Active Vocabulary

Date: 2015-10-07; view: 504.

Unit 2. Enhanced Oil Recovery

C. Discussion

Topics:

1. Oil and Gas Production.

2. Methods which allow to increase the recovery of crude oil.

Enhanced oil recovery is the third stage of hydrocarbon production during which sophisticated techniques that alter the original properties of the oil are used. Enhanced oil recovery can begin after a secondary recovery process or at any time during the productive life of an oil reservoir. Its purpose is not only to restore formation pressure, but also to improve oil displacement or fluid flow in the reservoir.

In primary production, no extraneous energy is added to the reser­voir other than that required for lifting fluids from the producing wells. Most reservoirs are developed by numerous wells; and as pri­mary production approaches its economic limit, perhaps only a few percent and no more than about 25 percent of the crude oil has been withdrawn from a given reservoir.

The oil industry has developed methods for supplementing the pro­duction of crude oil that can be obtained mostly by taking advantage of the natural reservoir energy. These supplementary methods, collec­tively known as enhanced oil recovery technology, can increase the recovery of crude oil, but only at the additional cost of supplying ex­traneous energy to the reservoir. In this way, the recovery of crude oil has been increased to an overall average of 33 percent of the original oil.

The three major types of enhanced oil recovery operations are chemical flooding (alkaline flooding or micellar-polymer flooding), miscible displacement (carbon dioxide [CO2] injection or hydrocarbon injection), and thermal recovery (steam flood or in-situ combustion). The optimal application of each type depends on reservoir temperature, pressure, depth, net pay, permeability, residual oil and water saturations, porosity and fluid properties such as oil API gravity and viscosity.

Chemical flooding is a general term for injection processes that use special chemical solutions. Micellar, alkaline and soap-like substances are used to reduce surface tension between oil and water in the reservoir, whereas polymers such as polyacrylamide or polysaccharide are employed to improve sweep efficiency. The chemical solutions are pumped through specially distributed injection wells to mobilize oil left behind after primary or secondary recovery. Chemical flooding is a major component of enhanced oil recovery processes and can be subdivided into micellar-polymer flooding and alkaline flooding.

The general procedure of a chemical flooding includes a preflush (low-salinity water), a chemical solution (micellar or alkaline), a mobility buffer and, finally, a driving fluid (water), which displaces the chemicals and the resulting oil bank to production wells. The preflush and the mobility buffer are optional fluids.

Miscible displacement is a general term for injection processes that introduce miscible gases into the reservoir. A miscible displacement process maintains reservoir pressure and improves oil displacement because the interfacial tension between oil and water is reduced. The effect of gas injection is similar to that of a solution gas drive.

Miscible displacement is a major branch of enhanced oil recovery processes. Injected gases include liquefied petroleum gas (LPG), such as propane, methane under high pressure, methane enriched with light hydrocarbons, nitrogen under high pressure, and carbon dioxide [CO2] under suitable reservoir conditions of temperature and pressure. The fluid most commonly used for miscible displacement is carbon dioxide because it reduces the oil viscosity and is less expensive than liquefied petroleum gas.

Miscible displacement is also called miscible gas drive, miscible drive or miscible flood.

Thermal recovery is a general term for injection processes that introduce heat into a reservoir. Thermal recovery is used to produce viscous, thick oils with API gravities less than 20. These oils cannot flow unless they are heated and their viscosity is reduced enough to allow flow toward producing wells.

During thermal recovery, crude oil undergoes physical and chemical changes because of the effects of the heat supplied. Physical properties such as viscosity, specific gravity and interfacial tension are altered. The chemical changes involve different reactions such as cracking, which is the destruction of carbon-carbon bonds to generate lower molecular weight compounds, and dehydrogenation, which is the rupture of carbon-hydrogen bonds.

Thermal recovery is a major branch of enhanced oil recovery processes and can be subdivided in two types: hot fluid injection such as steam injection (steam flood or cyclic steam injection) and hot water-flooding and in-situ combustion processes.

Fig 1. Enhanced oil recovery