Investigation of possible wellbore cement failures during hydraulic fracturing operations

Abstract

We model and assess the possibility of shear failure, using the Mohr-Coulomb model -along the vertical well by employing a rigorous coupled flow-geomechanic analysis. To this end, we vary the values of cohesion between the well casing and the surrounding cement to representing different quality levels of the cementing operation (low cohesion corresponds to low-quality cement and/or incomplete cementing). The simulation results show that there is very little fracturing when the cement is of high quality.. Conversely, incomplete cementing and/or weak cement can causes significant shear failure and the evolution of long fractures/cracks along the vertical well. Specifically, low cohesion between the well and cemented areas can cause significant shear failure along the well, but the same cohesion as the cemented zone does not cause shear failure. When the hydraulic fracturing pressure is high, low cohesion of the cement can causes fast propagation of shear failure and of the resulting fracture/crack, but a highquality cement with no weak zones exhibits limited shear failure that is concentrated near the bottom of the vertical part of the well. Thus, high-quality cement and complete cementing along the vertical well appears to be the strongest protection against shear failure of the wellbore cement and, consequently, against contamination hazards to drinking water aquifers during hydraulic fracturing operations.

Relative permeability for water and gas through fractures in cement

“…Force (444 N and 667 N) was applied to both of the cylinders with the offset plates until a shear fracture occurred. This simulates a shear fracture which could occur with incomplete cementing of a wellbore combined with high pressures [17]. For the multiple fractures the cement sample was rotated 90° and fractured again, to increase the aperture and the number of fractures.…”

Sealing Failure Mechanism and Control Method for Cement Sheath during Hydraulic Fracturing

“…Researchers have generally focused on the integrity of the cement sheath at the target location, which is usually the production layer or horizontal section during drilling, perforation, fracturing, or production . The SCP refers to the down-hole gas passing through a certain seepage channel, such as a radial crack, micro-annulus, casing thread, or packer, to the wellhead. , The occurrence of the SCP indicates the failure of the wellbore barrier; however, an analysis of the integrity of the cement sheath at the production layer cannot ensure the sealing failure mechanism. For shale gas wells, the fluid column pressure acts on the inner wall of the entire casing; however, the well structure, in situ stress, and formation mechanics parameters are different at different depths, which lead to different stress states and failure forms.…”

Full‐Life‐Cycle Analysis of Cement Sheath Integrity

“…In addition, hydraulic fracturing has been employed more and more extensively for unconventional resources such as shale gas. e high casing pressure encountered in hydraulic fracturing tremendously challenges the well integrity [5][6][7]. e sealing failure could be in different types including shear or tensile failure of cement sheath [8] and microannulus at the cement-casing interface (first interface) or cement-formation interface (second interface) [9][10][11].…”

“…After drilling, a well sequentially experiences cement injection, cement setting, completion, and production stages. Main mechanisms causing the failure of well integrity in these stages include the shrinkage of cement sheath due to cement hydration [9], the increase in internal casing pressure during hydraulic fracturing [7], and the increase in casing temperature when producing deep oil/gas [4].…”