Perforation job consists of creating holes in the near-wellbore formation by shooting the perforation agent through the formation in order to establish fluid communication between the wellbore and hydrocarbon reservoir. During this process, some degree of damage occurs in the rock surrounding the perforated tunnel. Determining perforation-induced damage to the reservoir rock and its ultimate impact on the well productivity is of particular interest to the industry.
A discrete element-based numerical simulator was employed in this work to evaluate stress redistribution and failure zones around a perforation tunnel in a 2D plane. Formation was simulated as an assembly of densely packed particles bonded together. Macro-mechanical properties of the rock were estimated by changing the micro-parameters in several simulated biaxial tests. The perforation was simulated by shooting an external body penetrating through the wellbore wall in a high velocity, which generates the perforation tunnel as a result of the particle bonds being broken. Shear and tensile failures are expected to develop around the perforation tunnel.
The extent of the damaged zone (EDZ) and the depth of penetration (DOP) in a perforation job are important parameters which are influenced by formation mechanical properties, in-situ stress magnitude and direction and the perforation agent size and velocity. In this paper the effect of these parameters are studied by conducting number of sensitivity analyses. The results indicated that the EDZ reduces as formation strength increases and this effect is lesser if formation is perforated at maximum horizontal stress direction. Also, it was seen that both EDZ and DOP reduce as formation grain size increases. Larger perforation agents resulted in a lesser DOP but wider EDZ.