Design of a Gravity Assisted Immiscible Gas Injection Program for Application in a Vuggy Fractured Reef

ABSTRACT

The potential of gravity assisted immiscible gas injection was evaluated for a waterflooded vuggy fractured reef. Extensive laboratory work was carried out for the study of the effects of wettability and heterogeneities on recovery. Tests were performed on 2-D glass micromodels, unconsolidated systems and reservoir core plugs. The recovery due to gas injection was correlated to the recovery due to waterflooding for each test. The laboratory results indicate that an incremental recovery of up to 20% original oil in place (OOIP) should be expected from gas injection even from a heterogeneous fractionally wet formation. In this paper, an overview of the laboratory and preliminary design work is presented. A discussion of the mechanisms that take place during the gravity assisted immiscible gas injection in a fractured vuggy reservoir with fractional wettability is also included.

Introduction

Gas injection is the oldest of the fluid injection processes. The intention of recent gas injection projects has been to increase the ultimate recovery and therefore gas injection can be considered an enhanced oil recovery process.

Gas injection may be either a miscible or an immiscible displacement process. In this paper only immiscible gas injection is considered. Of particular interest to this work is the immiscible injection gas in the carbonate reefs of Western Canada. The primary problems with gas injection in carbonate reservoirs (1) are the high mobility of the displacing fluid and the wide variations in permeability. Much greater control over the injection process must be exercised than is necessary with waterflooding. The benefits obtained by the gas injection are dependent upon horizontal and vertical sweep efficiency of the injection gas. The sweep efficiency depends oil the type of porosity system present and on the gasflood design. In general, immiscible gas injection better suited to vertical displacement schemes than horizontal displacement schemes.

Oil fields that have low oil saturation in either primary or secondary gas cap are prime candidates for improved recovery by gas injection into the gas cap (1). Further shrinkage of oil is reduced by gas injection. Inasmuch as it maintains a relatively high pressure gradient in the oil phase, relative permeability to oil remains high and oil is produced faster and in greater quantity.

Considerable insight and a large number of demonstration experiments dealing with the use of gravity assisted immiscible gas injection (GAIGI) as a novel EOR process is given in Kantzas et al. (2, 3) and in Chatzis et al. (4). In these references description of the pore level mechanisms are given and extremely high recoveries of waterflood residual oil are reported for unconsolidated systems. Also high recoveries are reported for Berea sandstone. It was explained that when oil spreads on water, the invasion of gas displaces residual oil from large pores in water-wet systems. This oil forms a thin film which becomes thicker as the displacement evolves. An oil bank is finally formed which flows downwards and sweeps the reservoir. This approach extends gas injection strategies to reservoirs which are too shallow for miscible floods. Also it avoids the use of any expensive surfactant systems.

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