An experimental study on the influence of pressure on permeability and seismic velocity in the Montserrat geothermal field (West Indies)

Volcanic regions are prime areas for harnessing geothermal energy. However, the geological complexity of these regimes can hinder the efficient exploitation of this renewable resource. As useful as geophysical surveys are in understanding the subsurface, models can be unconstrained due to the lack of realisitic data. Here, we present the results of a new laboratory study measuring permeability and P-wave velocity on altered lavas, volcaniclastic sediments and limestones samples obtained from three cores retrieved from the MON-3 well, drilled in the Montserrat geothermal field. By measuring these parameters at increasing effective pressure varying from 10 to 70 MPa, we found permeability stress senstivity factors varying from 0.006 to 0.122 MPa⁻¹ and increases in P- wave velocity between 2 and 20 %. The highest permeability stress senstivity factor and largest increase in P-wave velocity found in altered lavas were interpreted to be as a result of the easy closure and deformation of slit shaped microfractures and smectite clays characteristic of low bulk modulus. Conversely, lowest permeability stress senstivity factors and smallest increases in P-wave velocity found in volcaniclastic sediments, were coincident with intergranular pore geometries and higher bulk modulus minerals such as quartz and illite-smectite that may enhance the rigidity of the rock. Additionally, by measuring permeability and P-wave velocity on samples cored perpendicular (vertical) and parallel (horizontal) to the axis of the core, we found higher permeability senstivity factors (0.001–0.05 MPa⁻¹ higher) and P-wave velocity increases (1–4 % higher) on horizontal samples, which is consistent with the preferential closure of horizontally oriented pore spaces. From this experimental study, we provide implications for enhancing geothermal energy recovery in Montserrat. Overall, our findings can be utilized to help improve on geophysical and numerical models of volcanic geothermal regimes.