Reservoir Mid-Term

Modules 1-8

Give 3 examples of unconventional hydrocarbon deposits

1. Oil sands

2. Shale oil/gas

3. Coal Bed Methane

Name the 3 classifications of petroleum recovery methods

1. Primary

   • Reservoir is producing under its natural energy

2. Secondary

   • Pressure Maintenance Methods are needed

     • Injection of water, gas, or both to maintain reservoir pressure

3. Tertiary (EOR Methods)

   • Chemical Flooding, Thermal Recovery Methods (ie. SAGD) etc.

Name 3 natural drive mechanisms for an oil reservoir and 1 natural drive mechanism for a gas reservoir.

Oil Reservoir:

1. Solution Gas Drive

2. Water Drive

3. Gas Cap Drive

Gas Reservoir:

1. Gas Expansion Drive

What is petroleum?

A complex mixture of naturally occurring hydrocarbons

What is the latin root of “petroleum” ?

“petro” → rock

“oleum” → oil

• ie. “Rock Oil”

What are the 2 theories of petroleum origin?

1. Organic Theory

   • Solid “kerogen” is formed from buried organic materials and cooked and broken down under high P&T into liquids and gases.

2. Inorganic Theory

   • Not produced from living organisms but from carbon and hydrogen within the earth combining under the right P&T to create hydrocarbons.

Label the diagram of a hydrocarbon cap and seal

What is the simplest hydrocarbon combound?

Methane (CH₄)

What types of rocks are reservoir rocks?

Sedimentary

Which types of sedimentary rocks make up roughly 75% of all reservoir rocks?

Siltstone and shale

Name the 2 ways that reservoirs can be classified

Based on:

1. Reservoir Trap

2. Reservoir Fluids

Name the 3 types of Reservoir Traps

1. Structural

2. Stratigraphic

3. Combination

Traps formed when tectonic movement exerts forces on the subsurface strata structure. Strata is deformed and generate folds and faults.

Structural Traps

Traps formed as a result of depositional conditions in the sedimentary process.

Stratigraphical Traps

• Reef traps

• Unconformity traps

• Lens traps

Based on the phase diagram of reservoir fluids, petroleum reservoirs can be classified in what 3 ways?

1. Single Liquid Phase

2. Single Gas Phase

3. Two Phase (liquid and gas)

What is Reservoir Engineering?

Reservoir Engineering is the study of hydrocarbon reservoirs and their rock and fluid properties, in order to:

• calculate reserves

• forecast production

• design recovery schemes that maximize recovery and minimize cost

Maximum possible reservoir porosity is estimated at what percentage?

38%

Cubic packing (theoretical) → 48%

What type of recovery method can often produce up to 85% of the gas in a gas reservoir?

Primary

• ie. Gas expansion drive

Name the 3 main Core Analysis tests

1. Porosity

2. Permeability

3. Fluid Saturations

Define:

• Bulk Volume

• Grain Volume

• Pore Volume

Bulk Volume (BV): Total volume of a portion of a reservoir

• BV = GV + PV

Grain Volume (GV): Volume of the solid parts (ie. rock) in the bulk volume

• measured experimentally

Pore Volume (PV): Volume of the empty space in the bulk volume

• PV = BV - GV

• PV = (m_{wet core} - m_{dry core})/ρ_f

Refers to the affinity of fluids to adsorb on the rock surface

Wettability

Volume of hydrocarbon within pore volume

Hydrocarbon Volume

Ratio of Fluid Volume to Pore Volume

Fluid Saturation

• ie. S_o or S_g

Ratio of Pore Volume to Bulk Volume

Porosity (Φ)

What type of logging method can be used to measure Total Porosity? Explain how it works.

Acoustic Well Log

• Acoustic tool generates sound waves and sends them through the formation from source to receiver.

• Tool measures Sonic Transit Time → translates it into Formation Density and subsequently Formation Porosity.

  • Sound waves travel faster through solid rock than fluid in pore spaces.

  • The denser (less porous) the formation, the faster the sound wave travels.

  • The more porous the formation, the slower the sound wave travels.

What type of logging method can be used to measure Effective Porosity? How does it work?

Neutron Density Log

• Tool bombards the formation with neutrons.

• Measures dispersion and reflection back to tool.

  • The more porous the formation, the slower the neutrons rebound back to the tool.

  • The less porous, the faster the neutrons rebound back to the tool.
    

Porosity ________ as depth increases.

decreases

• due to higher overburden pressure from the weight of overlying rocks

Name some factors that affect the porosity of reservoir rock

1. Sorting

2. Cementation

3. Packing

4. Grain shape

5. Existence of fractures

6. Depth

Porosity formed when Reservoir Rock was originally created or consolidated (i.e. space between deposited grains).

Primary Porosity

Porosity formed after rock was created or consolidated.

Secondary Porosity

• Includes: effects of fractures, faults, dissolution, dolomitization, etc.

Give the grain densities of:

• Sandstone

• Limestone

• Dolomite

Sandstone = 2.65 g/cm³

Limestone = 2.7 g/cm³

Dolomite = 2.8 g/cm³

The minimum water saturation (S_w) in pore space

Irreducible Water Saturation (S_wirr)

The minimum oil saturation (S_o) in pore space

Residual Oil Saturation (S_or)

The minimum gas saturation (S_g) in pore space with connected paths filled with gas that flows by itself as a separate phase

Critical Gas Saturation (S_cg)

What 4 data points would you need in order to calculate the initial hydrocarbon volume in a reservoir?

• Reservoir average thickness

• Reservoir average area

• Average porosity

• Average fluid saturation

Which method used to calculate reservoir volume make use of contour maps of the reservoir?

Planimetric

Original hydrocarbon volume in place before any production

Resource

Volume of hydrocarbons that are economically recoverable using existing technology

Reserves

Total vertical reservoir rock thickness measured from top to bottom of the formation

Gross Thickness

In a reservoir, this is the total sum thickness of the layers that meet a porosity cut-off requirement

Net Thickness

The sum thickness of the productive layers of the reservoir

Net Pay Thickness

• ie. layers of the reservoir with:

  • Φ ≥ Φ cut-off

  • S_HC ≥ S_HC cut-off

Represents the reservoir storage capacity of hydrocarbons (O or G)

Saturation-Porosity-Thickness (SΦh)

Represents the reservoir pore space capacity of the rock

Porosity Thickness (Φh)

A special closed system with no energy crossing the boundary

Isolated system

A system with a fixed amount of mass. No mass enters or exits the system boundary.

Closed System (or Control Mass)

• Energy (heat or work) may cross the system boundary

What type of system is assumed for a reservoir?

Closed system

• vapour + liquid within closed boundary

The dividing surface between Liquid and Vapour Phases.

Interface

What would be the effect of increasing the temperature in the reservoir while keeping the pressure constant?

Vaporization of some molecules from liquid to vapour phase

What would be the effect of increasing the pressure in the reservoir while keeping the temperature constant?

Condensation of some molecules from vapour to liquid phase

What is the phase affinity for light hydrocarbons? For heavy hydrocarbons?

Light hydrocarbons → affinity for vapour phase

Heavy hydrocarbons → affinity for liquid phase

When there is no longer a driving force to push molecules from one phase to another, what state is the system said to be in?

Equilibrium

Name 4 types of equilibrium

1. Thermal Equilibrium

   • Entire system has constant T

2. Mechanical Equilibrium

   • Entire system has constant P

3. Chemical Equilibrium

   • Chemical composition (mole fraction) of phases within the system remains constant

4. Phase Equilibrium

   • Mass of phases within the system remains constant

When is a system said to be in thermodynamic equilibrium?

When it is simultaneously in:

1. Thermal Equilibrium

2. Mechanical Equilibrium

3. Chemical Equilibrium

The general term used for any mathematical equation that relates the pressure and temperature to the volume of a system

Equation of State

• ie: Ideal Gas Law, Van der Waals, etc.

What type of diagram represents equations of state in plot form?

Phase Diagram

What are the 3 assumptions used for the Ideal Gas Law?

1. High T, Low P

2. No interactions b/w gas molecules

3. No volume occupied by gas molecules

What two parameters does Van der Waals Equation introduce to the Ideal Gas Law?

1. Volume occupied by gas (b)

2. Attractive forces b/w gas molecules (a/V²)

Label this single component phase diagram with the following labels:

• Solid

• Liquid

• Gas

• Triple Point

• Critical Point

• Bubble Point/Dew Point Line

The region of a two phase diagram where both liquid and vapour are present

Phase Envelope

In a two phase diagram, is the bubble point and dew point line the same?

Negative

In a two-phase diagram, what single phase will exist at a high T and low P ?

• What about at a high P and low T ?

High T, Low P → single phase vapour

Low T, High P → single phase liquid

The point at which the bubble point line meets the dew point line on a two-phase diagram

Critical Point

The maximum pressure at which 2 phases coexist on a two-phase diagram

Cricondenbar

The maximum temperature at which 2 phases coexist on a two-phase diagram

Cricondentherm

A fluid that exists above the phase envelop on a two-phase diagram

Supercritical Fluid

Name the type of oil represented by each liquid-shrinkage curve on the graph

In this type of reservoir, when the pressure falls below the dew point pressure, some of the gas in the reservoir condenses into liquid hydrocarbons.

Retrograde Gas Condensate

In phase diagrams for the various types of gas reservoirs, which type has separator conditions (ie. P&T) that lie outside the phase envelope?

Dry Gas

• because there is only one phase (gas) present at surface conditions

On a P-T phase diagram for a pure substance, what is the name of the point at which the boundaries between the three phases - solid, liquid, and gas - intersect?

Triple Point

A test designed to simulate a pressure depletion process for black oil, which enables the R_so and B_o to be measured

Differential Liberation Test

An experimental procedure used to estimate the bubble point of a reservoir fluid

Flash Expansion Test

Refers to crude oil that contains dissolved gas in solution under high pressure, as it exists naturally in the reservoir

Live Oil

• live oil mass varies with the amount of gas dissolved in the oil

Crude oil that has no dissolved gas in it

Dead Oil

• the mass of dead oil is fixed because there is no dissolved gas in the oil

Refers to the reduction in the volume of crude oil when it is brought from reservoir conditions to surface conditions during production

Oil shrinkage

The point at which oil is fully saturated with gas

The Bubble Point

When the reservoir pressure is greater than the bubble point pressure, what type of oil is present?

Undersaturated Oil

• all the gas is dissolved in the oil

• no free gas in the reservoir

When the reservoir pressure is less than or equal to the bubble point pressure, what type of oil is present?

Saturated Oil

• at P_res=P_b the first bubble of gas comes out of the oil

• at P_res<P_b more solution gas comes out of oil as free gas, and oil and free gas coexist

Describe the volume and phase change during production

As the °API increases, the specific gravity of the oil __________.

decreases

As the °API increases, the density of the oil __________.

decreases

When pressure increases, R_so ___________.

increases

When temperature increases, R_{so ____________.}

decreases

• because gas tends to vaporize out of the oil at higher temperatures

At STP, the R_so will be _________.

zero (virtually)

When R_so increases, the dead oil °API _{____________.}

increases

When R_so decreases, the S.G. of the dead oil _{____________.}

increases

When R_so increases, the S.G. of the gas _{____________.}

increases

Explain the rising B_o with rising pressure until the P_b, and then the slight drop off in B_o with increasing pressure beyond the P_b

• Below P_b (Saturated Oil): B_o increases as pressure increases because more gas is absorbed into the oil with increasing pressure, expanding the volume of live oil.

• At P_b (Bubble Point): B_o is at its maximum, as the oil is fully saturated.

• Above P_b (Undersaturated Oil): B_o decreases slightly as pressure increases due to oil compressibility, while there is no longer any gas available to dissolve in the oil.

What is the pressure requirement in an oil reservoir in order for a gas cap to exist?

A gas cap only forms when the reservoir pressure is below the bubble point pressure because gas comes out of solution and accumulates.

If the reservoir pressure is above the bubble point, the oil holds all the gas in solution, and a gas cap cannot exist.

At STP, B_o is ___________.

1

• because at surface, only dead oil is present and so (dead oil)/(dead oil) = 1

When the dead oil °API increases, the B_{o ____________.}

increases

When the dead oil S.G increases, the B_{o ____________.}

decreases

When the gas S.G increases, the B_{o ____________.}

increases

The presence of dissolved gas ________ oil viscosity.

reduces

How does the viscosity of dead oil relate to the viscosity of live oil?

μ_{dead oil} > μ_{live oil}

@ P ≤ P_b, μ_{live oil} ________ when P increases.

decreases

• because there is still gas remaining to be dissolved, which will decrease the viscosity of the live oil

@ P > P_b, μ_{live oil} ________ when P increases.

increases

• because there is no more gas to be dissolved, and the oil compresses, leading to a higher viscosity

Will a high shrinkage oil have a high or low B_o ?

High

If an oil shrinks a lot as it is produced to surface, does the oil have a high or low shrinkage factor?

Low

• ie. shrinkage factor of .10 means that the oil will occupy 10% of the volume that it occupied in the reservoir

• shrinkage factor of .75 means that the oil will occupy 75% of the volume that it occupied in the reservoir

Therefore, low shrinkage factor = relatively large amount of shrinkage

What is the liberated gas volume at bubble point pressure?

0

What is the free gas volume in the reservoir at a pressure above the bubble point pressure?

0