Experimental study on the sensitivity of deep-buried limestone porosity to effective stress in groundwater reservoirs

Abstract

The sensitivity of rock porosity to effective stress is critical for calculating reserves in both hydrocarbon and groundwater reservoirs and for clarifying transport transformations between surface water and groundwater in hydropower projects. This study addresses the pressing need to understand the impact of porosity sensitivity to effective stress in the deep pressurised aquifers of hydropower stations on the long-term safety and stability of extra-high dams post-impoundment. An effective stress-porosity experimental curve for deep-buried limestone is established based on variable confining pressure seepage experiments conducted on specimens sourced from on-site drilling at the Xiluodu reservoir, the world’s fourth largest, with a dam height of 285.5 m. The differences in the response of limestone porosity to effective stress between hydrocarbon and groundwater reservoirs is explored by comparing existing limestone effective stress-porosity curves for hydrocarbon reservoirs worldwide. First, the applicability and limitations of various fitting methods for effective stress-porosity curves are compared using measured data. Subsequently, a modified exponential fitting method that accounts for dead-end pores is proposed, significantly enhancing the curve-fitting accuracy for groundwater reservoirs. Finally, a probability distribution model of the measured effective stress-porosity data, conforming to a normal distribution, is constructed via the Monte Carlo method. This model is used to derive a statistically significant effective stress-porosity fitting expression for Xiluodu limestone based on limited experimental data. This study provides parameters and a microscopic research basis for understanding water transport in aquifers and regional deformation processes induced by impoundment, such as river valley contraction and settlement deformation, and for informing the rehabilitation of overexploited aquifers.