Perforation job consists of creating holes in the near-wellbore formation by shooting the perforation agent through the formation in order to provide a means for hydraulic communication between the wellbore and hydrocarbon bearing formation. Perforation itself may alter the porosity and induce damage to the rocks in the vicinity of the wellbore to some extent, which consequently results in reduced production. Due to difficulties associated with conducting laboratory experiment under in-situ stress conditions to study a perforation scenario and inefficiency of commonly used numerical simulators suitable for continuous medium to model such a dynamic process, in this work a discrete element method (DEM) code was used for modeling purposes. Using particle flow code (PFC) porosity and different in-situ stress changes were simulated in 2D to distinguish the level of rock damage along or/and around a perforation tunnel. The formation was simulated as an assembly of densely packed particles bonded together. The perforation was simulated by shooting an agent penetrating through the wellbore wall with a high velocity to create a perforation tunnel. The results are presented in this paper and the advantages and disadvantages of the DEM code and 2D numerical approach are explained. The simulation results obtained for different scenarios of operating conditions and rock mass properties will also be compared with previous models.
DEM based codes have been used in a wide range of petroleum applications including sanding prediction and wellbore stability, modeling of hydraulic fracturing and fluid leak-off in a fracpack [1–3]. However, this technique has rarely been used for modeling perforation phenomenon [4, 5]. The results of recent studies demonstrate the possible applications of DEM numerical codes for modeling a perforation job. In this paper, it is shown in particular how porosity may change around the perforation tunnel during a short period. This, to the best knowledge of the authors, has not been the subject of any study in the past. In contrary to a small number of numerical studies, considerable experimental works have been carried out to investigate the effect of operating conditions and rock parameters on the quality of perforation job. The results of these experiments show that formation porosity [6], ultimate strength [7–10], wellbore pressure [8], acoustic velocities and bulk modulus [11–13], grain size [14], lithology [15], heterogeneity [16], and confining and effective stresses [10, 12, & 17] are the important factors that may affect the job performance. Amongst these parameters, few available publications discussed whether or how porosity may change around the perforation tunnel as a result of perforation, yet the available literatures present dissimilar conclusions. Some papers report that as a result of perforation the rock grains are subjected to a severe shock-type pressure (in orders of millions of psi) in a very short time period (microseconds) and accordingly metamorphous rock structure with lower porosity may be formed [18]. In these studies the porosity reduction also is considered due to grains compaction and/or creation of planar fractures around the perforation hole, and also validated by experimental results.
