Kazbulat Shogenov

Reservoir quality and petrophysical properties of cambrian sandstones and their changes during the experimental modelling of CO2 storage in the Baltic basin.

The objectives of this study were (1) to review current recommendations on storage reservoirs and classify their quality using experimental data of sandstones of the Deimena Formation of Cambrian Series 3, (2) to determine how the possible CO2 geological storage (CGS) in the Deimena Formation sandstones affects their properties and reservoir quality and (3) to apply the proposed classification to the storage reservoirs and their changes during CGS in the Baltic Basin. The new classification of the reservoir quality of rocks for CGS in terms of gas permeability and porosity was proposed for the sandstones of the Deimena Formation covered by Lower Ordovician clayey and carbonate cap rocks in the Baltic sedimentary basin. Based on permeability the sandstones were divided into four groups showing their practical usability for CGS (‘very appropriate’, ‘appropriate’, ‘cautionary’ and ‘not appropriate’). According to porosity, eight reservoir quality classes were distinguished within these groups. The petrophysical, geochemical and mineralogical parameters of the sandstones from the onshore South Kandava and offshore E6 structures in Latvia and the E7 structure in Lithuania were studied before and after the CO2 injection-like alteration experiment. The greatest changes in the composition and properties were determined in the carbonate-cemented sandstones from the uppermost part of the South Kandava onshore structure. Partial dissolution of pore-filling carbonate cement (ankerite and calcite) and displacement of clay cement blocking pores caused significant increase in the effective porosity of the samples, drastic increase in their permeability and decrease in grain and bulk density, P- and S-wave velocity, and weight of the dry samples. As a result of these alterations, carbonate-cemented sandstones of initially ‘very low’ reservoir quality (class VIII), ‘not appropriate’ for CGS, acquired an ‘appropriate’ for CGS ‘moderate’ quality (class IV) or ‘very appropriate’ ‘high-2’ reservoir quality (class II). The permeability of the clay-cemented sandstones of ‘very low’ reservoir quality class VIII from the lower part of the E7 reservoir was not improved. Only minor changes during the alteration experiment in the offshore pure quartz sandstones from the E6 and E7 structures caused slight variations in their properties. The initial reservoir quality of these sandstones (‘high-1’ and ‘good’, classes I and III, respectively, in the E6 structure, and ‘cautionary-2’, class VI in the E7 structure) was mainly preserved. The reservoir sandstones of the Deimena Formation in the South Kandava structure had an average porosity of 21%, identical to the porosity of rocks in the E6 structure, but twice higher average permeability, 300 and 150 mD, respectively. The estimated good reservoir quality of these sandstones was assessed as ‘appropriate’ for CGS. The reservoir quality of the sandstones from the E7 offshore structure, estimated as ‘cautionary-2’ (average porosity 12% and permeability 40 mD), was lowest among the studied structures and was assessed as ‘cautionary’ for CGS. Petrophysical alteration of sandstones induced by laboratory-simulated CGS was studied for the first time in the Baltic Basin. The obtained results are important for understanding the physical processes that may occur during CO2 storage in the Baltic onshore and offshore structures.

Experimental modeling of CO2‐fluid‐rock interaction: The evolution of the composition and properties of host rocks in the Baltic Region

The objective of this study was to determine the influence of the possible CO2 geological storage in the Baltic Region on the composition and properties of host rocks to support more reliable petrophysical and geophysical models of CO2 plume. The geochemical, mineralogical, and petrophysical evolution of reservoir sandstones of Cambrian Series 3 Deimena Formation and transitional clayey carbonate caprocks of Lower Ordovician Zebre Formation from two offshore structures in Latvia and Lithuania and two onshore structures in Latvia, induced by laboratory-simulated CO2 geological storage, was studied for the first time in the Baltic Region. The geochemical, mineralogical, and petrophysical parameters were measured in 15 rock samples, before and after the alteration experiment. The diagenetic alterations of reservoir rocks were represented by carbonate cementation in the top of the onshore South Kandava structure, and quartz cementation and compaction, reducing the reservoir quality, in the deepest offshore E7 structure in Lithuania. The shallowest E6 structure offshore Latvia was least affected by diagenetic processes and had the best reservoir quality that was mainly preserved during the experiment. Carbonate cement was represented by calcite and ankerite in the transitional reservoir sandstones of very low initial permeability in the upper part of the South Kandava structure. Its dissolution caused a significant increase in the effective porosity and permeability of sandstones, a decrease in the weight of samples, bulk and matrix density, and P and S wave velocities, demonstrating short-term dissolution processes. Only slight geochemical changes occurred during the experiment in offshore reservoir sandstones. Minor dissolution of carbonate and clay cements, feldspar and some accessory minerals, and possible minor precipitation of pore-filling secondary minerals associated with slight variations in rock properties, demonstrating both short-term and long-term processes, were suggested. As a novelty, this research shows the relationship between diagenetic alterations of the Cambrian Series 3 Deimena Formation reservoir sandstones and their changes caused by the CO2 injection-like experiment.

Petrophysical and numerical seismic modelling of CO2 geological storage in the E6 structure, Baltic Sea, offshore Latvia

Time-lapse numerical seismic modelling based on rock physics studies was for the first time applied to analyse the feasibility of CO2 storage monitoring in the largest Latvian offshore geological structure E6 in the Baltic Sea. The novelty of this approach was the coupling of the chemically induced petrophysical alteration effect of CO2-hosting rocks measured in laboratory with time-lapse numerical seismic modelling. Synthetic seismograms were computed for the E6 structure, where the sandstone reservoir of the Deimena Formation of Cambrian Series 3 (earlier Middle Cambrian) was saturated with different concentrations of CO2. The synthetic seismograms obtained after CO2 injection were compared with the baseline. The following four scenarios were considered: (1) a uniform model without the alteration effect; (2) a uniform model with the alteration effect; (3) a plume model without the alteration effect; and (4) a plume model with the alteration effect. The presence of CO2 in the reservoir layers can be detected by direct comparison and interpretation of plane-wave synthetic seismic sections, and is clearly observed when one displays the difference between the baseline and post-CO2 injection synthetics. The normalized root-mean-square imaging techniques also clearly highlight the time-lapse differences between the baseline and post-injection seismic data. The laboratory-conducted alteration of the petrophysical properties of the reservoir had a strong influence on the reflected signals in the seismic sections. The greatest difference was revealed on seismic sections with 1% CO2 saturation, increasing the detectability of the stored CO2. The difference decreased with an increase in CO2 content. The saturation of CO2 could be qualitatively estimated up to a value of 5%. Higher saturation produced a strong signal in the repeatability metrics but the seismic velocity varied so slightly with the increasing gas content that the estimation was challenging. A time shift or push-down of the reflectors below the CO2 storage area was observed for all scenarios. According to changes in the amplitude and two-way travel times in the presence of CO2, reflection seismics could detect CO2 injected into the deep aquifer formations even with low CO2 saturation values. Our data showed the effectiveness of the implemented time-lapse rock physics and seismic methods in the monitoring of the CO2 plume evolution and migration in the E6 offshore oil-bearing structure. The new results obtained could be applied to other prospective structures in the Baltic region.