During a perforation job an opening is made in the formation to act as a conduit for hydrocarbon to be produced from the reservoir and flow into the wellbore. This process induces some degree of damage to the surrounding rocks which increases the wellbore skin and consequently decreases production. In this study the perforation job is simulated numerically using PFC2D which is a 2D DEM code to evaluate the length of perforation tunnel (LPT) and the extent of damaged zone (EDZ). Formation is modeled as an assembly of bonded particles with assigned micro-mechanical properties. Furthermore; formation macro-mechanical properties were obtained by performing a series of simulated biaxial tests. High velocity perforation agent was shot against the formation which generated the perforation tunnel as a result of particles displacement and bonds breakage. The results showed that LPT and EDZ are influenced by perforation direction, particle size distribution, formation porosity, and magnitude of in-situ stress. Perforation into formations with smaller particle size ratios and lower porosities results in less LPT and EDZ. The LPT reaches it maximum limit in the direction of maximum horizontal stress. In addition, the higher the value of isotropic horizontal stress, the lesser would be the LPT and the EDZ.
During past few decades, perforation devices have been improved from bullet agents to high performance perforation agents of shaped charges [1]. However, the previous studies have not been able to fully understand the mechanical interaction of perforation agent and formation. Several experimental studies that tried to enlighten this phenomenon have shown that many factors including the formation porosity [2], ultimate strength [2, 3, 4], and confining and effective stresses [4, 5, 6] are the important parameters affecting the perforation job quality. The presence of in-situ stresses makes the interaction more complicated and difficult to model experimentally. This is perhaps the reason why many of experimental works have been carried out under ambient pressure. The importance of in-situ stresses will be clearer when geomechanical rock properties and perforation parameters carried out in ambient and confined stress conditions are compared. For example, the correlations between the formation ultimate compressive strength (UCS) and the LPT for unconfined [4] and confined [2] conditions are quite different. In contrary to experimental efforts, less numerical attempts have been practiced for simulation of the perforation process. The reasons for this may be due to the difficulties encountered in this kind of simulations. The problems associated with numerical simulations based on continuum approaches could be modeled more efficiently using Distinct Element Method (DEM). This method was firstly introduced by Cundall and Strack [7]. PFC2D is a DEM based code which is used here for modeling both perforation agent and formation. It generates an assembly of circular particles bonded together to construct the model. The code has been used to simulate several rock mechanics problems such as biaxial experiments of a rock specimen in laboratory [8], failure around a circular opening under biaxial compression [9], and hydraulic fracturing [10].
