Estimation of fractures aperture and critically state of stress in wellbore condition

UDK: 550.8.072
DOI: 10.24887/0028-2448-2022-7-121-125
Key words: Barton-Bandis model, critically stressed fractures, mechanical aperture, formation microimager, fractured reservoir
Authors: S.V. Zhigulskiy (Gazpromneft STC LLC, RF, Saint-Petersburg), A.С. Gunkin (Saint-Petersburg Mining University, RF, Saint-Petersburg)

This paper describes the approach for assessing the critical stress state and mechanical aperture of the fracture system in the near-wellbore zone, which takes into account the direction of the wellbore in relation to the revealed fracture system and the direction of the regional stress as well as the contribution of the wellbore pressure to the transition of a fracture to a critically stressed state with a subsequent increase in aperture.

Critically stressed state of a fracture was studied according to two criteria, such as dry friction criterion and Barton nonlinear shear strength criterion. The main differences between two criteria is about friction coefficient, in the case of dry friction criterion the more important are stresses (normal and shear) while in the case of the Barton nonlinear shear strength criterion shows that the importance is understanding of joint roughness coefficient, due to which the relationship between shear and normal stress becomes non-linear, also for different rocks strength will change, which approaches through such a parameter as the joint compressional strength (JCS).

The Barton-Bandis model was adopted as the fundamental model of fracture opening. This model allows the investigation of fracture aperture considering surface roughness and dilation due to shear displacement.

The simulation result is presented as a certain sinusoid (a fracture contour along the well surface, obtained from the results of the interpretation of the reservoir microscanner). The sinusoid has two attributes: fracture aperture in mm and critically stressed index. This allows us to explore how the critical stress state of a fracture in the near-wellbore zone changes when the pressure inside the well changes. This approach can be applied to select the optimal wellbore trajectory and reduce the risk of lost circulation in fractured reservoirs.

References

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