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Impact of Initial Wettability and Injection Brine Chemistry on Mechanical Behaviour of Kansas Chalk


The injection of seawater-like brines alters stiffness, strength and time-dependent deformation rates for water-saturated chalks. This study deals with the mechanical effects and oil production upon brine injection through wettability-altered samples. The results from two test programs are presented: (a) ‘Wettability determination program’ and (b) ‘triaxial test program’. Kansas chalk samples were saturated by a mixture of oil and water and aged over time at 90 °C. The wettability index of the altered samples was estimated using chromatographic separation tests by co-injecting sulphate ions that adsorb on the water-wet mineral surfaces and non-affine tracer. A good repeatability was observed. In the triaxial test program, unaged water-wet and aged mixed-wet samples were hydrostatically loaded to 1.5 times yield stress so stiffness and strength could be determined. The samples were kept at the same stress level over time to monitor the volumetric creep. After a stagnant flow period of 15 days, MgCl2 brine and seawater were flushed through the samples so the oil production and ion concentration of the effluent water could be obtained. The combined observations of the bulk volume, oil volume and estimated solid volume (from effluent analyses) enabled us to calculate pore volume and thereby oil saturation with time. The mixed-wet samples were found to be stiffer and stronger than the water-wet samples, and when the stress was kept at 1.5 times yield the creep curves overlapped. During the flow-through period, the changes in ion composition are insensitive to the presence of oil, and ongoing water weakening for mixed-wet samples is the same as in the water-wet samples. Further, we found that oil was only produced during the first 2–3 pore volumes (PVs) injected. Afterwards, no oil was produced even though the chemical reactions took place and pore volume reduced.

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The authors acknowledge the Research Council of Norway and the industry partners, ConocoPhillips Skandinavia AS, Aker BP ASA, Eni Norge AS, Equinor ASA, Neptune Energy Norge AS, Lundin Norway AS, Halliburton AS, Schlumberger Norge AS, Wintershall Norge AS, and DEA Norge AS, of The National IOR Centre of Norway for support.

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Experimental Uncertainty

The experimental uncertainties that are linked to the key experimental findings and corresponding conclusions, and an evaluation of magnitude of the error are given below:

  1. a.

    Measurement of brine volumes to estimate \( S_{wi} \) produced during the wettability alteration procedure: A volumetric burette was used to collect the brine on the downstream side of the Hassler cell. The accuracy of this burette is in the order of 0.1 ml.

  2. b.

    Dilution of samples during IC analysis: A given variation of 2% corresponding to a variation of 5 × 10−4 mol/l is expected from the ion chromatography.

  3. c.

    Measurement of oil volumes through the separator: During the first 15 days of bypass open and with stagnant fluids inside the mixed-wet cores, we compactify the grain by pore volume reduction, which results in expulsion of oil and water from the pores. The dead volume of the outlet tubing to the separator was less than 1 ml. If some oil had produced during the first 15 days of compaction into this tubing, this oil should have been produced immediately when the brine flooding through the cores started on the 16th day, but in all cases, it took more than this dead volume to be flooded into the core to get the first drop of oil out into the separator. Hence, it was concluded that no oil was produced during the first 15 days of compaction.

  4. d.

    Area estimate from chromatographic separation test: The errors linked to the estimation of areas between thiocyanate and sulphate for the reference water-wet cores and the mixed-wet cores were found to be 0.03 × 10−3 PV/g and 0.02 × 10−3 PV/g, respectively.

  5. e.

    Pycnometry: The solid volume measurements using pycnometry were done twice for a few selected samples to check for the repeatability. The standard deviation for the solid volume measurements was estimated by the Gas Pycnometer itself and was found to be in the range of ± 0.002 cm3 to ± 0.009 cm3.

  6. f.

    Specific surface area: SSA determination was carried out two times on most of the samples to check for the repeatability of the results. The repeatability was found to be very good with a variation of ± 0.05 m2/g.

  7. g.

    Porosity determination by saturation: The dry mass and the wet mass for all samples before and after test were determined using a weighing machine. The error linked to that machine is in the order of ± 0.002 g.

  8. h.

    Bulk volume estimate as a function of time: The lengths and diameters of the cores before and after test were measured using a Digital Caliper and the error associated with this caliper is in the range of 0.01 mm to 0.02 mm. Further on, the radial strain was measured only at the middle of the core, and not along the whole length. Hence a factor X was introduced to relate axial strain to volumetric strain and was assumed to be constant throughout the duration of the test. The error linked to the calculation of X was estimated to be ± 0.08, which means that the error linked to the estimation of bulk volume with time is also the same, i.e. ± 0.08 cm3.

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Sachdeva, J.S., Nermoen, A., Korsnes, R.I. et al. Impact of Initial Wettability and Injection Brine Chemistry on Mechanical Behaviour of Kansas Chalk. Transp Porous Med 128, 755–795 (2019).

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  • Improved oil recovery
  • Water flooding
  • Chalk compaction
  • Wettability
  • Geomechanics
  • Oil production measurements