Sample Paper on Injection of Immiscible Gas into Carbonate Reservoir

Injection of Immiscible (Non-Hydrocarbon) Gas into Carbonate Reservoir

Introduction

Many oil and gas industry players have long proposed the injection of immiscible (non-hydrocarbon) gasinto groundwater aquifers and reservoirs as one possible answer to the reduction of excessive greenhouse gas emission into the earth’s atmosphere, leading to global warming(Khan 3). This is done by a process of Enhanced Oil Recovery (EOR), which is the recovery of oil through the injection of fluid that is foreign to the reservoir(Ebrahimi et al. 211).Carbon Dioxide(CO2) has been used for purposes of enhancing oil recovery (EOR) since the 1950s. The use of (high pressure) Nitrogen in this respect is more recent,considered as an “enhanced oil recovery process for a naturally fractured thick carbonate reservoir containing light oil and underlain by an infinite aquifer” (Ebrahimi et al. 212). However, these two gases (CO2 and Nitrogen)do not behave the same way.

This paper examines the process of injection of immiscible (non-hydrocarbon) gas into carbonate reservoir, including the differences between CO2and Nitrogen into carbonate reservoir.

Efficiency of Microscopic and Macroscopic Immiscible Gas Displacement

  1. Gas/Oil Viscosity and Density Contrast

Different gases and oils have different viscosities and densities, which can contribute to gas/oil displacement inefficiencies. At reservoir conditions, gases and oils have viscosities of approximately 0.02 cp and 0.5 cp respectively. Generally, the densities of gases are one-third of the densities of oil. Gases are considerably lighter compared to oils(Khan 23).

  1. Capillary Pressure and Relative Permeability of Gas/Oil

Typically, commercial laboratories employ special routine procedures for core analysis to measure capillary pressure and relative permeability of gas/oil. Gas/oil capillary pressure, for instance, is measured with either centrifuge equipment or porous-plate. To gather data on gas/oil relative permeability, laboratories use the viscous displacement method where gas displaces oil. One may also use the centrifuge method by which helps to obtain data/information on capillary pressure as well as relative permeability simultaneously.

Gas, as a non-wetting phase in the displacement process, first flows through the largest pores, while water flows through the smallest pores.According to Hagoort (cited in Masjed 17), initial saturation of water significantly influences the relative permeability of oil during a gas/oil displacement.

  1. Mobility Ratio

Mobility ratio refers to the displacing phase’s mobility divided by the displaced phase’s mobility (Fred 142), in this case oil. In simple calculations, mobility ratio is calculated where the relative permeability values end. Often, all displacements of oil by gas are at unconducive mobility ratios, typically ranging between the values 10 to 100 and at times even more.

  1. Efficiency of Linear Gas/Oil Displacement

The gas/oil linear displacement efficiency refers to the percentage of oil volume recovered, and is calculated for any gas injection period by integrating the volume of gas-invaded zone as a factor of gas saturation (Sg). The values for the efficiency of gas/oil displacement are the consequent of saturation profiles that are generated with the use of fractional flow curves.

  1. Factors that Affect the Efficiency of Gas/Oil Displacement

Fractional flow and equations for material balance can help understand the factors that affect the efficiency of gas/oil displacement. These factors include: initial saturation conditions; relative permeability ratios; fluid viscosity ratios; capillary pressure; formation dip; and factors of permeability, injection rates and density difference, among others. Some of these are discussed below:

Laboratory tests as well as mathematical analyses indeed show thatinitial saturation conditions do have significant impact on the efficiency of gas displacement. For instance,the gas saturation decreases the amount of displacement oil wheninjection of gas starts after the reservoir pressure has fallen to a level below the bubble-point. Moreover, when the saturation of free gas is beyond the breakthrough saturation, oil banks do not form. Instead, the production of oil is accompanied by immediate and perpetually increasing production of gas (16). Oil viscosity affects fractional flow. Formation diptogether with gravity significantly improves the behavior of fractional flow, particularly where permeability is sufficiently high and rates of withdrawal are below gravity-stable conditions (Khan 7).

  1. Unfavorable Mobility Ratio Causes Instabilities in Viscous Flow

The displacements that happen at unconducive mobility ratios are unstable, thereby leading to viscous fingering. This is true for all gas/oil displacements, although more so when displacement happens horizontally. Such instabilities can have significant impacts on the displacement process. Although these figures are mainly on miscible displacement, the patterns they display also apply for immiscible displacement. In both cases, a small interruptions in the field of flow grew into a viscous finger after the perturbation stopped, leading to viscous fingering. In real reservoir situations, viscous fingering can result from the two physical aspects of: the variety of permeability heterogeneity styles; and the fact that gas is less dense than oil in a cross-section process of immiscible gas/oil displacement(Fred 142).

Compositional Effects of Gas/Oil duringDisplacement of Immiscible Gas

Regardless, generally, there are two types of compositional effects/interactions duringthe displacement of immiscible gas: swelling and stripping compositional effects. Moreover, the implications of the compositional effects during immiscible gas displacement vary depending on the composition of the gas to be injected and the oil into which the gas is injected as well as the pipelines and other surface facilities available in specific field circumstances.

  1. Swelling Compositional Effects

Essentially, this effect is when gas dissolvesinto and gets saturated in the oil phase. In this respect, if oil does not get saturated with gas under the pressure conditions in the reservoir or pressure in the reservoir increases as a consequence of gas injection, the volume of gas that dissolves in the oil gets saturated at the same pressure. As a result of the increased gas volume in the oil solution, the formation volume factor (FVF) of the oil also increases. This is what is known as swelling, and can significantly increase the efficiency of the gas/oil displacement process.

  1. Stripping Compositional Effects

Stripping (also known as vaporizing)is when various oil components transfer to the oil phase. In this regard, the lean injected gas vaporizes hydrocarbon oil components. Often, the injected gas is a lean natural gas, mostly the residue gas from nearby plants that process gas and constituted primarily of methane. Typically, the propane among other heavier hydrocarbon components have been condensed the gas produced. Ethane is produced from the gas at the time. When such lean injected gas comes into contact with the reservoir oil and in the reservoir conditions, they vaporize the oil’s various hydrocarbon components until the gas and oil phases reach compositional equilibrium. The stripping (or vaporizing) effect enhances the process of recovering hydrocarbons from the reservoir of oil. However, this impact varies depending on the type of oil. For instance, lighter oils have greater percentage of vaporization of oil components.

General Techniques of Immiscible Gas/Oil Displacement

  1. Types of Gas-Injection Operations

Usually, there are two types of injection operations: crestal and pattern gas injection. These classifications are dependent upon the location of wells for gas injection, among other factors.

  1. CrestalGas Injection

This is also called gas-cap or external injection and utilizes injection wells located in higher structural positions, including both primary and secondary gas cap. Generally, this technique is used in reservoirs that possess crucial structural relief or thick columns of oil that have good vertical permeability. To help this technique work better, injection wall must be in positions to provide strategic areal distribution as well as have maximum advantage of gravity-assisted drainage.

  1. Pattern as Injection

Also called internal or dispersed gas injection, pattern gasinjection consist of injection walls geometrically arranged to ensure uniform distribution of injected gas throughout the reservoir’s oil-productive portions. This technique mainly applies to reservoirs with low structural relief and low permeabilities as well as reservoirs that have low permeability.

  1. Optimum Time for the Initiation of Gas Injection Operations

This depends on a balance between the availability of gas market, risks and environmental considerations, among other factors that influence project economics. In relation to oil recovery and improvements on thee producing characteristics of reservoirs, gas injections operations are more favorable where the reservoir is at the oil bubble-point pressure or even slightly below.

  1. Efficiencies Relating to Recovery of Oil by Immiscible Gas Displacement

Continue gas injection increase recovery efficiencies. However, the rate of recovery declines after the occurrence of gas breakthrough. Ultimately, oil recovery efficiency depends on the economic considerations, including the cost of compression of gas as well as the availability and volume of potentially more expensive alternatives (such as Nitrogen) or lean residue gas.

Vertical or Gravity Drainage Displacement of Gas

In many cases, there is an original gas cap, so that the gas gets injected into the interval of gas cap for a cross-sectional view of anticlinal reservoir that has gas cap over oil column that possess dip angle and thickness (Juanes&Patzek 17). This is vertical drainage gas displacement and the force of gravity works in this situation; it tries to give the downward process of gas/oil displacement some stability by ensuring the gas stays on top of the oil as well as counteracting the process of gas/oil viscous displacement that is unstable. When the rate of the production of oil stays below the critical rate, the gas/oil contact (i.e.GOC) moves downward at the same rate.

Methods for Calculating Immiscible Gas Displacement

  1. Modifications of Displacement Equations

Whether basic displacement equations are applicable depends on whether the underlying assumptions that govern a particular reservoir area reasonable. In this respect, modifications of equations seek to reduce the demand of having to make assumptions.

  1. Methods for Evaluating Sweep Efficiency

One of the methods for evaluating sweep efficiency is the mathematically proposed concept of substitution index (SI). This is considered a practical method that has proved successful before, particularly in evaluating sweep efficiency during reservoir development, such as polymer flooding or water-flooding. The SI is said to reflect the degree of exchange in the pore between the initial oil generated from the microscopic view and the injected water. The SI also describes the performance of the sweep of the whole field from a microscopic view.

  1. Calculating the Performance of Immiscible Gas Injection

The best method for predicting immiscible gas injection performance (i.e. when there is enough data to characterize the fluids and rocks in the reservoir) is numerical simulation. Most mathematical calculation methods stem from- and are variations of- the fractional flow theory, developed by Muskat over 60 years ago (Juanes&Patzek 21).

Summary and Conclusions

Enhanced Oil Recovery (EOR) is a diverse area. Gas EOR, particularly the injection of immiscible gas into carbonate, is a complex process that involves various issues. This paper explores many of these issues. Due to the word-limitations on this paper, this exploration takes only brief look at these aspects: viscosity and density; compositional effects; geological considerations; gas displacementtechniques; calculationmethods for gas displacement, among others.

Works Cited

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Displacement in South Pars Oil Zone, University of Tehran, 2006. Print.

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Displacements with Gravity. Presented at the SPE Western Regional/AAP Pacific Section Joint Meeting, Long Beach, California, 19-24 May. Web, 26 June 2015

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