advance reservoir engineering

Reservoir +

Rock Properties +

Fluid Properties +

Overview

1 Advance RE Dr. M.Ali

Reservoir

To form a commercial reservoir of hydrocarbons, a geological formation must possess three essential characteristics:

Sufficient void space to contain hydrocarbons (porosity). Adequate connectivity of these pore spaces to allow

transportation over large distances (permeability).

A capacity to trap sufficient quantities of hydrocarbon to prevent upward migration from the source beds(cap rock).


Reservoir Migration of hydrocarbon from source rock to reservoir rock and trapped by cap rock.


Reservoir

Rock Body Porous & Permeable Contains Oil (and/or) Gas + Water


Rock

Grains + Pores + Cementing Material


Porosity Porosity is the storage capacity of reservoir rock A rock can be made up of small grains or large grains

but have the same porosity Porosity depends on grain packing, not the grain size


Porosity classification


Permeability

The ability of the rock to transmit fluids through its pores

Darcys Law


Permeability classification

Absolute Perm. The ability to flow or transmit fluids through a rock, conducted

when a single fluid, or phase is contained in the pores (under the action of an applied pressure gradient).

Effective Perm. The ability to preferentially flow or transmit a particular fluid

when more than one immiscible fluids are present in the reservoir

Relative Perm. Relative permeability is the ratio of effective permeability of a particular

fluid at a particular saturation to absolute permeability of that fluid at 100 % saturation. It is normally assumed that the effective permeability is the same for all fluids at 100% saturation, this permeability is denoted as the (absoulte) permeability of the system.


Permeability and Grain size

Large Grain Size = High Permeability Small Grain Size = Low Permeability


Reservoir Fluids

Oil

Gas

Water

Oil Volume +

Gas Volume +

Water Volume

= Pore Volume


Fluid Saturations

Fluid Volume

Fluid Saturation = ----------------------

Pore Volume

So + Sw + Sg = 1


Fluid Saturation in Pores

Hydrocarbon Volume = x (1- Sw) Water Volume = x Sw Grain volume or Matrix = 1-


Water-Wet System Pertaining to the adhesion of a liquid to the surface of a solid. In water-wet conditions, a thin film of water coats the surface of the formation matrix, a condition that is desirable for efficient oil transport.


Oil-Wet System Pertaining to the preference of a solid to be in contact with an oil phase rather than a water or gas phase. Oil-wet rocks preferentially imbibe oil. Generally, polar compounds or asphaltenes deposited from the crude oil onto mineral surfaces cause the oil-wet condition.


Hydrocarbons A naturally occurring organic compound comprising hydrogen and

carbon. Hydrocarbons can be as simple as methane [CH4], but many are highly complex molecules, and can occur as gases, liquids or solids. The molecules can have the shape of chains, branching chains, rings or other structures. Petroleum is a complex

mixture of hydrocarbons. The most common hydrocarbons are natural gas, oil

Methane Propane

CH4 C3H8


Aromatics A type of compound containing hydrogen and carbon atoms arranged in a symmetrical 6-carbon ring structure with single (C-C) and double (C=C) bonds alternating around the ring. Rings are single, multiple or fused and can have other chemical groups attached in place of hydrogen. Benzene, C6H6 is the simplest single-ring aromatic, napthalene, C10H8, the simplest fused-ring aromatic and toluene is the simplest aromatic, having an alkyl side chain, C6H5-CH3. Xylene, a common oilfield chemical, has two methyl side chains, C6H4-(CH3)2.


Phase Diagram A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium.


Reservoir Pressure The pressure of fluids within the pores of a reservoir, usually hydrostatic pressure, or the pressure exerted by a column of water from the formation's depth to sea level. Because reservoir pressure changes as fluids are produced from a reservoir, the pressure should be described as measured at a specific time, such as initial reservoir pressure.


Reservoir Temperature The reservoir temperature represents the temperature of the formation. It increases with reservoir depth and differs widely depending on the reservoir location's geothermal gradient. Although the thermal gradient varies from place to place, it averages 25 to 30 oC/km [15 oF/1000 ft].


Capillary Pressure

The pressure difference existing across the interface separating two immiscible fluids in capillaries (e.g. porous media).

One fluid wets the surfaces of the formation rock (wetting phase) in

preference to the other (non-wetting phase) Gas is always the non-wetting phase in both oil-gas and water-gas systems. Oil is often the non-wetting phase in water-oil systems.

Pc = pnwt - pwt

Pc = capillary pressure

Pnwt= pressure in nonwetting phase

pwt = pressure in wetting phase


Capillary Tube

Considering the porous media as a collection of capillary tubes provides useful insights into how fluids behave in the reservoir pore spaces.

Water rises in a capillary tube placed in a beaker of water, similar to water (the wetting phase) filling small pores leaving larger pores to non-wetting phases of reservoir rock.

Air-Water System


The height of water in a capillary tube is a function of:

Adhesion tension between the air and water

Radius of the tube

Density difference between fluids

h = Height of water rise in capillary tube, cm

aw = Interfacial tension between air and water, dynes/cm

= Air/water contact angle, degrees

r = Radius of capillary tube, cm

g = Acceleration due to gravity, 980 cm/sec2

aw = Density difference between water and air, gm/cm3


Combining the two relations results in the following expression for capillary tubes:

From a similar derivation, the equation for capillary pressure for an oil/water system is:

Pc = Capillary pressure between oil and water

ow = Interfacial tension between oil and water, dyne/cm

= Oil/water contact angle, degrees r = Radius of capillary tube, cm


Capillary Curve


Drainage and Imbibition capillary Pressure curve


Reservoir Flow Forces

psi/ft u,k0.00633

= Force Viscous

Gravity Force = 144

, psi/ft

S, psi/ft dS

dP = PForce = Capillary

cc

To analysis of flow behavior in terms of force , there are three major forces; viscous, gravity and capillary forces:

where, S is the saturation of interest. Inspection of these equations gives us some understanding of the forces. For example, we could say that

1. If there is no flow then viscous force = 0.

2. If there is no density difference (single phase, for example) then gravity force= 0.

3. If there is no saturation gradient(single phase, for example) then capillary force= 0.


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