Completing offshore horizontal wells often requires killing the wells immediately after perforating before pulling the gun out of the hole and installing the rest of the completion hardware. Perforating practice varies from operator to operator. Some operators prefer to perforate in a clear fluid and spot a kill fluid for well control afterwards. While others perforate directly in a solid containing kill fluid. Both methods are done in overbalance. A two-year extensive research program was carried out to quantify the formation damage caused by these perforating practices.
The testing was conducted under downhole conditions in a large-scale laboratory setup, which physically simulates the wellbore and the reservoir, as well as the perforating and production processes. Tests were designed to study the pressure dynamics during perforating; to gain understanding of the effect of the perforating pressure dynamics on fluid loss control and formation damage; to select the most suitable kill fluid; and finally to provide overbalanced perforating design guidelines for production optimization.
The core test results showed that perforating design not only determines the skin damage caused by debris and grain crushing, but also plays a key role in controlling fluid loss, which strongly influences the permeability and productivity of the reservoir. Therefore, proper perforating design not only involves selecting the right shaped charges and gun hardware, but also it needs to take into account the perforating pressure dynamics, fluid formulation, and completion sequence.
In addition to the core flow testing, petrographic studies were performed on the perforated core samples to allow visual observation of the rock alterations caused by perforating and kill fluids.
This paper presents the findings in formation damage mechanisms during overbalanced perforating and kill processes and provides recommended practices in both job design and fluid selection for this type of operation in long horizontal wells.