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Research Papers: Oil/Gas Reservoirs

Emergence of Delamination Fractures Around the Casing and Its Stability

[+] Author and Article Information
A. Dahi Taleghani, W. Wang

Craft and Hawkins Department of
Petroleum Engineering,
Louisiana State University,
Baton Rouge, LA 70803

1Present address: Shell, Co., Houston, TX 77002-4916.

Contributed by the Petroleum Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received May 12, 2016; final manuscript received May 18, 2016; published online June 27, 2016. Editor: Hameed Metghalchi.

J. Energy Resour. Technol 139(1), 012904 (Jun 27, 2016) (11 pages) Paper No: JERT-16-1211; doi: 10.1115/1.4033718 History: Received May 12, 2016; Revised May 18, 2016

Casing support and zonal isolation are principal objectives in cementing the wells; however, the latter objective always raises the most concern particularly when there is a potential formation fluid migration into the cement sheath. Wellbore integrity is highly dependent upon the integrity of the interfacial bonding between the cement and the formation as well as the bonding between casing and cement. A closer look at the common cement strength test data, performed routinely in the labs, reveals a complicated behavior that cannot be simply modeled using a single parameter, i.e., the interfacial strength. Here, we used cohesive interface constitutive equation to model the behavior of cement interfaces. Formation of microannulus is modeled by utilizing an axisymmetric poroelastic finite element model enriched with cohesive interfaces to simulate initiation of the failure zone and possible broaching of the failure zone along the wellbore to shallower zones. We demonstrated that it is possible to use data produced from routine tests, such as the push-out test, to determine not only the shear strength but also the normal fracture energy and the stiffness of the cement interface. Cohesive interface properties are calibrated such that simulated test results match with the measured response of the specimens. In the next step, we used these parameters to anticipate well-cement behavior for the field-scale problems. A sensitivity analysis is provided to show the role of each parameter in initiation and development of the failure zone. Interestingly, the shear strength, which is commonly measured from push-out tests, is not the only parameter determining the size of the fracture, but other parameters such as normal strength show equally important influence on initiation and propagation of the failure zone. The proposed approach provides a tool for more accurate predictions of cement integrity in the subsurface conditions to quantify the risk of wellbore integrity issues.

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Figures

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Fig. 1

Potential mechanisms for fluid migration outside the casing are demonstrated. Casing is shown in dark beige, cement sheath in yellowish brown, formation in tan, cracks in blue, and debonding or incomplete cementing in white (see online version for color).

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Fig. 2

Linear softening law for a typical cohesive element under (a) normal and (b) shear tractions are demonstrated

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Fig. 3

A schematic of the push-out tester is shown here. Core sample is located at the center surrounded by cement and casing, respectively.

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Fig. 4

Loading curve of pull-out test calibrated with different traction separation laws

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Fig. 5

Loading curve of pull-out test for different interface shear strength is shown in this graph. Higher shear strength leads to larger maximum force.

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Fig. 6

Loading curve of pull-out test are shown for different normal fracture energies in this graph. The effect of normal toughness on loading curve is significant, but our simulations show no effect of shear toughness on loading curve in this experiment. To measure, shear toughness of cement interface new lab experiment should be designed.

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Fig. 7

Well configuration and dimensions are not shown. The red bubble indicates the leakage point (see online version for color).

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Fig. 8

A schematic picture of the finite element model is shown with its dimensions, boundary conditions, and loading for the axisymmetric model

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Fig. 9

The finite element mesh for the axisymmetric geometry is demonstrated. Due to the large gradient of stress and displacements, the elements in the vicinity of the wellbore and cement liner are adaptively refined. Moreover, interface layers between casing and cement or cement and formation are attached by the two-dimensional cohesive elements for displacements and pore pressure.

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Fig. 10

Normal tractions along the cement-rock and cement-casing are drawn. A considerable difference exists between stresses on both sides of the cement liner in the failure zone.

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Fig. 11

Shear traction along the cement liner at end of failure process. Large shear stress concentration observed at the end of failure zone indicates shear failure in development of delaminating fractures.

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Fig. 12

Damage index along cohesive zone at the end of the failure shows the extension of the failure zone that is in accordance with calculated shear stress distribution

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Fig. 13

Fracture length development due to increase of excessive pressure for cases with different shear strength but similar critical energy and normal strength is shown above

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Fig. 14

Propagation of delamination cracks due to increase of excessive pressure for cases with different normal strength but similar critical energy and shear strength are shown above

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