BP Norway: Zonal Isolation in Openhole Chalk Formation

Abstract

The Valhall field has been producing oil for 25 years. The completion strategy of the field has continuously been developed over the years, with cased hole completions with multiple propped fractures as main completion strategy since the mid 1990s. This completion technique was also chosen for the Valhall Flank Project initiated in 2001. The later stages of the project several of the well targets were located on the borders of the field, where the reservoir formation is considerably thinner and has a lower porosity and higher reservoir pressure than in the centre of the field. Using multiple propped fracture completion technique for these targets was not regarded attractive, and to access the reserves in these targets in a cost effective manner it was decided that the optimal design for the "thin pay" targets was a concept with pre-perforated liner in open hole. This paper describes the completion design evolution on Valhall and the considerations made for choice of completion method for the later stages of the Valhall Flank Project. An overview of the swell packer technology, in addition to the specific packer design for Valhall, is given and it is discussed how this technology provided opportunities to isolate several parts of the well if excessive water production occurred. Future applications for swell packer technology are also discussed. Introduction One of the main concerns with the pre-perforated liner concept versus cased hole completions was the lack of zonal isolation. As the well targets on the Valhall Flanks moved further towards the boundaries of the reservoir, it was an increasing risk of excessive water cuts. Means providing opportunity to isolate parts of the well were evaluated, and it was decided to include swell packers in the well S-8, drilled and completed as an open hole dual lateral with pre-perforated liner. The Valhall field The Valhall field is located in the North Sea approximately 290 km offshore southern Norway in 69m of water. The field is located in the southwestern corner of the Norwegian continental shelf. The field was discovered in 1975. Decision to develop the field was made in 1978 and the field started producing in October 1982. The initial development consisted of a 3-platform complex (quarters, drilling and process/compression platforms). In 1996, a fourth platform was added (wellhead platform) to provide additional slots for infill drilling. In 2000, a water injection program from an additional injection platform was approved. To provide efficient access to the flank areas in the North and the South, installation of two new wellhead platforms was approved in 2001. The Valhall field is an over-pressured, under-saturated Upper Cretaceous chalk reservoir and is a NNW-SSE trending anticline. The primary reservoir is the Tor Formation with secondary reservoir from a unit within the Hod Formation. The thickness of the Tor formation varies abruptly ranging from 0 to 80m. The reservoir quality varies considerably with some of the best porosities (up to 50%) and permeability (1 to 10 mD) developed in the thickest areas [1]. Completion design evolution Initial testing at Valhall gave experiences with severe chalk production and casing deformations due to the weakness of the Tor formation. It was apparent that completion design would be a critical factor to successfully produce the field. The completion strategy has continuously been developed over the years with focus on limiting solids production and wellfailures. Details of the completion history can be found in Barkved, O. et al [1]. In short terms, the completion designs have evolved from "Up and Under Fracturing" in the early 80s and "Propped Fractured Gravel packs" in the late 80s, to horizontally drilled wells in the early 90s (Acid Fractured, Acid Matrix Stimulated, Direct Perforated Unstimulated, Cased-off Openhole with inflatable packers). From 1995 to date, the main completion strategy on Valhall has been horizontal cased hole completions with multiple propped fractures. The strategy was developed due to the relatively disappointing performance of the earlier horizontal wells. The multiple propped fracture wells have been completed with 3 to 12 zones, with the number of zones in a well depending principally on the length of reservoir section. The benefits from these completions have been high and sustained rates, typically producing 5,000 to 8,000 BOPD the first year1.

Effect of HCl Acid and Brines on Water-Swelling Packers

“…It is suggested that using swelling packers can replace cementing and provide better zonal isolation (Kennedy et al 2005;Jorgensen et al 2007). Using swelling packers with a preperforated liner solves this problem.…”

Lab Investigation of Oil Swelling Elastomers for Smart Well Completions

“…Oil swelling elastomer will swell faster in lighter oils compared to heavier ones. 7,8 The objectives of the present study are to perform lab studies to: (1) Investigate oil swelling packers in different crude oils, (2) Examine the effect of oil gravity and viscosity on the performance of oil swelling packers, and (3) Assess the effect of HCl acids and brines on the swelled elastomers Common acid used in carbonate formations is HCl at various concentrations.…”