From Simulator to Field Management: Optimum WAG Application in a West Texas CO2 Flood - A Case History
Authors: Sharma Arun K. , Clements Lucille E. 
Published: December 2007

Using field data, the processes by which compositional simulation results are effectively integrated into an active CO2 flood operating policy are presented as a case history. The economic success or failure of a CO2 flood often hinges on two basic requirements: effective CO2-oil contact and displacement, and economic use of CO2. An inexpensive and effective tool to help achieve these requirements is the use of Water Alternating Gas (WAG) cycles. But how do we use WAG ratios to best displace oil in a timely manner while combatting the effects of reservoir heterogeneity and adverse mobility ratios on sweep efficiency? Reservoir simulation is often used to optimize WAG ratios. The final challenge, however, is to transfer small-scale, homogenized simulation results to the field in such a way as to affect the bottom line; namely decrease operating expenses and increase oil production.

For the Mallet Unit (a West Texas CO2 flood), compositional simulation results are scaled to the field in accordance to a reservoir characterization. An overall operating philosophy is developed and implemented. A data collection process is developed using integrated software and hardware to provide a basis upon which field surveillance can be conducted.

Finally, overall results are reviewed including accelerated tertiary oil production response and reduced gas processing costs, yielding very favorable project economics.

Introduction

Carbon Dioxide has proven to be a very effective miscible injectant leading to the nearly complete mobilization of residual oil. The possibility exists, therefore, to conduct a tertiary flood with high displacement efficiency since CO2 has the ability to completely displace and mobilize oil from the pore space. The principle challenge is to contact as much targeted pore space as possible. In fact, overall recovery in CO2 floods is generally lower than predicted or seen in core displacements because of poor sweep efficiency at the field level. Much work has been conducted to understand the processes which rob tertiary floods of their potential recoveries. It has been reported that gravity segregation leading to miscible injectant override, viscous fingering and worst of all high permeability streaks can lead to areal and vertical channeling.

To improve areal and vertical sweep efficiency, Caudle and Dyes suggested decreasing the mobility ratio of the system by simultaneously injecting water with the injectant gas. Additional investigators promote the use of Water-Alternating-Gas (WAG) injection where gas and water are injected in consecutive alternating slugs.

The use of WAG strategies has become paramount in the operation and economic maintenance of CO2 floods. It has been found to be an effective and inexpensive tool in the control of gas production throughout the life of the flood. Although gas production is expected as a CO2 flood matures, it is the premature cycling of CO2 from injectors to producers without an associated tertiary response which is extremely detrimental to the economic viability of a CO2 flood. P. 319