Fundamentals and Practical Aspects of Gas Injection

This chapter describes the different methods and the application of gas injection in the oil and gas industry and environmental purposes. For this purpose, first the worldwide hydrocarbon distribution is studied which categorized as conventional and unconventional resources. Also, the graphical distribution is reported using the latest statistics of conventional and unconventional hydrocarbon resources. Second, the steps of recovery of oil reservoirs are studied which consist of primary, secondary and tertiary oil recovery. Gas injection is performed for improvement of oil recovery in the secondary and tertiary oil recovery. Environmental protection and reduction of greenhouse gases is another goal of gas injection. Third, various relevant aspects including, historical evolution, screening criteria, sources of gas and economic of gas injection are investigated.

This chapter introduces the PVT challenges of gas injection. First, the phase diagram of various gases, oil samples and mixtures of oil and gas are investigated. In the next part, the important PVT experiments are discussed in details. These experiments consist of CCE (constant composition expansion), DL (differential liberation), CVD (constant volume depletion), flash separation and swelling test. Some calculation must be considered for designing the gas injection process as it is discussed in Sect. 2.4. In Sects. 2.5, 2.6 and 2.7, three cases are studied about change of phase behavior due to gas injection, MMP calculation of gas injection and asphaltene precipitation due to gas injection, respectively. The optimum design of gas injection and PVT challenges associated with gas injection are presented in Sects. 2.8 and 2.9, respectively.

This chapter deals with basic concepts of oil and gas flow in porous media. In the first section (oil flow), the basics of oil flow in porous media and different flow equations, including viscous, Darcy, and Brinkman flow are introduced. Then, based on a new work the boundary between these kinds of flow are reported. Then, the well-known diffusivity equation, combination of continuity equation and Darcy velocity, is introduced. The relative permeability as an important concept in two-phase flow through porous media is described as well. After that, the forces affecting oil flow in porous media are introduced as well as different dimensionless numbers which reflect the relative importance of forces during oil flow. The gas flow through porous reservoir can occur as single or multiphase. A second phase releases as a result of pressure decline in reservoir and starts flowing when its saturation exceeds a minimum value, known as critical saturation. At these conditions, the relative permeability concept applies to take into account the multiphase flow. Also, the classic Darcy’s law equation for flow in porous media may fail in certain flowing conditions, and the non-Darcy flow equations need to be studied and applied for proper modelling of gas flow in porous medium. In addition, the role of wettability alteration on production enhancement of gas condensate reservoirs is a novel approach in gas reservoir performance. Also, the active forces during CO₂ storage and the challenges of gas flow in unconventional gas reservoirs are two important subjects. These subjects will be covered in the second section (gas flow) of this chapter.

In this chapter, some of the key aspects of UGS as a sustainable energy supply infrastructure were reviewed. This type of gas injection is associated with the reuse of depleted oil and gas formations as a natural underground storage volumes with proved reservoir characteristics and cap rock integrity, as well as available surface facilities and wells. The operation has its distinct features in terms of planning, gas injection/withdrawal, and etc. Some of the common criteria for screening the UGS were also reviewed and discussed in this chapter.

Gas injection into the gas cap which is known as pressure maintenance or crestal gas injection is done to increase the reservoir pressure. Different types of gas may be injected in this method including producing gas, N₂, CO₂ etc. The injected gas is chosen base on the field development studies. Each of these gases has some advantage and disadvantages. Gas injection in naturally fractured reservoirs is a challenge which needs more investigation on this subject. This chapter summarizes the basic concepts of pressure maintenance and active mechanisms during pressure maintenance in naturally fractured reservoirs. Also, this chapter provides the essential concepts in simulation of pressure maintenance in fractured reservoirs.

This chapter provides a comprehensive study about gas recycling into the oil and gas reservoirs. First, the concept of gas recycling is studied. In general, gas recycling is mainly called to the re-injection of produced gas into the gas condensate reservoirs in order to maintain the reservoir pressure above the dew point pressure. But, the re-injection of produced gas into the oil and unconventional reservoirs is also called gas recycling. This chapter addresses the advantages and disadvantages of this method, gas recycling mechanisms in fractured reservoirs, the operation design and well patterns, economical aspects, studies and projects worldwide. Also, a detailed study of the re-vaporization mechanism during gas injection is investigated. In the case study, it is shown that gas recycling can significantly improve the gas and condensate production from a gas condensate reservoir.

Proper design of surface facilities in a gas injection project is of great importance from both engineering and economic points of views. A process engineer frequently deals with standards, protocols and codes in designing facilities available in the literature. In this chapter, engineering facets and basic designs of gas injection surface facilities including pipeline, compressor, intercooler and separator were introduced and discussed. A step by step procedure for initial design of different facilities were shown, and the fundamental concepts and equations for design of each facility were presented, supporting with case studies in each section so as to ensure the design effectiveness. All of the case studies were extracted from the field data, and thus, predicting these data with reliable agreement proves the proper design of facilities.

In this chapter, several methods will be discussed which can be used to predict the water content of gases. Most of these methods are based on equations of state and rigorous thermodynamic models. Different methods of predicting water content of acid gas systems are evaluated based on the literature experimental data. In addition, the water content diagrams compatible with the experimental data for pure CO₂, H₂S, CH₄ and their mixture are presented. These charts use for facility type calculations and trouble shooting. In another section, the gas solubility concepts will be reviewed. This section contains hydrocarbon gas solubility in water, non-hydrocarbon gas solubility in water, the calculation of gas solubility in water using Henry’s law constant and the effect of salinity on gas solubility. At the end of this chapter, a brief review has been conducted on the activity models.

Gas injection operations are faced with important challenges. These challenges must be studied carefully before the operation in order to increase the gas injection efficiency. This chapter provides a review of the most important issues for designing the different gas injection methods. The compatibility of fluids and rocks is discussed after the introduction part. In Sect. 9.3, corrosion in the different gas injection methods such as carbon capture and sequestration (CCS), acid and flue gas injection are investigated. Gravity override as one of the vigorous problems of gas injection is discussed in Sect. 9.4. Gas mobility control procedures are introduced in Sect. 9.5. In Sect. 9.6, cap rock integrity as one of the important issues in the gas injection operation is studied in detail. Phase trapping during the gas injection and HSE are discussed in Sects. 9.7 and 9.8, respectively.

After primary and secondary recovery, substantial amounts of hydrocarbon remain entrapped by capillary forces. This phenomenon, known as capillary trapping. Different factors affect the microscopic trapping mechanism such as pore structure, wettability, capillary pressure, interfacial tension, relative permeability, initial saturation, and other properties of the rock and fluids. Understanding the amount of trapped phase is vital for different applications such as enhance oil recovery (EOR), enhanced gas recovery (EGR), and carbon capture and storage (CCS). In EOR and EGR, the purpose is reducing residual oil saturation, while for CCS, it is opposite, i.e. the target is maximising the amount of trapped CO₂. There are two main capillary trapping mechanisms, snap-off and by-passing which are described in detail in this chapter. The laboratory methods and empirical mathematical models that are used to measure and predict the trapped phase saturation are discussed in this chapter. Furthermore, there are various techniques for the mobilization of the trapped hydrocarbon phase. Gas injection is one of the effective methods to remove the trapped phase. This chapter discusses some of the key aspects of phase trapping and mitigation methods.