Abstract
Most oil and gas wells are cased and perforated. Perforating is usually the most important part of completing a well because it has a major impact on well productivity. However, conventional perforating studies use single-shot perforations into cylindrical core targets, which do not provide information on the interaction between phased multiple perforations.
Four laboratory ‘Block Tests’ were performed during 1991 and 1992, in a joint venture between BP Exploration, Schlumberger, Elf Aquitaine and Oryx Energy. A 4 3/4" (121mm) wellbore was drilled through each of the 25 ft3 (0.7m3) blocks of Berea sandstone. Each block was loaded into a large ‘stress frame’ and placed under confining stress before a 3 1/2" (89mm) diameter casing was cemented in place. The four tests included underbalance and overbalance perforating, well killing, clean-up and acid stimulation. A pack-off tool was used to measure the flowrate from individual perforations.
The differences between perforations and the interactions between them dominated the behaviour of the large sandstone blocks. Variability in the apparently homogeneous Berea sandstone, over just a few inches, meant that no two perforations behaved in exactly the same way. The work showed how the most productive perforation can become the least productive after killing a well, and how a poorly designed kill pill can give clean-up from very few perforations. Underbalance and overbalanced perforating did not give the expected results.
By simulating the downhole situation with real phased perforations, the Block Tests improve the understanding of what happens downhole, allowing better design of well operations.
