An Analysis of CO2 WAG Coreflood by Use of X-ray CT
Authors: Yamamoto Junji , Satoh Tohru , Ishii Hiroshi , Okatsu Komei 
Published: December 2007

Abstract
Two kinds of coreflood tests were conducted to investigate CO2 WAG (Water Alternating Gas) under the reservoir condition. Flow behavior of fluids in the core during these tests was monitored by X-ray CT scanner and cross sectional images of saturation distribution in the core were post-processed from CT data.
In the first test, 2.82PV of CO2, 1.47PV of water and 1.28PV of CO2 were injected sequentially into water flooded core. At the end of each stage, almost no oil was observed in the effluent. From fluid saturation analysis based on CT values, oil bank in the core was not confirmed and no unfavorable phenomenon such as gravity segregation was observed during initial CO2 injection stage. High trapped gas saturation at the end of water injection stage was observed which might be a cause of early CO2 breakthrough in second CO2 injection stage.
In the second test, a parallel core model was used to simulate a layered reservoir with different permeabilities and no cross-flow. CO2 and then water was injected to investigate any improvement in sweep efficiency by WAG process. From the processed CT profiles, injected CO2 behavior was visually understood and the movement of the displacement front in the lower permeability core toward the outlet by water injection was clearly confirmed.
From these results with X-ray CT monitoring, CO2 WAG process was suggested to be effective to improve sweep efficiency especially in the layered reservoir with different permeabilities.

Introduction
X-ray CT application to core analysis including EOR corefloods has been conducted for more than 10 years by major oil companies, research centers and universities to visualize porosity distribution and flow behavior in the core. Japan National Oil Corporation (JNOC) and Japan Petroleum Exploration Co. Ltd. (JAPEX) has also started the related activities to this X-ray CT technologies and the efforts has been made mainly for detecting and analyzing the fracture system in the core by use of the second generation X-ray CT. After investigating the core analysis method including measurement of porosity distribution and the basic fluid saturation analysis technique, JNOC introduced a new medical X-ray CT system with faster scanning time per slice for monitoring fluid flow in the core. JNOC and JAPEX have conducted the studies about the analysis of flow behavior in the core using this new system for these several years. After establishing the basic procedure to analyze flow behavior in the core, this technique was decided to apply for CO2 coreflood.
This coreflood test was conducted as a part of Joint Study by JNOC and JAPEX in Sarukawa Oil Field in Japan named "CO2 Flood-Sarukawa Project". Features of the target reservoir in Sarukawa Field are a multi-layer system with different permeabilities and an inclined reservoir system with 20 to 30.

Experimental Outline
Two plug cores of tuffacious sandstone reservoir rock taken from Sarukawa Oil Field were connected in series as a composite core for these tests. Filter papers were inserted to keep contact with the core faces or end-plug faces of a core holder for reducing capillary end effect. The core was wrapped with aluminum sheets, then the upstream end-plug and the core were enclosed with heat-shrink tubing to prevent CO2 leakage through rubber sleeve while applying confining pressure.
Recombined crude with gas was used for these experiments. For the monitoring purpose by X-ray CT scanner, NaI (5 wt.%) was added as a dopant in NaCl brine (25,000ppm).
P. 443^