Reservoir final exam

recovery stages

• primary: natural drive mechanisms & fluid expansion

• secondary: water flooding

• tertiary:EOR

displacement efficiency factors

• capillary pressure

• relative permeability

displacement efficiency, E_D

a measure of the effectiveness of the displacing fluid to mobilize the oil at those places where the displacing fluid contacts the oil reflected in:

• magnitude of residual oil saturation, S_or

• regions contacted by the displacing fluid

• time scale in which S_or is reached

sweep efficiency, E_S

a measure of how effective the displacing fluid sweeps out the reservoir volume (areally and vertically)

displacement efficiency formula

E_S = E_VS x E_AS

E = E_D x E_S

water flooding purposes

• maintain reservoir pressure

• displace oil from a water injection well to the production well

viscous drive

displacement of oil by applying a viscous force between the injected water and the oil in place

fractional flow

describe the relative movement of different fluid phases within a system - the relative flow velocity of water with respect to the total flow velocity

wave classification

• Spreading waves: wave becomes more diffuse on propagation

• Sharpening waves: wave is self sharpening and becomes less diffuse →the wave will become a shock, even if the initial condition is diffuse

• Mixed waves: Like the Buckley-Leverett wave

• Indifferent waves: neither spreading nor sharpening – might appear as shocks in the absence of dissipation

shock construction methods

• equal area

• wedge construction

equal areas method

based on the principle of conservation of mass and involves finding the shock front position by balancing areas on the fractional flow curve

shock waves

the properties of a surface of discontinuity of a solution of a first-order quasi-linear hyperbolic system of partial differential equations.

aerial vs. vertical sweep

sweep efficiency factors

• reservoir geometry

• formation heterogeneity

• displacement instability

• well pattern & placement

heterogeneity types

• vertical

• areal

fluid movement in vertical reservoir

A reservoir is rarely homogeneous in vertical direction, thus, the injected fluid will seek the path of least resistance to flow and will move through the reservoir as irregular front. It will travel more rapidly in the more permeable zones and less rapidly in the tighter zones. => reduction in vertical sweep efficiency and and an early breakthrough.

Lorenz coefficient, L

a single parameter that describes the degree of heterogeneity within a pay zone selection (0: homo - 1: het)

unfavourable (unstable) displacement

Middle layer is advanced front & has a higher fluid mobility than in the other layers → lower injection pressure, accepts more fluid, meaning front is running away from the main front

favourable (stable) displacement

Middle layer is advanced front & has a higher resistance flow than in the other layers → higher injection pressure, meaning main front is catching up with middle layer

two layer system

• low mobility ratio (M<1): favorable mobility ratio → high perm layer initially invaded faster than low permeable layers, but flow in high-K slows down with time due to increasing resistance to flow.

• high mobility ratio (M>1): unfavorable → high-K is invaded more easily and flow resistance is decreasing with time → displacement is unstable.