The influence of alteration and fractures on gas permeability and mechanical properties of the sedimentary and volcanic rocks of the Acoculco Caldera Complex (México)

Petrophysical and mechanical properties of limestones and volcanic altered and unaltered rocks were estimated by a series of laboratory measurements. The aim of this work was to study the changes in matrix permeability and the influence of an induced vertical fracture. The phenomenon was studied by direct comparison of different properties (porosity, UCS, permeability) and by a multiple linear regression analysis. Porosity, UCS, and matrix permeability of limestones and non-altered volcanic rocks showed homogeneous values. However, the permeability increased from poor to moderate after fracture induction (10–17 to 10–14 m²). Altered volcanic specimens showed a more complex pore system. Thus, considerable variations in all measured properties were identified. For example, porosity ranged from 6 to 50% and Permeability ranged from 10–14 to 10–12 m². Comparison plots including UCS, porosity, and matrix permeability showed a good fitting (R²=0.75). Multiple linear regression analysis in fractured specimens has a moderate R² (0.55) The model that better explains the permeability evolution of fractured specimens has the fracture permeability as the response variable and the porosity values as independent variables. The usage and repeatability of these equations in other zones should include more singularities of every single specimen as pore size, pore distribution, and fractures.