General Solution for Tidal Behavior in Confined and Semiconfined Aquifers Considering Skin and Wellbore Storage Effects
Xuhua Gao, Kozo Sato, Roland Nicholas Horne
ESSOAr, February 2020
Tidal analysis provides a cost-effective way of estimating aquifer properties. Tidal response models that link aquifer properties with tidal signal characteristics, such as phase and amplitude, have been established in previous studies, but none of the previous models incorporate the skin effect. It is found in this study that the skin effect and the wellbore storage effect can have significant influence on the results of tidal analysis and should be included in tidal response models. New models are proposed with skin and wellbore storage effects fully incorporated, so that aquifer information can be assessed more accurately based on tidal analysis. The models can be applied to confined aquifers with only horizontal flow or semiconfined aquifers with both horizontal flow and vertical flow. For confined aquifers, the new model indicates that positive skin leads to larger phase lag between the tidal response the the theoretical tide, and negative skin can reduce the phase lag or even cause a phase advance. For semiconfined aquifers, both the skin effect and the vertical flow affect the phase difference between the tidal response and the theoretical tide, and with the proposed model, contribution from these two sources can be separated and analyzed independently, making it feasible to evaluate semiconfined aquifer properties considering both factors. Increasing wellbore storage causes larger phase lag or smaller phase advance for both types of aquifers. Real-world examples for confined and semiconfined aquifers are analyzed respectively to demonstrate practical applications of the proposed models.
Time-lapse analysis of pressure transients due to ocean tides for estimating CO2 saturation changes
Sato, K., and Horne, R.N.
International Journal of Greenhouse Gas Control Volume 78, November 2018, Pages 160-167
This study proposed and examined a practical technique for analysing CO2 storage responses using offshore pressure transients affected by the ocean tide. The gravitational attractions of the solar-system bodies cause ocean tides, and the pore pressure exhibits diurnal and semidiurnal fluctuations in response to such tidal phenomena. The pressure-fluctuation amplitude is related to the loading efficiency, which is a function of reservoir elastic properties and fluid saturations. Therefore, the loading efficiency can be used to estimate the in situ pore compressibility and the CO2 saturation. Applying the tidal-signal analysis in a time-lapse manner, one may see temporal changes in CO2 saturation and consequently describe the dynamic behavior of sequestered CO2. In the analysis at the offshore CO2 storage site in Tomakomai, Japan, a temporal decrease in CO2 saturation was detected during the shut-in period, which is caused primarily by CO2 migration away from the well. The proposed methodology essentially requires only continuous pressure data, which are routinely available during CO2 storage operations, and thus, can be a cost-effective and labour-saving monitoring technique.