Geothermal energy is expected to play a major role in decarbonizing heat supply in the Netherlands. Knowing the geomechanical behavior of a reservoir and seal can be crucial when engineering in the subsurface[5]. That is why this thesis focuses on characterizing the geomechanical behavior of the upper Delft Sandstone. Due to limited prior research and known lithological heterogeneity, this study seeks to quantify the constitutive behavior of the upper Delft Sandstone Member. Another goal is to measure the static moduli of the upper Delft Sandstone Member and to try to predict these moduli in parts of the reservoir where no cores are available.
From 87 m of core from well DEL-GT-01, 39 suitable samples were tested. Petro-physical measurements and classification into sandstones and shales were followed by UCS (15 samples), triaxial (13 samples), and Brazilian Disc Tests (11 samples). These provided strength, yield stress, Young’s modulus, Poisson’s ratio, and tensile strength.
Results show weak correlations between porosity and strength, but stronger samples had higher Young’s modulus and yielded later. Coarse, organic-rich sands were weaker, while P- and S-wave velocities correlated with strength parameters. Triaxial tests confirmed increasing yield point with confining pressure and lower friction angles in shales.
A linear model predicted static Young’s modulus in parts of the reservoir where no cores have been taken from dynamic moduli with an average absolute error of 2.6 GPa. Uncertainty remains due to limited sample data and high reservoir heterogeneity (REV larger than sample scale). The results in this study were compared to a geomechanical research of the IJsselmonde formation, which is a very similar formation. The data was not found to be very comparable, due to slightly different formations and different test techniques. Nevertheless, this data was the most comparable data out there.
Answering the main research question is and will remain difficult, because one can never know the geomechanical behavior of the complete reservoir. This study however did make a good start. It is now known that the upper Delft Sandstone member is moderately weak to strong. A correlation within the sands was also found with the grain size, or the amount of organic grains. It is not certain which of the two had the greatest effect. The ranges of dynamic moduli are known and we know what we need to test if we want to reduce the uncertainty within certain correlations.
This study shows the importance of extensive geomechanical data collection since the geomechanical behavior is not that easily explained.