Enhanced Oil Recovery

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INTRODUCTION

Oil and Gas refers to the naturally occurring liquid and natural gas specifically made up of long chain hydrocarbons and various organic compounds found beneath the surface of the earth in entrapments called reservoirs; the presence of oil and gas in these reservoirs is the reason humans survive everyday and carry out their daily activities effectively. Different activities are usually carried out to ensure that the oil and gas present in the reservoirs continue to support humans through their day-to-day activities; such activities include exploration, development, production and finally, abandonment and reclamation. This process is what is referred to as “the oil and gas process”. On completion of this process, numerous efforts are made to increase the quantity of oil that can be extracted again from the oil well and we refer to these efforts as “recovery”. There are three main types of recovery, primary recovery (solution gas, gas cap and natural water drive) secondary recovery (gas injection and water flooding) and tertiary recovery (enhanced oil recovery EOR, polymer flooding and steam flooding). In this article, analysis on the “enhanced oil recovery” technology and trends will be emphasized upon.

ENHANCED OIL RECOVERY

Enhanced oil recovery becomes very vital when oil production has to be increased to obtain a recovery percentage of at least 75% (Rigzone, 2009). This can only achieved by using any of the four basic methods of EOR; these methods include chemical flooding, miscible gas displacement, thermal recovery and microbial EOR. Among these four mentioned, “Thermal methods are the oldest EOR methods, they have been developed over the last thirty years” (Elsevier, 1981). Miscible gas displacement also called “gas injection” by some engineers refers to the process of injecting CO2, natural gas and Nitrogen into a reservoir; “in miscible gas displacement, the gas is injected at or above the minimum miscibility pressure (MMP) which causes the gas to be miscible in oil” (Bandar, 2007). Chemical flooding also called “chemical injection” involves injecting chemicals such as a polymer directly into the reservoir to enhance the oil recovery. Thermal recovery introduces heat to the reservoir to reduce the viscosity of the oil. Many times, steam is applied to the reservoir, thinning the oil and enhancing its ability to flow (Rigzone, 2009). Over 50% of the tertiary recovery method employed by the United States of America is the thermal recovery method.

MISCIBLE GAS DISPLACEMENT

The most common gas employed when “gas injection” is being used is the CO2 (carbon dioxide) gas. The two major reasons why carbon dioxide is used are because it is miscible with crude oil and also it is the cheapest of all the other miscible fluids present. Carbon dioxide injection into the reservoir based on previous experiences has shown that a recovery of up to 15% of the oil that was originally in the reservoir is achievable. With the injection of the carbon dioxide gas, changes occur in the reservoir such and temperature and pressure changes. Through the changes in temperature and pressure, carbon dioxide can form a solid, liquid, gas and supercritical fluid (Teledyne, 2007). Furthermore, when carbon dioxide has been injected into the reservoir, it begins to form a homogeneous mixture with the crude oil thus, the light hydrocarbons, which are present in the crude, mix with the carbon dioxide gas and this dissolves the oil. Upon miscibility of the carbon dioxide gas and the crude, the physical forces separating the liquid phase and the gaseous phase gives way and this helps the carbon dioxide gas move the oil from the rocks towards the wells for production. According to the USDOE, a very good example of an applied carbon dioxide gas injection technique is the Wasson. Field's Denver Unit CO2 EOR project which has resulted in more than 120 million incremental barrels of oil through 2008 (2010). Moreover, carbon dioxide...
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