Abstract
Seawater injection for Enhanced Oil Recovery (EOR) purposes can increase the hydrocarbon recovery factor in carbonate reservoirs but are also responsible for weakening their mechanical strength. This study investigates the geomechanical behavior of oil-bearing reservoir chalk subject to reactive flow at reservoir conditions. The obtained results are compared with previous results from experiments on water-saturated outcrop chalks.Two unwashed oil-bearing reservoir chalk cores from the Eldfisk Field in the North Sea are mechanically tested in a triaxial cell. The cores are saturated with NaCl brine prior to testing, in addition to residual oil. The cores’ axial and radial deformations were monitored during hydrostatic stress loading and creep under constant 50 MPa stress, at 130 °C. During the test, the cores were flooded with 0.657 M NaCl, 0.219 M MgCl2, 0.219 M MgCl2 + 0.130 M CaCl2 and synthetic seawater (SSW). The average creep strain increased from 0.03% to 0.04% per day radially and from 0.03% to 0.06% axially after changing from the NaCl brine to SSW brine. Flooding MgCl2 brine after NaCl increased the average compaction rate from 0.05% to 0.09% per day radially and from 0.04% to 0.07% per day axially. Adding CaCl2 to the MgCl2 brine reduced the average compaction rate from 0.05% per day during MgCl2 injection to 0.02% per day both radially and axially, comparable to reports from outcrop chalk experiments. The brine composition-dependent creep compaction in the core flooding experiments was explained by dissolution of primary calcite, confirmed by Ion Chromatography, and precipitation of secondary Mg-bearing minerals, seen by Scanning Electron Microscopy (SEM) analytics.Generally, the aforementioned results describing geomechanical behaviors of oil-bearing reservoir chalk cores under hydrostatic stress and thermochemical influence in this study are comparable to those from previous studies on outcrop chalk, thus supporting many years of laboratory research as applicable in the reservoir chalk context.
Highlights
Developing Improved Oil Recovery (IOR) techniques is central in the efforts for sustainability and resource optimization on the Norwegian Continental Shelf (NCS)
Hydrostatic loading and creep compaction with continuous brine injection was performed on two Eldfisk reservoir chalk cores
Thereafter, the geomechanical behavior of Eldfisk cores have been compared with previous results obtained from onshore outcrop chalks
Summary
Developing Improved Oil Recovery (IOR) techniques is central in the efforts for sustainability and resource optimization on the Norwegian Continental Shelf (NCS). Implementing the seawater injection at Ekofisk Field in 1987 contributed to reservoir re-pressurization and an increase in oil pro duction rates. Despite re-pressurization of the reservoir, high compaction rates continued in certain areas of the field (Schroeder et al, 1998; Hermansen et al, 2000), indicating that effective stress alone was not the only compaction driving mechanism. Studies demonstrate a two-step water effect on chalk: an abrupt, instantaneous weakening occurring at the first water-chalk contact, followed by a creep-like deformation at a lower, stable rate (Schroeder et al, 1998; Korsnes et al, 2006). The initial, dramatic water weakening step ex plains much of the continued compaction of Ekofisk field, but the focus of this study is rather on the role of water chemistry in water weakening of chalk. The initial, dramatic water weakening step ex plains much of the continued compaction of Ekofisk field, but the focus of this study is rather on the role of water chemistry in water weakening of chalk. Korsnes et al (2006) demonstrated that while water weakening occurs regardless of the water composition, the magnitude of induced weakening was more pronounced when injecting seawater than in case of distilled water
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