Seawater in Chalk: An EOR and Compaction Fluid
- Tor Austad (U. of Stavanger) | Skule Strand (U. of Stavanger) | Merete V. Madland (U. of Stavanger) | Tina Puntervold (U. of Stavanger) | Reidar I. Korsnes (U. of Stavanger)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2008
- Document Type
- Journal Paper
- 648 - 654
- 2008. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 5.3.1 Flow in Porous Media, 4.1.2 Separation and Treating, 4.3.1 Hydrates, 6.5.2 Water use, produced water discharge and disposal, 5.4.1 Waterflooding, 5.3.4 Integration of geomechanics in models, 4.1.5 Processing Equipment, 5.8.7 Carbonate Reservoir, 5.2 Reservoir Fluid Dynamics
- 8 in the last 30 days
- 1,058 since 2007
- Show more detail
- View rights & permissions
|SPE Member Price:||USD 10.00|
|SPE Non-Member Price:||USD 30.00|
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 SO4 2-, 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.
The average oil recovery from carbonate reservoirs is less than 30% worldwide, which is far less than that from sandstones. The carbonates are usually highly fractured, and approximately 90% of the reservoirs are neutral to oil-wet, which prohibits oil displacement by water injection. Half of the world's proven oil reserves are present in carbonates, and, therefore, the enhanced-oil-recovery (EOR) potential is very high.
Chalk is the dominant oil-containing carbonate formation in the North Sea, and it is characterized as fragmentary parts of calcite skeletons produced by plankton algae known as coccolithophorids. The properties of the biogenic sediments were maintained because of an early invasion of oil, which stopped the further recrystallization of the material into limestone or dolomite. Because of the soft nature of the biogenic sediment, the reservoirs are usually naturally fractured. The permeability of the matrix blocks is low, approximately 2 md, and the porosity can be very high, nearly 50%. The reservoir temperatures are high, in the range of 90 to 130°C. During the primary-production phase purely by pressure depletion of the Ekofisk field, compaction and subsidence occurred, 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 approximately 50%. The seawater appeared to imbibe into the chalk matrix efficiently, even though the wetting conditions vary from moderately water-wet in the Tor formation to slightly oil-wet in the upper Ekofisk formation (Thomas et al. 1987). It was also observed that the compaction did not stop in the waterflooded areas even though the reservoir was repressurized to initial conditions. Thus, seawater appeared to have a water-weakening effect on the chalk.
There is no doubt that the seawater has a special interaction with chalk at high temperatures, which has an impact on oil recovery and rock mechanics. In the present paper, we will provide a short summary of our studies during the last 12 years to determine the chemical mechanism behind this important interaction.
|File Size||1 MB||Number of Pages||7|
Carlberg, B.L. 1973. Solubilityof Calcium Sulfate in Brine. Paper SPE 4353 presented at the SPE OilfieldChemistry Symposium, Denver, 24-25 May. DOI: 10.2118/4353-MS.
Delage, P., Cui, Y.J., and Schroeder, C. 1996. Subsidence and CapillaryEffects in Chalks. Paper presented at Dans EUROCK 96, Prediction andPerformance on Rock Mechanics and Rock Engineering--ISRM InternationalSymposium, Torino, Italy, 1291-1298.
Heggheim, T., Madland, M.V., Risnes, R., and Austad, T. 2004. A Chemical InducedEnhanced Weakening of Chalk by Seawater. J. of Petroleum Science andEngineering 46 (3): 171-184. DOI: 10.1016/j.petrol.2004.12.001.
Hellmann, R., Gratier, J.P., and Renders P. 1996. Deformation of Chalk byPressure Solution. Proc., V.M. Goldschmidt Conference. Heidelberg,Germany, 1: 248.
Hirasaki, G. and Zhang, D.L. 2004. Surface Chemistry of Oil RecoveryFrom Fractured, Oil-Wet Carbonate Formations. SPEJ 9 (2):151-162. SPE-88365-PA. DOI: 10.2118/88365-PA.
Korsnes, R.I., Strand, S., Hoff, Ø., Pedersen, T., Madland, M.V., andAustad, T. 2006a. Does the Chemical Interaction Between Seawater and ChalkAffect the Mechanical Properties of Chalk. InEUROCK 2006 MultiphysicsCoupling and Long Term Behaviour in Rock Mechanics, eds. A. Van Cotthem, R.Charlier, J.-F. Thimus, and J.-P. Tshibanqu, 427-434. London: Taylor &Francis.
Korsnes, R.I., Madland, M.V., and Austad, T. 2006b. Impact of BrineComposition on the Mechanical Strength of Chalk at High Temperatures. InEUROCK 2006 Multiphysics Coupling and Long Term Behaviour in RockMechanics, ed. A. Van Cotthem, R. Charlier, J.-F. Thimus, and J.-P.Tshibanqu, 133-140. London: Taylor & Francis.
Korsnes, R.I., Madland, M.V., Austad, T., Haver, S., and Røsland, G. 2008.The effects oftemperature on the water weakening of chalk by seawater. J. of PetroleumScience and Engineering 60 (3-4): 183-193. DOI:10.1016/j.petrol.2007.06.001.
Newman, G.H. 1983. The Effectof Water Chemistry on the Laboratory Compression and PermeabilityCharacteristics of Some North Sea Chalks. JPT 35 (5):976-980. SPE-10203-PA. DOI: 10.2118/10203-PA.
Rao, D.N. 1996. WettabilityEffects in Thermal Recovery Operations. Paper SPE 35462 presented at theSPE/DOE Improved Oil Recovery Symposium, Tulsa, 21-24 April. DOI:10.2118/35462-MS.
Risnes, R., Madland, M.V., Hole, M., and Kwabiah, N.K. 2005. Water Weakening ofChalk--Mechanical Effects of Water-Glycol Mixtures. J. of PetroleumScience and Engineering 48 (1-2): 21-36. DOI:10.1016/j.petrol.2005.04.004.
Standnes, D.C. and Austad, T. 2000a. Wettability Alterationin Chalk 1. Preparation of Core Material and Oil Properties. J. ofPetroleum Science and Engineering 28 (3): 111-121. DOI:10.1016/S0920-4105(00)00083-8.
Standnes, D.C. and Austad, T. 2000b. Wettability Alterationin Chalk 2. Mechanism for Wettability Alteration From Oil-Wet to Water-WetUsing Surfactants. J. of Petroleum Science and Engineering 28(3): 123-143. DOI: 10.1016/S0920-4105(00)00084-X.
Strand, S., Standnes, D.C., and Austad, T. 2003. Spontaneous Imbibition of AqueousSurfactant Solutions Into Neutral to Oil-Wet Carbonate Cores: Effects of BrineSalinity and Composition. Energy Fuels 17 (5): 1133-1144.DOI: 10.1021/ef030051s.
Strand, S., Høgnesen, E.J., and Austad, T., 2006a. Wettability Alterationof Carbonates--Effects of Potential Determining Ions (Ca2+ andSO42-) and Temperature. Colloids and Surfaces A:Physicochemical and Engineering Aspects 275 (1-3): 1-10.DOI:10.1016/j.colsurfa.2005.10.061.
Strand, S., Standnes, D.C., and Austad, T. 2006b. New Wettability Test forChalk Based on Chromatographic Separation of SCN- andSO42-. J. of Petroleum Science and Engineering52 (1-4): 187-197. DOI:10.1016/j.petrol.2006.03.021.
Thomas, L.K., Dixon, T.N., Evans, C.E., and Vienot, M.E. 1987. Ekofisk Waterflood Pilot.JPT 39 (2): 221-232; Trans., AIME, 283.SPE-13120-PA. DOI: 10.2118/13120-PA.
Zhang, P. and Austad, T. 2006. Wettability and OilRecovery From Carbonates: Effects of Temperatures and Potential DeterminingIons. Colloids and Surfaces A: Physicochemical and EngineeringAspects 279 (1-3): 179-187. DOI:10.1016/j.colsurfa.2006.01.009.
Zhang, P, Tweheyo, M.T., and Austad, T. 2007. Wettability Alterationand Improved Oil Recovery by Spontaneous Imbibition of Seawater into Chalk:Impact of the Potential Determining Ions Ca2+, Mg2+, andSO42-. Colloids and Surfaces A: Physicochemicaland Engineering Aspects 301 (1-3): 199-208.DOI:10.1016/j.colsurfa.2006.12.058.