Challenges in Heavy Crude Oil - Grane, an Overview
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18234, "Challenges in Heavy Crude Oil - Grane, an Overview," by J.A. Haugan, E.S. Jenssen, and S. Hatlem, Hydro Oil & Energy, prepared for the 2006 Offshore Technology Conference, Houston, 1-4 May.

Innovative solutions were needed to pro-duce the heavy oil from the Grane field. First production was on 23 September 2003. Grane is the first heavy-oil development on the Norwegian continental shelf. Close cooperation between Hydro's operations group and project-development team, the contractors, and partners formed the basis for completing the Grane facilities.

Introduction

A predrilling campaign comprising 12 wells allowed production startup shortly after completion of the offshore commissioning phase of the production facilities. The drainage strategy of the reservoir uses gas injection with gas imported through the North Sea gas-transportation system. Estimated reserves of more than 1.2×108 m3 will be drained by 31 production wells. The strategy is to reach the expected 60% recovery factor by drilling multilateral wells.

Fig. 1 shows the 22 000-tonne process, drilling, and accommodation installation that exports Grane crude oil to the Sture Oil Terminal on Norway's west coast through a 212-km-long, 28-in.-diameter pipeline. Hydro's high-pressure, multiphase flow loop at the research center in Porsgrunn was used to design the topside process and process equipment in cooperation with contractors. The operations comply with Hydro's zero-discharge philosophy.

Grane Field

Discovery of the Grane field in 1991 showed the importance of investigating new exploration and development methods. The Grane reservoir bears little resemblance to most other oil fields on the Norwegian continental shelf. Most of the fields contain relatively thin oil zones in reservoir-rock types of Jurassic age. The Grane oil is heavy and is in sandstone from the Tertiary period. The 92-km2 Grane Unit field is 180 km west of the Norwegian island of Karmoy.

Grane is one of the largest known oil discoveries under development and production on the Norwegian continental shelf. At the end of 2005, only 15% of the expected reserves had been produced during the first 2 1/2 years of operation. According to the original design, the processing capacity should be 3.4×104 m3/d, and it should flow through the 212-km-long pipeline from Grane to the Sture terminal north of Bergen. In the last weeks of 2005, production reached 3.74×104 m3/d.

Heavy Crude Oil at Low Pressure

Because the reservoir is at a depth of 1700 m below the seabed, and water depth is 128 m, this development might seem simple with regard to Norwegian offshore oil and gas fields. Grane, however, has been described as a challenging field because of the oil gravity (19°API) and low reservoir pressure (≈170 bar). Natural gas must be compressed for injection to move oil out of the reservoir. Injection gas is imported from the Heimdal Gas Center, 50 km away, through an 18-in.-diameter pipeline. This solution will provide a far greater recovery compared to injecting water for pressure support, which is the usual drive mechanism. After approximately 20 years of operation, the injected gas can be produced and sold. Fig. 2 shows the Phase 1 production strategy.