Mechanistic Study Of Carbon Dioxide Flooding Using Horizontal Wells
Authors: Lim M.T. , Pope G.A. , Sepehrnoori K. 
Published: March 2010

Abstract
A finite-difference, equation-of-state, compositional simulator has been used to conduct a mechanistic study of carbon dioxide flooding using horizontal wells. We have used the reservoir and fluid properties of an actual West Texas carbonate reservoir that is currently being waterflooded. The phase behaviour and properties of the CO2 used in these simulations are typical of those of multiple-contact-miscible (MCM) field conditions of West Texas. Although the equation-of-state MCM calculations require much more computer time, comparison with a first-contact- miscibility (FCM) displacement demonstrates the importance of the more realistic phase behaviour description since the injectivity and oil recovery were significantly different for the MCM than for the FCM simulations. All simulations were conducted using permeability fields that have been conditioned with core data taken from two wells in the field. We have compared the performance of the process using a vertical injector and a vertical producer with that using a horizontal injector and a vertical producer combination. We have also investigated the impact of CO2 slug size, skin factor, and relative permeability parameters. These studies show significant sensitivity of the tertiary injectivity of CO2 and brine to the wellbore skin factor and to relative permeability. In tight reservoirs such as that considered in this study, injectivity has a major impact on the efficiency of the process. Although the oil recovery after a given number of injected pore volumes is relatively insensitive to many of the variables investigated in this and earlier studies, the oil recovery at a fixed time such as 20 years varies tremendously since it is limited by the amount of CO2 that can be injected in this time period. This study adds significant new insights and conclusions about the feasibility of using CO2 with horizontal wells.

Introduction
During the last 15 years, the petroleum industry has experienced a rapid increase in the number of horizontal wells being drilled and completed because recent advances in drilling technology have resulted in substantially reduced drilling and completion costs of horizontal wells(l). Drilling cost figures compiled by Joshi(2) showed that horizontal well costs have declined from about six to eight times that of vertical well costs to about two to three times. In some cases where extensive drilling experience has been acquired, the costs of drilling horizontal wells are reported to be about the same or even lower than vertical well costs(3). Another factor contributing to the upsurge in horizontal well activity is the fact that horizontal wells can deliver two to ten times the performance of conventional wells, mainly because of their larger surface areas exposed to flow(l). Finally, horizontal wells offer solutions to the problem of producing oil or gas in reservoirs where conventional technology either fails or is uneconomic. Some examples are(l):reservoirs where conventional wells have low productivity,reservoirs with vertical fractures,oil reservoirs where recovery is limited by water or gas coning, and (4) thick continuous heavy oil and bitumen reservoirs where steam-assisted gravity drainage is practical.