Drilling and completing long horizontal wells in the North Sea oil fields are extremely costly. When perforating these long horizontal sections (>1000 m) deployment of the perforation guns on drill pipe has been regarded as the safest and most cost efficient method. The well is then kept in overbalance until the completion is installed.

In order to maintain well control during the perforation operation a kill fluid with fluid loss control capability to seal off against the formation is required. However, perforating with the kill fluid in the wellbore presents a great challenge in minimizing formation damage. Therefore, optimizing the perforating design and properly choosing the completion fluids are crucial tasks to ensure good well productivity. A thorough laboratory-testing program has been performed to reevaluate perforating strategy and completion fluids selection for several fields in the North Sea. The fluid systems investigated include oil-based mud, low solid oil based mud, and kill pills formulated from formates and bromide brines.

The laboratory results have shown that if the perforating is properly designed and executed, formation damage can be reduced to a minimum even during overbalance perforating. In this work the major damage appears to be from the kill fluids. When using a water based kill pill, the reduction in relative permeability appears to be an important damage mechanism. Therefore the productivity is influenced strongly by the fluid loss control capability. In contrast, the filtrate invasion from the oil-based mud does not alter relative permeability, therefore the productivity impairment for these tests is less pronounced.

Furthermore, zinc and steel cased charges have been used to investigate their compatibility with kill fluids and their impact on formation damage. These studies have shown that interaction between zinc and CaBr2 based kill pill can cause failure of fluid loss control and excessive invasion of filtrate leading to reduced productivity.

This paper presents some of the results of the testing program.

You can access this article if you purchase or spend a download.