Experimental and Numerical Investigation on Oil Displacement Mechanism of Weak Gel in Waterflood Reservoirs

Abstract

The production performance of waterflood reservoirs with years of production is severely challenged by high water cuts and extensive water channels. Among IOR/EOR methods, weak gel injection is particularly effective in improving the water displacement efficiency and oil recovery. The visualized microscopic oil displacement experiments were designed to comprehensively investigate the weak gel mechanisms in porous media and the numerical simulations coupling equations characterizing weak gel viscosity induced dynamics were implemented to understand its planar and vertical block and movement behaviors at the field scale. From experiments, the residual oil of initial water flooding mainly exists in the form of cluster, column, dead end, and membranous, and it mainly exists in the form of cluster and dead end in subsequent water flooding stage following weak gel injection. The porous flow mechanism of weak gel includes the preferential plugging of large channels, the integral and staged transport of weak gel, and the residual oil flow along pore walls in weak gel displacement. The profile-control mechanism of weak gel is as follows: weak gel selectively enters the large channels, weak gel blocks large channels and forces subsequent water flow to change direction, weak gel uses viscoelastic bulk motion to form negative pressure oil absorption, and the oil droplets converge to form an oil stream, respectively. The numerical simulation indicates that weak gel can effectively reduce the water-oil mobility ratio, preferentially block the high permeability layer and the large pore channels, divert the subsequent water to flood the low permeability layer, and improve the water injection swept efficiency. It is found numerically that a weak gel system is able to flow forward under high-pressure differences in the subsequent water flooding, which can further improve oil displacement efficiency. Unlike the conventional profile-control methods, weak gels make it possible to displace the bypassed oil in the deep inter-well regions with significant potential to enhance oil recovery.

Keywords: displacement efficiency; numerical simulation; oil recovery; weak gel.

Figure 1

Visual connection diagram of micro-displacement experimental device.

Figure 2

Water–oil displacement stage.

Figure 3

Distribution of residual oil after water flooding. (a) Cluster residual oil; (b) dead end residual oil; (c) columnar residual oil; (d) membranous residual oil.

Figure 4

Weak gel injection stage (The arrow is the demonstration of aquauous weak gel displacement orientation, and the red arc is the swept area).

Figure 5

Weak gel group (The red circle is the weak gel group).

Figure 6

Subsequent water flooding stage (The arrow is the demonstration of subsequent water displacement orientation).

Figure 7

Oil displacement in water flooding stage.

Figure 8

Oil displacement in weak gel injection stage.

Figure 9

Oil displacement in subsequent water flooding stage.

Figure 10

Preferential plugging of large channels.

Figure 11

Weak gel transport.

Figure 12

Subsequent water flow redirection.

Figure 13

Negative pressure oil absorption.

Figure 14

Accumulation of oil droplets into oil stream.

Figure 15

A 3D numerical model of reservoir.

Figure 16

Aqueous phase viscosity distribution at different stages of weak gel injection.

Figure 17

A 3D distribution of weak gel mole fraction.

Figure 17

A 3D distribution of weak gel mole fraction.

Figure 18

Planar distribution of weak gel mole fraction.

Figure 19

A 3D streamline distribution of initial water flooding.

Figure 20

A 3D streamline distribution of subsequent water flooding.

Figure 21

Planar streamline distribution of initial water flooding.

Figure 22

Planar streamline distribution of subsequent water flooding.