Underbalance perforation is one of the best practices to insure less damage to the perforation tunnels. However, perforation jobs typically place large dynamic loads on the downhole equipment. Previous works on calculation of required underbalanced pressure did not consider the risk of equipment damage due to perforating shock loads. Predicting the magnitude and transient behavior of such loads is a critical step in developing completion designs that avoid damage or destruction to tool strings and production equipment, such as bent tubing and upset packers.
In this paper, on the basis of considering the advantages and disadvantages of conventional methods, a new method of designing negative pressure value for underbalanced perforating has been developed combined with a prediction model, which is fitted by a large number of numerical simulation results. It proves that the packer is the vulnerable part due to shock loads during perforating and the downhole equipment is susceptible to perforating shock damage under the condition of underbalance. A model based on numerical simulation results is established to predict the peak pressure on the packers. With this model we can evaluate the sensitivity of peak pressure to changes in the number of perforating bullet, charge per hole, packer to gun length, formation pressure and negative pressure. The model is verified by a field example. The pressure data was collected by a pressure sensor installed at the bottom of the perforating gun during the well completion in the South China Sea. The predicted peak pressure by the model is accurate, within 10% of measured values.
The field application shows that the method is more reasonable, and the oil production after underbalanced perforation is higher, which is valuable for providing guidance in further evaluations of underbalanced perforating pressure with low perforating shock loads.
The objective of well perforating is to create the critical link between production tubing and hydrocarbon reserves by means of an oriented explosive energy generated by the guns. (Oliphant et al. 1947, Allen et al. 1954, Bell et al. 1959). When 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. In the meantime, some detonation energy will be released into the wellbore to form a dynamic impact load, which could result in some equipment damage accidents, such as bent tubing, casing damage or upset packer. The modern completion engineers often employ numerical simulation within the perforating planning workflow, the foundations of the simulation technique used in this software are described in Sanders et al. (2011), Baumann et al. (2011, 2013a, 2013b), Baumann and Brinsden (2014), and Giunta et al. (2012), Bale et al. (2016). These studies are mainly focused on the development of simulation software to predict impact loads downhole.
