Water-drive Oil Reservoirs Research Articles

How pH induced surface charge modification explains the effect of petrophysical and hydrological factors on recovery trends of water drive gas reservoirs

It is generally known that water drive oil reservoirs are more efficient than water drive gas reservoirs. In the petroleum literature, several studies have documented recovery trends based on petrophysical factors other than the wettability factor.Consistent with the high concentration of carbon dioxide in natural gas systems, the pH induced wetting transition reported in petroleum recovery research associated with low salinity water flooding is a potential factor that can impact gas recovery from natural gas systems. In this paper, we have reviewed extensively the fundamental aspects of pH induced wetting transition associated with surface charge regulation in different lithologies. In so doing, we have developed theoretical models that aid explanation of gas recovery trends from different lithologies associated with natural gas systems under aquifer drive. Most importantly, putting our models in the context of fundamental theories associated with pH induced surface charge modification, we have successfully correlated the literature based wetting transition trends of gas recovery to petrophysical properties of absolute permeability and rock type as well as hydrological effect associated with aquifer type.

A Study of the Gravity Assisted Tertiary Gas Injection Processes

Abstract Gravity assisted tertiary gas injection processes can produce a large amount of incremental tertiary oil from water drive oil reservoirs. These processes include the Double Displacement Process (DDP) and the Second Contact Water Displacement (SCWD) process. A transparent sandpack micromodel was developed to conduct a pore-level observation to investigate the microscopic mechanisms of the DDP and the SCWD processes. Observation of the two processes confirmed that oil films play a very important role in achieving high recovery efficiencies in the DDP. In the SCWD process, trapped gas reduces the possibility of residual oil being trapped in the centre of the pores in the second water flood. Moreover, reservoir simulations at reservoir scale were performed to investigate the macroscopic level mechanisms of the two processes. The results have shown that both processes are efficient methods to recover waterflood residual oil. Introduction A waterflood can only recover 40% - 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). Recoveries of 85% to 95% of the OOIP have been reported from field tests(2 –4). This tertiary recovery method involving the up-dip 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 Double Displacement Process (DDP) because it involves the use of gas to displace the oil remaining after a waterflood(2). The high recovery efficiency made the DDP such an attractive process that numerous laboratory studies(5 –13) of the DDP have been conducted in different media to investigate the mechanisms of the process. Kantzas et al.(5, 6) showed that gravity drainage played a very important role in this process. They suggested that reservoir wettability and spreading coefficient had a great impact on the gravity assisted tertiary gas injection process. A strongly water-wet porous medium and a positive spreading coefficient were preferable in this process, and the process efficiency was dependent on the spreading phenomenon. Oren et al.(8) studied the effect of the spreading coefficient on oil recovery using a network model. Their experimental results showed that oil recovery was significantly higher for positive spreading systems than it was for negative systems. Vizika et al.(14) and Mani and Mohanty(15) confirmed these results by conducting gas gravity drainage experiments in a sandpack and a network model. The incremental oil recovered by the process consists of two parts. The first part is the bypassed oil, which exists as a continuous oil phase in the regions of the reservoir unswept by water due to reservoir heterogeneity or well placement. The second part is the residual oil existing at the microscopic scale as isolated oil blobs in the water swept regions of the porous medium due to the capillary and surface forces. The bypassed oil is recovered because gas injection improves the sweep efficiency. The trapped oil is recovered by oil film flow.

Pore-Level Observation of Gravity-Assisted Tertiary Gas-Injection Processes

Summary 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 water-flood. 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.

Computer Program on Water Drive Oil Reservoir

The authors present here a computer program (FORTRAN IV) for calculations on eater drive oil reservoir. The program developed in this paper combines the calculations of determining the original oil in place and predicting the reservoir performance, and gives promise of treating a large quantity of these complex calculations with profitable speed and precision. To decide the optimum values of the original oil in place and the water influx in the program, the slope of curves on original oil vs. production time must be calculated, and those values in the case that the absolute value of the slope is minimum will be selected as the optimum value. Successively, the prediction of the field performance is carried out with the optimum values decided.