Seawater in Chalk: An EOR and Compaction Fluid

Abstract

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.

Wettability alteration in carbonates during “Smart Waterflood”: Underlying mechanisms and the effect of individual ions

“…Austad research group proposed the Multicomponent Ionic Exchange (MIE) theory for carbonate rocks [16][17][18][19][20][21][22][23]. The main role proposed for sulfate which adsorbs onto the positively charged calcite surface and reduces the positive surface charge.…”

Three-Phase Flow Modelling Using Pore-Scale Capillary Pressures and Relative Permeabilities for Mixed-Wet Media at the Continuum-Scale

“…Many of the world's remaining oil reserves are located in formations, for example, mixedwet to oil-wet fractured carbonate reservoirs, that require enhanced oil recovery techniques (EOR), such as water-alternating-gas (WAG) (Christensen et al 2001), chemical injection (Austad et al 2008) and thermal recovery (Al-Hadhrami and Blunt 2001). Such EOR techniques can lead to large three-phase flow regions and modify the wettability of the porous media locally.…”

Microscopic Scale Study of Individual Water Ion Interactions at Complex Crude Oil-Water Interface: A New SmartWater Flood Recovery Mechanism

“…It was also demonstrated that seawater can favorably affect oil/brine/rock interactions to alter wettability and eventually improve waterflood recoveries in chalk (pure calcium carbonate rock) based on spontaneous imbibition tests (Austad et al, 2005;Strand et al, 2006;Zhang and Austad, 2006;Zhang et al, 2007;Austad et al, 2008). These studies attributed the resulting improved oil recoveries to wettability alteration caused by the interplay of potential determining ions present in the seawater, mainly (SO 4 ) 2-, Ca 2ϩ , and Mg 2ϩ .…”