Pore-Level Observation of Gravity-Assisted Tertiary Gas-Injection Processes
- W. Ren (U. of Alberta) | R. Bentsen (U. of Alberta) | L.B. Cunha (U. of Alberta)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2004
- Document Type
- Journal Paper
- 194 - 201
- 2004. Society of Petroleum Engineers
- 5.3.4 Reduction of Residual Oil Saturation, 5.4.2 Gas Injection Methods, 5.2 Reservoir Fluid Dynamics, 5.5 Reservoir Simulation, 5.1 Reservoir Characterisation, 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 6.5.2 Water use, produced water discharge and disposal, 5.3.2 Multiphase Flow, 5.4.1 Waterflooding, 4.6 Natural Gas, 2.4.3 Sand/Solids Control, 1.6.9 Coring, Fishing, 1.2.3 Rock properties, 5.7.2 Recovery Factors, 5.4 Enhanced Recovery
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In waterdrive oil reservoirs, more than half the initial oil in place (IOIP) is trapped in the water-contacted zone after natural water influx or waterflooding. Gas injection into such reservoirs, with the assistance of gravity, interfacial tension, and oil-film flow, can cause the displacement of excess water and the redistribution of reservoir fluids in the pore space. As the result of such fluid redistribution, most of the residual oil can be recovered. Moreover, a second waterflood following the gas injection can recover the oil in a shorter period of time.
Gravity-assisted tertiary gas-injection processes include the double-displacement process (DDP) and the second-contact water-displacement (SCWD) process. The DDP consists of injecting gas into waterflooded oil zones. The SCWD process consists of submitting these gasflooded zones to a new water-displacement process. In this work, the DDP and the SCWD process were conducted in a transparent sandpack micromodel, and a pore-level observation was performed to investigate the microscopic mechanisms of the two processes.
Observation of the two processes confirmed that the oil films play a very important role in achieving high recovery efficiencies in the DDP. The oil film was seen clearly; such observation also showed that oil flowing through oil films and layers was driven not only by its own weight but also by the increasing volume of the gas. In the SCWD process, trapped gas reduces the possibility of the residual oil being trapped in the center of the pores. Consequently, residual oil can be recovered quickly by a second waterflood. Therefore, the SCWD process is suitable to apply in situations in which the source of gas is not sufficient and in which the formation has a high irreducible gas saturation.
A waterflood can recover only 40 to 60% of the IOIP in conventional oil reservoirs. However, it has been shown in the laboratory that nearly 100% of the IOIP can be recovered by tertiary gas injection in the presence of connate water.1 This tertiary recovery method involving the updip injection of gas into steeply dipping, high-permeability, strongly water-wet, light-oil reservoirs to recover the residual oil is called the gravity-assisted tertiary gas-injection process. It is also known as the DDP because it involves the use of gas to displace the oil remaining after a waterflood. The high recovery efficiency made the DDP such an attractive process that numerous laboratory studies2-10 of the DDP have been conducted in different media to investigate the mechanisms of the process. The media used in these studies include sandstone core, glass beads, and network models. Moreover, four field tests have proved the technical feasibility of the DDP.11-17 On the basis of the field data, a numerical simulation study has been conducted.18 The results of this study showed that the DDP is a promising tertiary-oil-recovery method and that it can be improved further by applying the SCWD process.
Besides gravity drainage, it has been suggested that oil-film flow plays a most important role in the process. Imagine a steeply dipping waterflooded reservoir in which gas is being injected into the crestal region of the reservoir. After a short period of gas injection, a gas cap appears, and an oil bank is formed ahead of the gas front. The gas front is stable and moves slowly downward to push the oil bank toward the producing wells. The oil in the gas-swept zone may form a thin oil film if the spreading coefficient of the oil is positive. The oil films re-establish the hydraulic continuity of the oil by connecting all the residual oil to the oil bank. Oil flowing through the oil films contributes to the development of the oil bank. When the oil bank reaches the production wells, oil production begins. The production characteristics of the process are that the main oil production takes place in a relatively short period of operating time and, after gas breakthrough, the oil is produced at a very low rate because the oil flows mainly through the oil films. Given sufficient time, the flow of oil through the oil films can result in very low oil saturation. However, the long production time at a low rate is detrimental to the economic success of the process. When such is the case, the SCWD process can be used, provided the conditions are favorable. The SCWD process was introduced by Lepski et al.19 to shorten the operating time of the DDP, and it is referred to as an extension of the DDP.
In investigating the microscopic mechanisms of the DDP, most studies used glass network models for pore-level visualization.2,4,5,9,20 The network model is a 2D etched-glass micromodel. The advantages of the network model are that the characteristics of simple porous media can be controlled carefully and that multiphase flow can be observed easily. However, owing to simplification of the model, some important features are lost. For instance, it is impossible to simulate the irregular spatial distribution of natural pore space, and complex multiphase flow in a 3D reservoir-rock system is much different from that in a 2D network system. Therefore, in some respects, the network model is not representative of reservoir rocks.
To achieve a better understanding of the pore-level mechanisms of the DDP and the SCWD process, a transparent 3D sandpack model was constructed. This model is called a transparent cell. The updip tertiary gas injection and the second waterflood were conducted in the cell to observe how fluids flow and, especially, to observe how the mobilization of the residual oil in a single pore takes place after gas invasion or after second water invasion. Of greatest interest is to see whether oil films and oil-film flow occur in this 3D model and to understand the mechanisms underlying the SCWD process in a single pore space. In addition, the water/gas, oil/gas, and oil/water interfacial tensions were measured to determine the spreading coefficient of the oil over the water.
The construction procedure for the cell was similar to the procedure used by Lepski et al.19 However, the cell was improved so that experiments under pressures of up to 50 psi could be conducted. Moreover, the cell was built in such a way that the experiments could be conducted more easily and safely.
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