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Research Papers: Petroleum Engineering

Finite Element Analysis of Thermally Induced Stresses in the Near-Wellbore Region During Invasion of Mud Into Fractures

[+] Author and Article Information
Ze Wang, Yuanhang Chen

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

Contributed by the Petroleum Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received August 31, 2017; final manuscript received November 20, 2017; published online January 31, 2018. Assoc. Editor: Ray (Zhenhua) Rui.

J. Energy Resour. Technol 140(5), 052909 (Jan 31, 2018) (10 pages) Paper No: JERT-17-1474; doi: 10.1115/1.4038783 History: Received August 31, 2017; Revised November 20, 2017

A severe lost circulation event is usually associated with emanation and propagation of pre-existing or drilling induced fractures from the wellbore. To combat lost circulation and prevent further fracture propagation, a thorough understanding of the stress state in the near-wellbore region with fractures is imperative. However, it is not yet fully understood how temperature variation during the invasion of mud affects pre-existing or newly initiated fractures. A three-dimensional (3D) finite element (FE) analysis was conducted in this study to simulate the transport processes and state of stresses in the near-wellbore region during invasion of mud into fractures. To account for thermal effects, a thermo-poroelasticity model was coupled with flow and heat transfer models in the fractures. This study included a series of sensitivity analyses based on different formation properties and mud loss conditions to delineate the relative importance of different parameters on induced thermal stresses. It also evaluated potential risks of reinitiating fractures under various downhole conditions. The results demonstrate how the stresses redistribute as nonisothermal invasion of mud takes place in fractures. It shows that a temperature difference between the formation rock and the circulating muds can facilitate fracture propagation during invasion of mud. These results due to temperature change can also diminish the enhanced hoop stresses provided by wellbore strengthening (WBS) and other lost circulation prevention methods. Such information is vital to successful lost circulation management. The conclusions of this study are particularly relevant when a substantial temperature difference exists between circulating fluids and surrounding rock, as commonly seen in high-pressure, high-temperature, and deepwater wells.

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Figures

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Fig. 1

Simplified diagram of (a) the cross section of the whole wellbore and (b) 3D geometry established in the finite element study (a quarter of the whole model with asymmetrical boundaries)

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Fig. 2

Comparison of the SIF obtained in this study and that obtained from the analytical solution. Fluid leak-off is not considered.

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Fig. 3

Comparison between hoop stress change at the fracture tip obtained in this study and the fracture gradient change under the cooling effect obtained from the analytical solution

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Fig. 4

Thermal impact on hoop stress along the wellbore wall

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Fig. 5

(a) Thermal impact on hoop stress along the fracture surface and (b) SIF at fracture tip. Both the linear elasticity model and poroelasticity model with constant pore pressure were considered.

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Fig. 6

Hoop stress variation along the wellbore wall under the thermal effect. Maximum stress change (90 deg to the fracture mouth) are compared with analytical model.

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Fig. 7

(a) Hoop stress along the fracture face and (b) SIF under the formation cooling effect

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Fig. 8

Hoop stress along the wellbore wall under the thermal effect (thermo-poroelasticity model)

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Fig. 9

SIF under the thermal effect; poroelasticity model with different horizontal stress anisotropies. Minimum horizontal stress is set to 3000 psi.

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Fig. 10

Hoop stress along the wellbore wall under the cooling effect with different (a) Young's moduli and (b) Poisson's ratios

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Fig. 11

SIF under the formation cooling effect for (a) changing Young's moduli (constant Poisson's ration of 0.2), and (b) changing Poisson's ratio (constant Young's modulus of 1 × 106 psi), thermo-poroelasticity model

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Fig. 12

(a) Hoop stress along the wellbore wall under the cooling effect and (b) SIF with different fluid leak-off rate

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Fig. 13

Geometry used to calculate stress intensity factor [22]

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Fig. 14

The line contour for j. Γ is shrinking onto 0 by definition (not shown in the plot) [29].

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