Equations of overburden pressure at laboratory: an overburden pressure measurement method in core samples

Abstract

This study proposes a laboratory-scale overpressure measurement equation to avoid the core damage caused by traditional trial-and-error methods, filling the gap in existing methods. To estimate the overburden pressure before any damage to the core sample, the lithology table was obtained via laboratory data and petrophysical and geological information obtained from the field for determining the grain and fracture situation in the core samples. Afterwards, the fluids were injected into the core samples placed inside the apparatus under overburden pressure 15.6 to 121 $°C$ by setting overburden pressure on various values (50–179 bar). The core samples utilized were mostly the two main groups of reservoir rocks, sand and dolomite/lime, or a combination of both with variable porosities (5–25 %), which whole data was obtained from the presented lithology table.

The experimental data was integrated with field data to obtain empirical equations to determine the value of overburden pressure in the cores with various porosities, considering the fluid and rock characteristics. An increase in the saturation of fluids (especially in water-bearing types), the densities of rocks and fluids, percentage of fractures (mostly in carbonate type), and the viscosity of fluids were along with an increment in the total overburden pressure exerted on the sample. Subsequently, this increment caused a reduction in permeability and damage to the core samples. Conversely, the increase of initial pore pressure (mostly in sandstone type) was along with a decline in overburden pressure, resulting in a significant decrease in permeability and more damage to the mostly-fragile under-pressure cores. The equations introduced here incorporated the effects of these variables and represented the behavior of porous media to raise the speed and accuracy of predicting overburden pressure in the laboratory.