Abstract

N2 and CO2 are the most common gases utilized in foam enhanced oil recovery (EOR) techniques. Foam is introduced in enhanced oil recovery (EOR) to mitigate the problems encountered during gas floodings such as high gas mobility, viscous fingering, and gravity override. CO2 exists at supercritical conditions at typical reservoir conditions. Also, CO2-foam has a common problem to become weaker above its supercritical conditions of 1100 psi and 31°C. As a result, the advantages of using CO2 foam diminished due to the weakness of CO2 foam at supercritical conditions. The mobility of gas is not effectively decreased resulting in low sweep efficiency.

However, N2 is found to form a stronger foam at the same HPHT conditions as compared to CO2.

This work test the CO2/N2 mixed foam performance in oil-saturated sandstone cores at reservoir condition after secondary recovery by water flooding. It investigated the effect of changing CO2/N2 ratio and foam quality on the recovery by CO2/N2 mixed foam. The experiments were carried out using a core flooding system with three pumps working simultaneously to test and evaluate the effects of different parameters. N2 is added to CO2 with optimized quantity to generate stronger and more stable foam under the supercritical state of CO2. It is reflected by high stabilized apparent viscosity compared to using CO2 foam alone and the increased oil recovery. The effect of CO2/N2 ratio and foam quality was evaluated to choose the optimum value of the parameters for maximizing oil recovery.

With the increasing number of enhanced oil recovery (EOR) CO2 projects worldwide in addition to the necessity of CO2 sequestration, this study results may provide an effective solution to improve and optimize the parameters for CO2/N2 foam injection at reservoir conditions which can produce more oil keeping as much as CO2 underground.

Introduction

Amongst the EOR technique, there is a rapid increase in the number of EOR projects. In the US alone, there are around 114 active commercial CO2 injection projects which are using CO2 over a quantity of 2 billion cubic feet for injection and for producing around 280,000 BOPD (April 19, 2010, Oil and Gas Journal). The industry has seen the success of CO2 gas injection projects have however the recovery from the CO2 gas injection process is very low. The main issue is due to the low viscosity of the gas resulting in low recovery and early breakthrough of CO2. The high mobility and gravity override reduces the macroscopic sweep efficiency significantly leaving a larger part of the reservoir being unswept by CO2.

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