An Analytical Modelling Study of Pressure Build-up at CO2 Injection Wells in Saline Formations

摘要

Geological storage of CO2 in deep saline formations is one of the promising ways to reduce emissions of CO2 into the atmosphere. This is because of the mature technology as a result of the experience in oil and gas extraction and the fact that saline formations offer large capacity and immediate availability. However, not all saline formations are suitable for CO2 storage. One of the important selection criteria for a suitable formation is the existence of sufficient injectivity. Injectivity simply shows how much CO2 can be injected into a saline formation for a constrained pressure build-up. One of the most important constraints in CO2 storage is to keep injection well pressure below formation fracture pressure. Otherwise, formation can be fractured which can create pathways for CO2 to escape from containment. Hence, significance of estimating injectivity should not be underestimated. Analytical models are preferred in preliminary studies to estimate pressure build-up.This thesis presents new analytical tools to estimate pressure build-up at CO2 injection wells in saline formations. The models developed in this thesis assume injection of CO2 at constant rate through vertical wells into homogenous and horizontal formations of constant thickness. They model the effects of relative permeability, CO2 dissolution in formation brine and drying-out on pressure build-up. Analytical solutions are presented for three types of formation outer boundary conditions: closed-boundary, constant pressure-boundary and infinite-acting formation. Using these models, transient pressure behaviour of fully-penetrating wells, partially-penetrating wells and multi-well injection are examined.The predictive capability of analytical models for each case is tested using numerical reservoir simulations. The results show a good agreement between the analytical and numerical computations. For partially-penetrating wells, a total skin factor which is composed of mechanical skin, partial penetration pseudo-skin and two-phase flow effects is introduced. Using the models developed for multi-well injection, the effects of number of injection and pressure relief wells as well as formation properties on total injection rate are studied.