SPE Reservoir Evaluation & Engineering
Volume 11, Number 4, August 2008, pp. 648-654

Summary

North Sea chalk reservoirs are characterized as being purely biogenic and naturally fractured, having low matrix permeability and very high porosity (30 to 45%). The reservoir temperature is usually high, more than 90°C, and the wetting conditions appear to be moderately water-wet to neutral. Even though the permeability contrast between the matrix and fractures is significant, the injection of seawater has been a great success with the Ekofisk field as an example (estimated oil recovery is now approaching 50%). Seawater improves the water wetness of chalk, which increases the oil recovery by spontaneous imbibition and viscous displacement.

During the primary production phase by pressure depletion, extensive compaction was observed and, at that time, it was regarded as an important drive mechanism for oil recovery. The compaction continued in the waterflooded areas even though the reservoir was repressurized by the injected seawater. The phenomenon has been described as water weakening of chalk, and production costs have increased because of the loss of wells and substitution of platforms.

This paper gives an overview of the chemical aspects of the interaction between seawater and the chalk. Surface active components in seawater, such as Ca²⁺, Mg²⁺, and SO₄²⁻, will play an important role in both wettability modification and rock mechanics. In that sense, injection of seawater into chalk must be regarded as a tertiary-oil-recovery technique. Chemical models describing the wettabilty alteration and enhanced water weakening of chalk by seawater are suggested and presented.

Background

The average oil recovery from carbonate reservoirs is less than 30% worldwide, which is far less than that from sandstones. The carbonates are usually highly fractured, and approximately 90% of the reservoirs are neutral to oil-wet, which prohibits oil displacement by water injection. Half of the world’s proven oil reserves are present in carbonates, and, therefore, the enhanced-oil-recovery (EOR) potential is very high.

Chalk is the dominant oil-containing carbonate formation in the North Sea, and it is characterized as fragmentary parts of calcite skeletons produced by plankton algae known as coccolithophorids. The properties of the biogenic sediments were maintained because of an early invasion of oil, which stopped the further recrystallization of the material into limestone or dolomite. Because of the soft nature of the biogenic sediment, the reservoirs are usually naturally fractured. The permeability of the matrix blocks is low, approximately 2 md, and the porosity can be very high, nearly 50%. The reservoir temperatures are high, in the range of 90 to 130°C. During the primary-production phase purely by pressure depletion of the Ekofisk field, compaction and subsidence occurred, which contributed to 40% of the drive mechanism. Water injection in the Ekofisk field started in 1987 in order to give pressure support and prevent compaction. Injection of seawater was a great success, and the oil recovery is now estimated to be approximately 50%. The seawater appeared to imbibe into the chalk matrix efficiently, even though the wetting conditions vary from moderately water-wet in the Tor formation to slightly oil-wet in the upper Ekofisk formation (Thomas et al. 1987). It was also observed that the compaction did not stop in the waterflooded areas even though the reservoir was repressurized to initial conditions. Thus, seawater appeared to have a water-weakening effect on the chalk.

There is no doubt that the seawater has a special interaction with chalk at high temperatures, which has an impact on oil recovery and rock mechanics. In the present paper, we will provide a short summary of our studies during the last 12 years to determine the chemical mechanism behind this important interaction.

© 2008. Society of Petroleum Engineers

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History

• Original manuscript received: 26 June 2007
• Meeting paper published: 4 December 2007
• Revised manuscript received: 3 January 2008
• Manuscript approved: 6 January 2008
• Version of record: 20 August 2008