GAS INJECTION FOR OIL RECOVERY A SIMULATION STUDY

ABSTRACT

Proper handling of gas produced is crucial to the development of sour reservoirs. Over years of research and practice, many methods of gas processing have been developed from the solid storage of sulfur to reinjecting the gas back into producing or depleted light oil reservoir for miscible flooding enhanced oil recovery. This paper seeks to investigate the use of gas to enhance oil recovery and its associated phase behavior problems.

In designing a miscible gas flooding project, the minimum miscibility pressure (MMP) is the key parameter that determines the impact on gas and oil mixing phase behavior. The MMP is the lowest pressure at which the displacement process becomes miscible upon contact with the reservoir fluid. There are various methods to determine the MMP. A laboratory experiment is the most accurate but time consuming and subject to fluid sample quality; while the Equation of State is poor in characterizing polar molecules like H₂S. For this study, empirical correlations are used to determine the MMP because the study focuses more on the general trend of how methane concentration affects the MMP of the process.

In this study, a gas injection model is developed using a compositional simulator with the aim to determine mechanistically how gas enhances oil recovery.  This model is used to evaluate the effect of some important parameters such as acid gas concentration, injection pressure and injection rates on oil recovery efficiency.

The result of MMP study shows that methane concentration has a significant impact on the MMP of the process. As methane concentration increases in the injection gas, the MMP of the process also increases. From this study, it was observed that increasing acid gas concentration decreases the MMP of the process as a result of an increase in gas viscosity, consequently extending the plateau period resulting in late gas breakthrough and increasing the overall recovery of the process. It is also seen that this increase in viscosity increases the volumetric sweep efficiency of the process which is an improvement to most gas injection enhances oil recovery (EOR).  

CHAPTER ONE

 INTRODUCTION

One of the major problems of developing a sour reservoir is how to handle the produced acid gas. The gases produced from a sour reservoir are sweetened to selectively separate the gases using different methods. Among these methods, amine extraction is the most commonly used in petroleum industries. The separation process results in the production of a waste stream containing CO₂ and H₂S, and this mixture is referred to as acid gas. There is a need for an environmentally-friendly and cost-effective method for dealing with this acid gas stream.

Over the years a lot of strategies have been developed to handle acid gas mixture, and most of them are primarily concerned with the reduction of the toxic hydrogen sulfide to an inert/non-toxic reactive product. The most common technique is the Claus reaction process where gases containing H₂S are catalytically converted to elemental sulfur (Bennion et al. 1999). Another method used to manage the acid gas mixture is to inject a compressed acid gas mixture into a subsurface reservoir for storage.

In this research, I studied the re-injection of the rich waste acid gas stream directly back into the producing or depleted light oil reservoir for the purpose of miscible flooding enhanced oil recovery through characterizing its phase behavior and numerical simulation. Gas injection for enhanced oil recovery (EOR) presents a cost-effective and environmentally-friendly solution for managing a sour reservoir. It eliminates current taxation or future liability associated with emission or surface storage of sulfur. This study will focus on mechanisms of miscible gas injection using sour gas. Gas is a blend of natural gas with hydrogen sulfide (H₂S) and carbon dioxide (CO₂)

1.1 Research aim

The main goal of this research is to develop a miscible gas flooding EOR with gas using a compositional simulator with the aim to study the mechanism of how acid gas enhances oil recovery and to understand how hydrocarbon composition in gas affects miscibility development and the minimum miscibility pressure.