Well-Productivity Improvement by Use of Rapid Overpressured Perforation Extension: Case History

Abstract

Abstract This paper describes overbalanced perforating and surging operations as a pretreatment to hydraulic fracturing for the Romeo interval at Prudhoe Bay. Operation conditions are presented and discussed, as well as surface and bottomhole pressure measurements. Well productivity and the amount of proppant placed are compared to results in offset wells where the technique was not applied. The paper shows how the use of the technique allows placement of small, highly conductive fractures in intervals that were not previously considered fracturing candidates due to the close proximity to the gas/oil contact (GOC).The paper also shows pressure transient analysis affirming the technique as a stand-alone stimulation. It is shown that the use of extreme overbalanced perforating and surging treatments prior to hydraulic fracturing produces a substantial increase both in the success rate and the efficiency of the hydraulic fracturing operation and in the production rate of the wells that are pretreated. Finally, a comparison between pressure data and a new radial fracture propagation model shows a good match. The model demonstrates that high-energy treatment can significantly increase the extension and the height of the fracture; this was corroborated by downhole pressure measurements recorded during one overbalance treatment and by well logs. Introduction In 1990 an aggressive hydraulic fracture stimulation program began in the Prudhoe Bay Unit's Sadlerochit reservoir. This paper is limited to the ARCO-operated portion of the field or the Eastern Operating Area (EOA). North Slope wells, like offshore wellbores, are drilled from a central location. These extended reach wells present difficulties for hydraulic fracturing operations because of shear stresses associated with wellbores that do not align with one of the principal in-situ stresses. The fracturing campaign was highly successful and has resulted in an incremental oil rate of 180,000 barrels of oil per day (BOPD) from 1990 through1993 - Two hundred thirty-three wells were hydraulically fractured in the EOA, with a comparable number fractured in the Western Operating Area (WOA). The primary hydraulic fracture stimulation zones are the Romeo and the Zulu. Of these EOA wells, 34% were stimulated in the Romeo formation, 56% in the Zulu formation. Even though the zones are in the same reservoir there are significant differences requiring different hydraulic stimulation approaches. This paper addresses the more difficult to fracture Romeo interval by first describing the problem of Romeo formation fracturing followed by a discussion of the extreme overbalance (EOB) operations done at Prudhoe Bay. The impact of these pre-treatments is discussed on subsequent fracturing treatments and on the well productivity. Several case histories are presented that further illustrate the technique. Stimulation benefit of stand-alone extreme overbalanced perforating treatments is also investigated using bottomhole pressure data. Finally, fracture propagation model predictions of surface and bottomhole pressure are compared with surface and bottomhole pressure recorded during the EOB with fast-response pressure transducers. Formation Description. The Zulu is a clean sand several hundred feet thicken cased between large shales. These thick shales adjacent to the Zulu allow large fractures to be placed without fracturing out of zone. On the other hand, the lower Romeo sands are less continuous, less permeable, finer grained, and are thin sand/shale sequences.