A Study of the Gravity Assisted Tertiary Gas Injection Processes
Authors: Ren W. , Bentsen R.G. , Cunha L.B. 
Published: March 2010

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.