Enhanced Oil Recovery Field Planning and Development Strategies Review

A Primer on Enhanced Oil Recovery

Guenther Glatz
October 28, 2013

Submitted as coursework for PH240, Stanford Academy, Fall 2013

Fig. ane: Illustrative cases for stable and unstable displacement.

Introduction

Oil remains the world's leading selection of energy, bookkeeping for 33% of global energy consumption, despite losing market place share 13 years in a row. In 2012, global oil consumption increased by 890,000 barrels per day, bringing total daily consumption to approximately 87 meg barrels. [ane] To satisfy demand and increase reserves, companies take been relying on enhanced oil recovery (EOR) methods for decades. [2]

Lifetime of a Reservoir

The life of a hydrocarbon reservoir is commonly divided into iii phases. During the initial phase, or principal recovery, oil is produced by natural drive mechanisms like supporting aquifers, solution gas drive, gas cap drive, or gravity drainage. Once natural drive mechanisms are depleted, secondary recovery techniques are put into action to maintain the reservoir pressure level. This includes techniques like water or gas injection. The lifetime of a reservoir is farther increased by special techniques during tertiary recovery. The third phase is commonly referred to as enhanced oil recovery; though, it should be noted that enhanced oil recovery is not limited to a certain phase of recovery, and that several definitions for EOR exist in the literature. [3-v] Improved oil recovery (IOR) has sometimes been used interchangeably with EOR, and while in that location is no formal definition for IOR, it is generally understood to summarize any approach improving oil recovery. IOR, therefore, encompasses EOR. [v]

Why EOR Methods Are Required

Generally, natural depletion of a reservoir allows for a very limited recovery of the oil in place. Depending on the blazon of reservoir, values are around 20%. During secondary recovery, another 15 to 20% might be added. [iv] Values for recovery at the tertiary stage can vary significantly depending on many factors. For instance, original production forecasts under primary depletion for the Suplacu de Barcau oil reservoir, a heavy oil reservoir located in Romania, were estimated to be around ix% for an 80 years fourth dimension frame. Through the introduction of a combination of thermal enhanced oil recovery methods, the cumulative extracted rough in 35 years corresponds to a current recovery factor of 44.6%. [6] We need to understand the mechanisms limiting main and secondary recovery to understand the significance of enhanced oil recovery. There are two primary reasons oil cannot be swept completely from a reservoir when displacing it with another fluid. First, we have trapping of oil on the pore scale. The mechanism is described by a snap-off model and can be elegantly summarized past the capillary number. [vii] The capillary number is a dimensionless number and defined as

where five is the velocity, μ the viscosity, σ the surface tension, and θ the wetting angle. [8,9] In essence, the capillary number is the ratio of mucilaginous versus capillary forces. A pocket-sized capillary number suggests that the motion of the fluid is dominated past capillary forces. Conversely, a large capillary number indicates a viscous dominated authorities. From a practical point of view, nosotros wish to increment the capillary number thereby reducing trapping. As pointed out by S. Thomas, nosotros would need an increase by iii orders of magnitude to halve the residual oil saturation. [5]

Looking at the capillary number one would assume any technique raising the production of velocity and viscosity does suffice. The production, however, is directly proportional to the pressure driblet, consequently limiting an injection well to fracture pressure. For case, chemical enhanced oil recovery methods focus specially on increasing the capillary number by reducing interfacial tension rather than increasing the product of velocity and viscosity. [x]

The second phenomena affecting recovery happens on the reservoir scale and is described by another dimensionless number, the mobility ratio. [9] Mobility itself is defined as the ratio of permeability κ and viscosity μ

The mobility ratio is then given as the ratio of the displacing phase mobility to the displaced phase mobility:

Chiliad

=

λdisplacing stage λdisplaced phase

From a sweep efficiency perspective, mobility ratios larger than 1 are unfavorable because information technology would result in an unstable displacement. Displacing viscous oil with h2o is an example for a mobility ratio larger than ane. The water will tend to finger through the reservoir yielding poor sweep efficiency. Mobility ratios smaller than 1 are preferred considering the injected fluid is and then able to displace the oil in a more piston similar manner. [11] Both cartoons in Fig. one requite an aeriform view of a reservoir with a simple injector-producer configuration. The oil is colored in green, water injected is colored in blue. The figure on the left illustrates the pasty fingering trouble for mobility ratios larger than 1. Once a finger has reached the producer it will get harder and harder to sweep the remaining oil. The water injected volition adopt the flowpath established by the finger. The effigy on the right illustrates the example for a mobility ratio smaller than 1. The water front is growing in a stable, radial manner with no fingers trying to get ahead of the front end. Ultimately, this will yield a ameliorate sweep of the reservoir.

EOR Methods - An Overview

Enhanced Oil Recovery methods are divided into three principal groups: chemical, miscible or solvent injection, and thermal. [9] All of them are subject to extensive research at Stanford.

As mentioned in a higher place, chemical methods focus on injecting interfacial active components such as surfactants thereby increasing the capillary number. [10] Co-ordinate to Lake, 1 of the most common methods is micellar-polymer flooding. [vii] Newer methods like alkaline-surfactant-polymer and surfactant-polymer target to modify both dimensionless numbers at the same fourth dimension. In the case for alkaline-surfactant-polymer, the polymer targets the mobility command consequence. The surfactant and the alkali combine forces to lower interfacial tension. [3]

Miscible or solvent injection relies on the injectants miscibility with the oil phase. This type of enhanced oil recovery method aims to subtract the viscosity of the oil and/or cause it to swell. [nine] With respect to the second dimensionless number, the mobility ratio, we can already foresee the challenges that proceed with miscible or solvent injection. The fluid injected usually has a lower viscosity and mucilaginous fingering is not unlikely. In add-on, depending on the density dissimilarity between the oil and the injectant, gravity over- or underride occurs reducing sweep efficiency. [9]

Thermal methods are particularly geared towards reservoirs with loftier viscosity oils. Adding thermal energy to the reservoir by injecting steam results in a decrease of the oil viscosity making information technology more mobile. In addition, residual oil is reduced and the presence of a gas phase in the reservoir causes light components in the oil to be distilled. [12] Kern River in Bakersfield is a prominent example for steamflooding. [13]

Fig. 2: Example of a successful combustion tube experiment.

Another possibility is to create the thermal free energy in identify past injecting air, combusting some of the oil. [14] This process is referred to as in-situ combustion and is part of the development strategy of the same Suplacu de Barcau oil reservoir. Interestingly, not every rough oil has the backdrop lending itself to this process. To determine whether or non a crude oil would exist possible candidate for in-situ combustion, extensive laboratory experiments have to exist carried out. Combustion tube experiments are of special interest at Stanford. A metal tube is filled with the reservoir matrix and oil mixture under investigation. Ane cease of the tube is and then heated in an inert atmosphere. Once ignition temperatures are reached, air is injected to initialize the combustion process. For a successful combustion run, the combustion front end volition slowly travel through the tube pushing the oil out. [14] Fig. 2 shows the result of a successful combustion tube run. Air was injected from right to left and we can conspicuously distinguish between the part swept past the combustion forepart, a transition zone, and the untouched part.

Summary

The demand for hydrocarbons is yet increasing. To keep up with demand, the hydrocarbon industry is constantly challenged to improve current recovery methods and observe new ways to produce oil. In this paper the mechanisms limiting master and secondary recovery were discussed to explain the significance of enhanced oil recovery methods. In essence, the limiting mechanisms can be summarized with 2 dimensionless numbers, the capillary number and the mobility ratio. Enhanced oil recovery methods aim to overcome these limitations attacking the challenges from different angles.

© Guenther Glatz. The author grants permission to re-create, distribute and display this piece of work in unaltered form, with attribution to the author, for noncommercial purposes merely. All other rights, including commercial rights, are reserved to the author.

References

[one] "BP Statistical Review of World Free energy," British Petroleum, June 2013.

[2] Eastward.C. Hammershaimb et al., "Recovery Efficiency of Enhanced Oil Recovery Methods: A Review of Significant Field Tests," I Petro 12114-MS, v Oct 83.

[3] J. Sheng, Modern Chemic Enhanced Oil Recovery: Theory and Practise (Gulf Professional person Publishing, 2010).

[4] A. Carcoana, Applied Enhanced Oil Recovery (Prentice Hall, 1992).

[5] S. Thomas, "Enhanced Oil Recovery - An Overview," Oil & Gas Sci. Technol. - Rev. IFP 63, 9 (2008).

[vi] A. Panait-Patică, D. Şerban and N. Ille, "Suplacu de Barcau Field - a Case History of a Successful In-Situ Combustion Exploitation," Ane Petro 100346-MS, 12 Jun 06.

[vii] L. West. Lake, Enhanced Oil Recovery (Prentice Hall, 1996).

[8] J. J. Taber, "Enquiry on enhanced oil recovery: Past, present and future," in Surface Phenomenon in Enhanced Oil Recovery Plenum, ed. by D. O. Shah (Springer, 1981).

[9] 5. Alvarado and E. Manrique, Enhanced Oil Recovery: Field Planning and Development Strategies (Gulf Professional Publishing, 2010).

[10] R. A. Fulcher, T. Ertkin and C. D. Stahl, "Result of Capillary Number and Its Constituents on Two-Phase Relative Permeability Curves," J. Petrol. Technol. 37, 249 (1985).

[11] J. Southward. Aronofsky, "Mobility Ratio - Its Influence on Flood Patterns During H2o Encroachment," J. Petrol. Technol. iv, 15 (1952).

[12] Yard. Prats, Thermal Recovery (Soc. Petrol. Eng, 1982).

[13] Thousand. R. Greaser and R. A. Shore, "Steamflood Functioning in the Kern River Field," One Petro 8834-MS, 20 Apr eighty.

[14] H. J. Ramey, Jr., "In Situ Combustion," One Petro WPC-14229, 13 Jun 71.

[xv] G. Glatz et al., "Kinetic Cell and Combustion Tube Results for a Central European Rough," One Petro 146089-MS, 30 Oct 11.

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Source: http://large.stanford.edu/courses/2013/ph240/glatz1/

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