed for stress analysis here. As the casing–cement–formation system has reached the equilibrium state after cement is set, the following hypotheses are assumed to simplify stress analysis:

  •     Tectonic stresses are assumed to be isotropic;

  •     As we focus on analyzing the effect of eccentricity on the stress of the cement sheath, the external loads on the surfaces of the cement sheath are treated to be equal to the stress calculated according to Equation (1);
  •     The cementing job is well done and no discontinuity exists in the cement sheath;

Since uniform tectonic stress is only considered in the model, the analytical model derived in this paper can only be used in a field with uniform crustal stress. The stress of the cement sheath caused by casing eccentricity in non-uniform tectonic stress is much more complex and needs to be studied in future.

Mechanistic Model of an Eccentric Cement Sheath

As shown in Figure 4, the eccentric cement sheath with double centers is simplified as an eccentric annulus and the bipolar coordinate system is employed to show the geometry parameters. The bipolar coordinate system is an orthogonal curvilinear coordinate system and the C1 = λi and C2 = −λi are the two focus points, corresponding to the points (0, λ) and (0, −λ) on the plane defined by the Cartesian coordinate system. λ is a positive real number.

The point M is an arbitrary point shown in the bipolar coordinate system. α and β are coordinate axes of the bipolar coordinate system [20]. The conversion of coordinates from the bipolar coordinate system to the Cartesian coordinate system is as follows:

Figure 4. Bipolar coordinate system for stress analysis in a cement sheath.

Figure 4. Bipolar coordinate system for stress analysis in a cement sheath.

In Figure 4, α = α1 and α = α2 are the inner and outer interfaces of the cement sheath, respectively. O1 is the center of the inner circle α1, with the radius r1; O2 is the center of the outer circle α2, with the radius r2; and δ is the distance between O1 and O2. The values of α1, α2 and λ are calculated according to r1, r2, δ, as described in Equation (A7). β is the angle between r1 and r2. Using geometry, β can be represented by λ, r1 and r2.

Using geometry, β can be represented by λ, r1 and r2

 

is the center of the circle αo; R is the radius of this circle; and φo is the angle of point M with respect to axis x.

Emanuel Martin
Emanuel Martin is a Petroleum Engineer graduate from the Faculty of Engineering and a musician educate in the Arts Faculty at National University of Cuyo. In an independent way he’s researching about shale gas & tight oil and building this website to spread the scientist knowledge of the shale industry.
http://www.allaboutshale.com

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