Seawater in chalk: An EOR and compaction fluid

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 degrees C, and the wetting conditions appear to be ability 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 all example (estimated oil recovery is flow approaching 50%). Seawater improves the water wetness of chalk. which increases the oil recovery by Spontaneous inhibition and viscous displacement. During the primary production phase by pressure depletion, extensive compaction was observed and, at that time, it was regarded as fit 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 all overview of the chemical aspects of the interaction between Seawater and the chalk. Surface active components in seawater. Such as Ca2+, ML2+, 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 Lis a tertiary-oil-recovery technique. Chemical models describing, the wettabilty alteration and enhanced water weakening of chalk by seawater are suggested and presented.