Minimizing Risk of Gas Escape in Gas Storage by In-Situ Measurement of Gas Threshold Pressure and Optimized Completion Solutions

Abstract

Abstract Underground gas storage operations and CO2 sequestration in aquifers relay on both proper wellbore construction and sealing function of the cap rock. The potential leakage paths are the migration along the wellbore due to poor cementation and flow through the cap rock. Although 60% of the gas storage wells undergo a costly work-over process to remedy missing cement seal only few operators consider appropriate completion solutions to minimize the risk of losses due to leaky annulus space. The leakage through the cap rock can occur by diffusion and two-phase migration. Modeling of two-phase flow requires the determination of the functional relationship between capillary pressure, relative permeability and saturation. The injected gas moves to the top of the formation below the cap rock due to gravity and density differences. Therefore, the ability of a cap rock to seal fluids is another key parameter for the successful gas storage or long term disposal of CO2 in addition to a proper completion. Capillary pressure data, which are critical for exact prediction of gas leakage through the cap rock are seldom available and yet necessary. In-situ method of gas entry pressure was developed and successfully implemented to help reducing uncertainties in gas leakage predictions. Zonal isolation of cap rock is performed followed by exchange of wellbore liquid by gas. Constant rate injection of gas is then conducted to determine the gas entry pressure into a fully water saturated cap rock. Simulations were performed using this gas threshold entry pressure to investigate the gas leakage amount through the cap rock and along the wellbore for both natural gas and CO2 storage models. It was shown that the uncertainty of predictions could be significantly reduced by using data obtained from in-situ gas threshold determination. Recommended completion solutions for both types of storage operations are presented, which minimize a risk of uncontrolled gas escape.