The presentation “Variation of formation water compositions in the reservoir: An example from the Pierce Field, UK North Sea” will be given by Ross McCartney, Oilfield Water Services Limited.
Formation water compositions are widely used by a range of operational functions, particularly during field appraisal and field development planning. However, often only one formation water sample is collected from the aquifer/water-leg, and little is known about the variation of formation water compositions across the field. This can add uncertainty to field development decisions and plans, and can lead to unforeseen operational challenges and costs if the composition of the produced formation water differs from that expected.
In this presentation I will summarise why formation water compositions vary in sedimentary basins at the basin scale and identify those additional factors that can cause formation water compositions to vary at the field scale. I will also briefly summarise what data sets can be used to characterise the variation of formation water compositions across a field.
I will then describe how three of these data sets (appraisal well DST water samples, production well produced water samples, and strontium isotope ratio residual salt analyses undertaken on core samples from both the oil-leg and aquifer) were used to identify variation in formation water compositions in the reservoir of the Pierce Field, UK North Sea.
The residual salt analysis (RSA) data showed that formation water strontium isotope ratios vary both with depth and laterally across the Forties reservoir. This is predominantly caused by leakage of formation water with high strontium isotope ratios into the Forties Formation at its base.
Formation water resistivity and strontium isotope ratio compositions from the appraisal well DST samples and production well produced water samples were evaluated, and these demonstrated that aquifer formation water resistivity and strontium isotope ratios are negatively correlated at Pierce.
The similar characteristics of the strontium isotope ratios from both the oil-leg and aquifer suggested that this correlation would also be valid in the oil-leg. The correlation was therefore used to calculate the formation water resistivity at all those locations where strontium isotope ratio RSA data were available, and these results were then used to update the formation water resistivity model for the oil-leg.
The revised Rw model has improved understanding of oil saturation and pressures in the field. New water gradients were developed for RFT pressures, and this helped minimize the uncertainty in the FWL. Log water saturations were recalculated using the new Rw model and although this did not change the average saturation in the reservoir, it did change the saturation in the transition zone. Finally, a new saturation height function was calculated independently from core and compared with the newly calculated log saturations; these were a good match which supported the use of the strontium isotope ratio-Rw correlation in the oil-leg.
Ross McCartney is the principal consultant for, and owner of, Oilfield Water Services Limited. He has 35 years of experience as a consultant in the oil, nuclear, and geothermal industries. His expertise is in the acquisition and use of formation-water and produced-water analyses to understand well and reservoir conditions in deep subsurface environments. He has authored or co-authored more than 50 technical papers and holds a BSc degree in geology from the University of Nottingham and a PhD degree in hydrogeochemistry from the Camborne School of Mines. He has served as a technical reviewer for SPE Journals, and as a member and Chair of SPE Conference and Applied Technology Workshop committees.