| dc.description.abstract | One of the principal recovery mechanisms in Naturally Fractured Reservoirs (NFRs)
is spontaneous imbibition. Understanding the parameters affecting spontaneous
imbibition is the first step to achieve optimum oil recovery in NFRs. The effects
of changing flow rate of injected water, wettability, rock properties, capillary pres-
sure, fracture width and fluids viscosities are studied using a simpli ed model.
Furthermore, a linear transfer function, used to model spontaneous imbibition, is
evaluated.
A numerical model developed at the University of Stavanger is used to describe
fracture matrix flow. Transport due to advection takes place in the fracture and
capillary forces influence flow in the direction perpendicular to the fracture. Two
dimensionless parameters describing fracture matrix flow are studied: alpha
which is the ratio of the time for flow in the fracture to the time for flow in the
matrix and beta which is the ratio of matrix pore volumes to fracture pore
volumes.
After studying the dimensionless parameters, the model is modi ed by intro-
ducing a linear transfer function of the form T = B phi (Seq - Sw ) to account for
the mass exchange between fracture and matrix. Numerical simulations are used
to compare the modi ed and the original model. Physical parameters affecting the
rate constant B are investigated.
As a result, it was found that when waterflooding takes place, a preferentially
water-wet (PWW) system produces signi cantly more oil than a preferentially oil
wet (POW) system.
Varying the dimensionless parameters respect to a base case, the following
observations were made. When alpha increases at constant beta , water imbibes deep in
the matrix improving oil recovery. On the contrary when alpha decreases, poor oil
recovery is observed. When increasing beta at constant alpha , the water injection rate is
reduced, improving oil recovery. When increasing beta and decreasing alpha water takes
more time to travel in the fracture, hence there is more time for imbibition to
occur, which improves oil recovery.
When the viscosity ratio is low (water viscosity/oil viscosity = 1/5), more water imbibes in the
matrix compared to the base case ( water viscosity/oil viscosity = 1). Reducing the water viscosity
increases oil recovery by spontaneous imbibition.
The linear transfer function evaluated can reproduce the oil recovery curve
produced by the original model. However, this transfer function cannot reproduce
the linear behaviour of the recovery curve before breakthrough. Regarding the
rate constant B, the following relation was observed B proportional to K/phi. Where K is the
matrix absolute permeability and phi is the porosity. | nb_NO |
