Advanced Waterflooding of North Sea Chalk Reservoirs

Jiasheng Hao*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Advanced, or smart waterflooding is a term denoting directed alteration of the ionic composition of the injected brine to achieve a better oil recovery. While the subject has been extensively researched during the past decades, the acting mechanisms of advanced waterflooding are not fully clarified, especially, for carbonate reservoirs. The purpose of this project is to summarize current research progress, further investigate promising mechanisms and explore previously uncovered aspects, especially, under the conditions of North Sea reservoirs.
The first part of the study is an updated and comprehensive literature review of smart waterflooding in carbonate reservoirs. The potential recovery mechanisms were classified into two groups: static and dynamic. The term “static mechanisms” refers to those that could happen without flow. Wettability alteration, double layer expansion, surface ion exchange, surface complexation, etc. belong to this group. On the other hand, the term “dynamic mechanisms” refers to those that occur during the flow, like fines relocation, flow diversion, emulsification, etc.. The numerous experimental works were categorized and the key information was extracted to identify the status of current research.
The second part of the project focuses on the kinetics of calcite dissolution and Ca-Mg exchange on chalk surfaces. These processes have been proposed by several researchers as the potential causes of additional recovery. The kinetics of such processes is essential to evaluate their significance in flow-through scenarios. Experimental works were carried out with commercial calcite powder and powders made from Stevns Klint outcrop and North Sea reservoir chalk. It was found that the dissolution of all three materials is similar. The equilibrium time is in the order of seconds and the equilibrium concentration is a few milligrams per liter. This means in the flooding experiments, calcite dissolution can only affect the inlet of the core, even after tens or hundreds of porous volume injected (PVI). Considering the amount of PVIs involved in common flooding experiments, it does not seem to be able to cause large additional recovery.
On the other hand, Ca-Mg exchange showed slow kinetics. The process kept going throughout the two weeks experimental period and did not stop at the end of the experiments. A two-layer model was proposed to describe the process and it fitted well with experimental data. The predicted exchange capacity of the calcite/chalk surfaces is in the order of 10-5 mol/m2/layer. These facts indicate that Ca-Mg exchange has only little impact on laboratory experiments. However, on the reservoir scale, where the range of time is much larger, it may be of more importance.
The third part of the thesis investigates the effect of flow diversion during smart waterflooding. Core flooding experiments were performed on sandstone and chalk reservoir cores. Based on the CT scanning images, homogeneous and heterogeneous cores were selected for both types of rock. It was found that the flow diversion mechanism can occur in both sandstone and chalk cores. However, the plugging agents that trigger this mechanism are different. The emulsions and precipitated fines worked in chalk, and most probably clay particles affected the recovery in the sandstone. The additional recovery was consistently observed from heterogeneous cores, which suggests that heterogeneity could amplify the effect of flow diversion. A simplified multi-layer model was employed to simulate the experiments. It was able to match well with experimental data, supporting the above-mentioned mechanisms.
The last part of the project studies the effect of chalk compaction on oil recovery. Core flooding experiments with simultaneous measurement of core length were conducted with Stevns Klint outcrop and North Sea reservoir cores. Again, heterogeneous and homogeneous cores were selected based on their
CT scanning images. The displacement of oil by water caused slight compaction in both types of rocks. This is consistent with the existing theory of water weakening of chalk. Subsequent injection of low salinity brines did not lead, neither to further compaction nor to additional recovery. Significant compaction caused by increasing of overburden pressure resulted in small additional recoveries in both heterogeneous and homogeneous outcrop cores and a considerable (compared to a homogeneous core) additional recovery from the heterogeneous reservoir core. A characteristic calculation showed that the additional production was correlated to the heterogeneity of the cores. The flow diversion mechanism was discussed in this context.
Overall, the current project shows that static mechanisms are not enough to explain the diverse experimental observations. Dynamic mechanisms that are related to the flow need further investigation.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages135
Publication statusPublished - 2020

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