Presenter
Carl Fredrik Gyllenhammar from CaMa GeoScience AS
Abstract
Introduction
Ever since Hawking postulated in 1963 that formation water salinity increases with depth, no one has opposed him. His conclusion is based on brine analysis from Union Count SE Arkansas. This presentation is explaining why the salinity offshore Norway often show decreasing salinity with depth as well as large variability; from 8675 ppm to 340000 ppm based on laboratory analysis og good water samples from 6305/5-1 drilled by NH in 1998 at Orman Lange and 1/6-6 operated by Shell in 1993.
Procedure
Salinity maps were initially made to search for Missed-Pay. Salinity trends to challenge the Rw (salinity) used by the operator to suggest a well was dry. Today, it is very important for CCS. The salinity data come from two sources.
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Deterministic log analysis using Interactive Petrophysics to calculate salinity in dry reservoirs. Interactive Petrophysics calculates the Rw Apparent that is the Rw that makes Sw=1. There are just over 1950 exploration wells. In average 4 – 5 reservoir zones in each well. Some wells have no reservoir zones, summing up to about 5000 salinity data points.
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Laboratory analysis of formation water. Produced water from DST is best. Second is MDT water samples from wells with oil-based-mud. An additional 500 data points. RFT samples from wells with water-based-mud are not used in this study.
Results/Observations
Based on the large variability in the laboratory water samples a large lateral variability was expected. But contrary to Hawkings postulations the salinity often deceases with depth offshore Norway. In the North Sea, Norwegian sea as well as the Barents Sea. With salinities often less than 20000 ppm, much less than ocean water at 3500 0ppm, I ask the question; Where is the fresh water coming from?
Morton and Land, 1987 suggested that the low salinity waters come from clay compaction. This has also been suggested by Bjørlykke in 1999 (and by Personal communication with him). Bjørlykke also oppose the idee that basement formation waters can migrate up into the overlying sediments as shown by Walter in 2016
There is no doubt that some of the low salinity waters come from clay compaction. But is that enough to explain the large quantities of fresh water needed? Interestingly, Morton and Land do point out in the same paper that the volume of low salinity water needed exceeds earlier estimated low salinity water expelled from clay compaction and illite to smectite transformation. They suggest that either the earlier calculations are in error, or the low salinity water may be sourced from underlying metamorphic units. In principle a fractured underlying basement.
High-precision lead (Pb) isotope data from many crude oil samples and black shales suggest that the oil must have been carried from the source rock to the reservoir following water migrating up from the underlying basement rock. The source of Pb can only be the underlying basement.
Lead concentrations in formation water are rarely measured. However, well 34/11-1 produced water from Jurassic sandstone at 4125 m depth with lead concentrations of 0.2–0.35 µg/L—approximately 1000 times higher than seawater.
About 100 exploration wells have penetrated basement. On the Utsira High the age of the sediment directly overlapping the basement range in age from 95 to 250 Ma, initially filled predominantly with meteoric water.
Conclusion
There is compelling evidence to suggest that the low salinity water is sourced from the underlying basement. Then mixed with high salinity water having been in contact with salt deposits. This could explain the large salinity variation laterally and vertically offshore Norway.
This suggests that the oil migration follow salinity and pressure differentials rather than paleo contour maps.
Supporting document (download from Google Drive)
Biography
Fred (Carl Fredrik) Gyllenhammar is a geoscientist consulting in the oil and gas industry. He started his academic career studying math, physics and chemistry at University of Grenoble in France in 1975. Ending up with an MSc in Applied Geophysics from University in Oslo. He started in Conoco in 1985 as a Wellsite Geologist. After 13 years he resigned and joined industry research project GeoPoP, at University in Durham. Submitting his PhD in 2001; “A critical review of currently available pore pressure methods and their input parameters. – Glaciations and compaction of the North Sea sediments.” Published as a textbook and used worldwide, including Schlumberger’s office in Houston. Joined BP research department in Sunbury in 2001 as a petrophysicist.
The last 30 years he has worked with prospect evaluation, doing the petrophysical as well as the seismic interpretation. Exploring for oil and gas in the North Sea (UK, Norway and Denmark), Egypt, Morocco, Tunisia, Azerbaijan and Kazakhstan.
He has patented a new well plug material to replace cement.