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The project was initiated by The National IOR Centre of Norway within the University of Stavanger and it aligns with the overall goal of the centre to improve oil recovery on the Norwegian Continental Shelf. It contributes to several of the centre’s tasks, such as development of IOR methods, IOR mechanisms, fluid flow simulations, and evaluation of economic potential and environmental impact. The chalk reservoirs are among the most prolific hydrocarbon fields in the North Sea, Ekofisk field alone accounting for approximately 10% of the produced net oil equivalents on the Norwegian Continental Shelf. Primary and secondary oil recovery methods induce a complex set of alteration in the reservoir properties. With more and more mature fields and reports of descending production trend, developing and implementing EOR techniques tailored to the specifics of each field is timelier than ever. Besides the mechanical changes due to fluctuations in the stress state of the reservoir during production, chalk interacts with the injected fluids, leading to chemical reactions that further affect its mineralogy, structure, and strength. It is therefore essential to understand the particularities of the reservoir rocks and the interplay of all these processes and their effect on fluid flow. The main purpose of this project is to study the relationship between the stress state, rock-fluid interactions on one side, and the permeability of compacting chalk cores on the other side, by investigating pore- and core-scale effects of stress state, temperature, and reactive fluid flow on the permeability of fractured chalk – an essential parameter in improved oil recovery, yet still insufficiently understood. A complementary objective is to describe the mineralogy and geochemistry of North Sea reservoir and non-reservoir chalk, as well as to model and predict permeability evolution in chalk under relevant North Sea reservoir conditions based on the interpretation of the experimental data. The approach of the project is therefore threefold: - factual: in-depth mineralogical and chemical characterization of the North Sea chalk and an evaluation of the mineralogical and geochemical impact of hydrocarbons and EOR fluids on reservoir chalk - experimental: geomechanical tests investigating permeability evolution in shear failing outcrop chalk exposed to thermochemical influence and the geomechanical and chemical response of North Sea reservoir chalk to reactive fluid flow - theoretical: simulation of permeability evolution investigating the role of fracture width, flooding rate and brine concentration on the mechanical and chemical compaction in water-wet fractured chalk and relative quantification of the mechanical and chemical contribution to permeability evolution The studies have been conducted on oil-bearing reservoir chalk from Ekofisk and Eldfisk fields in the North Sea, as well as on outcrop chalks saturated with various brines. The main finds are disseminated in four papers. A rare and coveted sample set consisting of chalk cores from several wells in the North Sea provided a unique opportunity to obtain mineralogical and geochemical data describing reservoir and nonreservoir chalks, as well as reservoir chalk before and after flooding with EOR fluids. Paper I (see subsection 4.1) shows that the mineralogical composition of the North Sea chalk is typical for regional marine deposits of the Upper Cretaceous, consisting of micritic carbonate matrix, microfossils and diverse authigenic and detrital minerals. Noncarbonate mineral phases consist mostly of quartz, illite, smectite, kaolinite. Carbon isotope ratios align well with primary trends for Upper Cretaceous stages; oxygen isotopes on the other hand, are far from primary, considerable deviation from the Upper Cretaceous trends is seen in all samples. Burial diagenesis, paleotemperature fluctuations, or meteoric water input with different thermal gradient can plausibly explain the disturbed oxygen isotopes. The presence of hydrocarbon fluids was most likely not the cause of the negative δ18O, as both the reservoir and the non-reservoir successions show similar oxygen isotope pattern. Dolomite is detected only in the reservoir cores, but it is interpreted to be diagenetic rather than anthropologic (i.e., due to EOR flooding) as it is present in both flooded and unflooded samples. Paper II (subsection 4.3) presents the results of geomechanical tests on reservoir chalk from the North Sea that facilitate a valuable and needed comparison between the response of reservoir chalk and outcrop chalk to mechanical and chemical compaction. Synthetic seawater (SSW) and simplified seawater (0.219 M MgCl2) injection through reservoir cores induced increased strain rates in the reservoir chalk, linked to retention of magnesium and production of calcium during flooding, which results in precipitation of secondary magnesium-bearing minerals or anhydrite. Adding calcium to the MgCl2 aqueous solution effectively reduced the creep strain rate, likely because the dissolution of primary calcite is inhibited. Most interestingly, the results from this study match very well the results gained from previous studies on water-wet outcrop chalks, thus validating decades of chalk research on outcrop chalk regarding brine-chalk chemical interactions and geomechanical behaviour. The experimental approach also includes a set of geomechanical tests on shear fractured outcrop chalk cores from the Cretaceous Niobrara Formation (Utah and Kansas, USA), often denoted “Kansas chalk”, exposed to deviatoric stress cycles and thermochemical influence, monitoring the effects of the test parameters on the permeability evolution of the cores. The results are presented in Paper III (see subsection 4.2). Deviatoric stress state together with low confining pressure induced shear fracturing at a steep angle (over 70°), close to flooding direction, corresponding to a simultaneous permeability increase. This was the main permeability-altering event. Subsequent deviatoric loadings had little effect on permeability in all tests, regardless of injecting brine (aqueous solutions of NaCl, Na2SO4, and synthetic seawater) and test temperature (50° C and 130° C). During creep, permeability generally declined slightly, or remained unchanged. The results indicate that once chalk has fractured under deviatoric stress conditions, the effective permeability is little responsive to compaction cycles and reactive flow, both at high and low temperature. Despite fracturing and exposure to different stress states, temperature and brine conditions, the end core permeability seems to remain within the same order of magnitude as the original value, ranging between the initial and double of the initial value. This indicates a notable insensitivity to changes in reservoir conditions. Theoretical predictions of permeability evolution in fractured and intact cores were obtained from simulations of reactive fluid flow through outcrop chalk. The model was calibrated to match nine previous geomechanical tests on intact chalk cores from Aalborg and Liège and four tests on pre-fractured Kansas and Mons chalk cores. Paper IV addresses the main finds from this study (see subsection 4.4). The model quantifies the permeability loss related to mechanical and chemical compaction. The simulations predict that porosity and axial strain are sensitive to fracture width, reactive brine concentration and injecting rate. However, while the permeability trend in time is affected by the brine composition, it remains insensitive to injection rate and fracture width. The results also underline that fracture permeability, in another order of magnitude compared to the matrix permeability, dominates the effective permeability evolution and that the mineral alterations and any significant permeability loss in the matrix are strongest at the core inlet. |
