New Underbalanced Perforating Technique Increases Completion Efficiency and Eliminates Costly Acid Stimulation

Abstract

Abstract Cased and cemented horizontal wells in the East Painter field in Wyoming have historically been perforated with modest underbalance pressures of 300 to 600 psi and then acid-stimulated to achieve economical production rates. The moderate success rate and expensive acid stimulations provided the motivation to evaluate other options for obtaining cleaner perforations. Conventional perforation-design techniques suggested that an underbalance of about 4000psi is needed to produce clean perforations for the low-permeability rock in this field. This would require a low wellbore pressure. An alternative is to use the PURE* (Perforating for Ultimate Reservoir Exploitation) system to create a high dynamic underbalance from a modest initial under- or overbalance. Two laboratory tests were conducted to evaluate these alternatives. The knowledge gained from these and other laboratory tests was used to modify the perforating techniques in the East Painter field to increase well production. Perforating gun design was changed, and the perforating process was modified to provide a substantial dynamic underbalance with only a modest initial underbalance. This new dynamic underbalance concept provides transient underbalances up to about 80 percent of the reservoir pressure without applying the equivalent static initial underbalance. This technique increased well performance, and eliminated the costly acid stimulation step by generating cleaner perforations and higher effective shot density. In this paper, details of the gun modifications and perforating process will be described along with specific well-performance data. Comparisons to previous perforating practices are also included. Introduction The Painter Reservoir Unit comprises two fields: Painter discovered in 1977 and East Painter discovered in 1979. The fields are located in the overthrust belt of southwestern Wyoming, Fig. 1. The two fields are plunging anticlines with a structural trend of north-northeast and south-southwest. They can be seen in the structure map and cross section, Figs. 2 and 3. Painter is currently in blowdown. Field-testing of the new underbalance perforating technique was done primarily in the East Painter field. The main hydrocarbon-producing formation is the Nugget Sandstone, an Aeoloian deposit, with a stratigraphic thickness of 900 feet. Under initial reservoir conditions there was a 1,000-ft gas cap - with a 165-ft transition zone (defined by gas-oil ratios) and a 220-ft column of oil. Porosities in the Nugget formation range from 8 to 16 percent with permeabilities ranging from 0.1–100 mD. The top of the Nugget formation is found at a depth of −3325 ft SS, approximately 10,500 ft measured depth. The rock is hard with a compressional strength of more than 20,000 psi. East Painter field currently has a reservoir pressure of 4,600 psi, near initial reservoir conditions as a result of pressure maintenance with nitrogen. As of April 23, 2002, current production from East Painter field is 5,591 BOPD, 71,142 MMscf/D from 18 active wells, 13 of which are horizontal.