Page 48 - Fundamentals of Enhanced Oil and Gas Recovery

36 Amirhossein Mohammadi Alamooti and Farzan Karimi Malekabadi

Two or more of the natural recovery mechanisms are frequently active at the time of
natural reservoir production, one of which is the main and dominant one. But this
main mechanism might change as natural production progresses. The nature of the
main mechanism has great influence on the amount of oil recovered from the reservoir.

1.15.4.1 Dissolved Gas Mechanism
In most reservoirs, dissolved gas drive mechanism plays an important role in the oil
recovery. This mechanism is especially effective in fractured reservoirs. When the inner
reservoir pressure reaches bubble-point pressure, it becomes a saturated reservoir.
Bubbles within the reservoir expand as pressure drops. Among especially influential
elements to improve recovery coefficient is high API degree of the oil or low oil vis-
cosity, high dissolved gas to oil ratio, and homogenizing structures. With the exception
of high pressure reservoirs, which are under-saturated, and reservoirs with strong aqui-
fer, all oil reservoirs are controlled through dissolved gas energy mechanism during
their first years of life.

1.15.4.2 Gravity Drainage Mechanism
The gravity drainage process can occur in the reservoirs in two forms: free gravity
drainage process and forced gravity drainage. Free gravity drainage occurs in reservoirs
with high permeability and proper oil layer thickness that have reached low pressure
levels, while forced gravity drainage occurs in reservoirs with dual porosity. In these
reservoirs, gas proceeds to the highly permeable area (the fracture), and oil is left
behind in the low permeable areas (the matrix). Pressure difference between matrix
and fracture fluids provides the required force to drive gas from the fracture to the
matrix and to displace and produce oil. Forced gravity drainage is demonstrated in
Fig. 1.19. If the height of the matrix is affected by factors such as permeable layers or
fracture and is thus limited, a high amount of unrecoverable oil is expectable in such
reservoirs. While free gravity drainage in highly permeable structures with thick layers
results in little unrecoverable residual oil in the reservoirs, this applies where oil recov-
ery in a particular reservoir is considerably less than the critical level. In the case of
gravity drainage processes, two basic points are noteworthy: (1). When the fluid
moves under the influence of gravity drainage with a uniform pressure in the homo-
geneous reservoir, the flow is usually in the vertical direction. In other words, if we
consider a homogeneous porous block that is in equilibrium with the displacing phase
around the block and is applied in the gravity drainage, the production rate of the dis-
placing phase would be independent of the opening or closure of the vertical margins
of the block. (2). In forced gravity drainage, if the oil and gas contact area in the frac-
ture exceeds the block or is in direct contact with the horizontal plane of the block,
it would have little effect on the production of oil with time.