Seawater in Chalk: An EOR and Compaction Fluid

Abstract

Abstract North Sea chalk reservoirs are characterized as pure biogenic, natural fractured, low matrix permeability, and very high porosity, 30–45%. The reservoir temperature is usually high, >90 oC, and the wetting conditions appeared to be moderate 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 appeared to improve 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 water flooded areas even though the reservoir was repressurized by the injected seawater. The phenomenon has been described as water weakening of chalk, and production costs are increased due to loss of wells and substitution of platforms. The paper will give an overview of the chemical aspects of the interaction between seawater and the chalk. Surface active components in seawater, like Ca2+, Mg2+, and SO42-, will play an important role both regarding wettability modification and rock mechanics. Chemical models describing the wettabilty alteration and enhanced water weakening of chalk by seawater are suggested and will be presented. Background The average oil recovery from carbonate reservoirs is less than 30% world wide, which is fare less that from sandstones. The carbonates are usually highly fractured, and about 90% of the reservoirs are neutral to oil-wet, which prohibits oil displacement by water injection. Half of the worlds proven oil reserves are present in carbonates, and the EOR-potential is therefore very high. Chalk is the dominant oil-containing carbonate formation in The North Sea, which is characterized as fragmentary parts of calcite skeletons produced by plankton algaes, known as coccolithophorids. The properties of the biogenic sediments were maintained due to an early invasion oil, which stopped further recrystallisation of the material into limestone or dolomite. Due to the soft nature of the biogenic sediment, the reservoirs are usually natural fractured. The permeability of the matrix blocks is low, about 2 mD, and the porosity can be very high, nearly 50%. The reservoir temperatures are high, in the range of 90–130 oC. During the primary production phase by pure pressure depletion of the Ekofisk field, compaction and subsidence took place, 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 about 50%. The seawater appeared to imbibe into the chalk matrix efficiently, even though the wetting conditions vary from moderate water-wet in the Tor formation to slightly oil-wet in the upper Ekofisk formation.1 It was also observed that the compaction did not stop in the water flooded areas even though the reservoir was repressurized to initial conditions. Thus, seawater appeared to have a water weakening effect on the chalk. It is no doubt that the seawater has a special interaction with chalk at high temperatures, which has impact on oil recovery and rock mechanics. In the present paper, we will give a short summary of our studies during the last 12 years to determine the chemical mechanism behind this important interaction.