A number of models for estimating oil well perforation depths have been proposed by different scholars. However, earlier derived models have not accommodated a wide range of perforation parameters, such as the number of perforating bullet, charge per hole, formation pressure, wellbore pressure and tubing length.
Based on numerical simulations, the perforation process of a single perforating shaped charge has been dynamically simulated, which includes the explosion, jet forming and penetration. According to the equivalent ideas and combined with a model based on numerical simulation results has been established to predict the peak pressure of the perforating, a new method to calculate jet perforation penetration depth has been proposed, which can evaluate the sensitivity of the pressure to changes to the number of perforating bullet, charge per hole, formation pressure, wellbore pressure and tubing length, with these are important factors of the penetration depth. The results of case study show that the predicted pressure by the model is accurate, within 10% of measured values by a pressure sensor installed at the perforating gun during a well completion job.
Through the above analysis, a novel method for the determination of the penetration depth has been proposed, which has important significance to provide guidance for the design of the perforating operations.
At present, the shaped charge jet perforation has been most widely used in the oil and gas well completion, which is to make the passage between the wellbore and the oil layers and is a key link in the exploitation of oil and gas fields. The penetration depth of perforation is not only the most basic data for evaluating the productivity of oil and gas, but also the basic condition of optimizing perforation scheme. As the perforating charge is detonated to form a jet, the casing and cement sheath are perforated, so as to form a flow channel from reservoir to wellbore for oil and gas. Meanwhile, some detonation energy will be released into the wellbore.
With the development and progress of the perforating technologies, a number of scholars have done research work for penetration depth and a lot of models for predicting oil well perforation depths have been developed. The effects of formation stress on shaped-charge penetration and perforated-target flow performance have been studied by test (Saucier R J. 1978). On the basis of this previous study, the formula of shaped-charge penetration of rock under formation stress has been established (Halleck P M. et al. 1988). A model takes into account change in density of jet as it stretches during penetration while maintaining a constant diameter has been presented (Walters et al. 1989). According to the logging data, the maximum and minimum perforating depths are figured out after the space between the perforating gun and the casing correction (Sun et al. 2004). The equivalent method to calculate jet perforation penetration depth and hand-hole size of casing has been proposed (He et al. 2008). Actual perforation tests have been done with different shaped charges, rock cores and pressure system conditions according to API standard inspection equipment. The results show that the corresponding relationships between penetration and the dose, rock strength, porosity, acoustic velocity and effective stress are obvious (Li et al. 2010). The intent of this study is to develop a new mathematical model that can extend the scope of previous models and provide room for factors like target strength, explosive load, gun-to-casing clearance, effective impact area of jet/bullet, and velocity of perforator already known to affect perforation gun performance (F.H. Boukadi et al. 2010). 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) (Nabipour, A. et al. 2010). In order to study penetration capability of shaped charge jet penetrating into a soil /concrete target, the response characteristics of the soil /concrete target under shaped charge jet penetration were analyzed (Xiao et al. 2013). Based on the characteristics of deep sandstone under complex geostress, the influence of complex geostress on penetration depth of shaped charge into sandstone was analyzed, and a prediction formula of penetration depth under complex geostress was developed (Ren et al. 2017).
