Reservoir Seals; How Reservoir Seals; How they Work and How they Work and How to Chose a Good Oneto Chose a Good One

Charles Christopher and Charles Christopher and James James IliffeIliffe

BP plcBP plc

Sealing ProcessesSealing Processes

What holds hydrocarbons and CO2 in the subsurface?Three sealing processes…– Capillary Seals– Pressure Seals– Permeability Seals

Buoyancy ForcesBuoyancy ForcesSeals hold back fluid pressure caused by the buoyancy of the fluids. Oil floats on water because it is less denseThe fluid pressure (B) increases by the density difference between the fluid (ρf) and water (ρw) times the height (h) times gravity (g)

hg**)( fwB ρρ −=

hρf

SEAL

ρw

Pressure

Fluid Properties: Density and Fluid Properties: Density and BouyancyBouyancy

Fluid Buoyancy Pressures

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7

Pressure (MPa)

Dept

h fro

m C

onta

ct (m

) Oil

CO2

Gas

At Same Res. P&TOil Density = 700 Kg/m3

Gas density = 400 Kg/m3

CO2 density = 500 Kg/m3

For 1000 m column Gas exerts the highest pressure, then CO2.Before we can assess how well seal behaves we have to understand the interfacial tension and wettability

Pressure exerted by CO2 is intermediate between oil and gas for an equivalent column.

O CO2 G

Capillary SealsCapillary Seals

rPc

θγ cos*2=

Washburn (1921)Pc = Capillary Pressure (MPa)γ = Interfacial tension (N/m)θ = wetting angle (°)r = pore throat radius

• Capillary entry pressure is directly proportional to the interfacial tension– the greater the IFT between the fluid in the seal pore throat and the fluid trying to enter the pore throat, the greater the seal capacity! Petroleum

Driving Force

Resistive Force

PetroleumCO2

Driving Force

Resistive Force

Driving Force

Resistive Force

Driving Force

Resistive Force

Pore throat

qΘ

Interfacial TensionInterfacial TensionIFT is a vital parameter in the capillary force calculation IFT is a function of fluid type, p and t. After Hildebrand 2003

Oil has an IFT of approximately 25 mN/m)

Pressure, Mpa

CH4

At 1.4 Km depth – a pressure of 14 MPa CO2 water IFT is 25 (mN/m)

Methane Gas has an IFT of 50 (mN/m)

Capillary Pressure, Pore Radius Capillary Pressure, Pore Radius and IFTand IFT

The smaller the pore throat – the higher the capillary Pressure – so the better the seal Seals are between 10 and 100 nm pore throats

NOTE:

LOG-LOG plot

Capillary Pressure and Pore throat radius

0.000000001

0.00000001

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

0.00001 0.0001 0.001 0.01 0.1 1 10 100

Capillary Pressure (MPa)

Rad

ius

(m)

OilGas

Reservoirs

Seals

Geological Barriers

mm

um

nm

Combining Buoyancy and IFTCombining Buoyancy and IFT

Fluid Buoyancy Pressures

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7

Pressure (MPa)D

epth

from

Con

tact

(m)

Oil

CO2

Gas

What is the theoretical pore throat to hold a 1000 m column of each fluid?19 nm – oil14 nm – gas10 nm – CO2

CO2 requires better seals than other fluids for a particular column

IFT and Pore throat radius

0.000000001

0.00000001

0.0000001

1 10 100

Capillary Pressure (MPa)

Rad

ius

(m)

Oil & CO2Gas

3 5 6

Pore size radius: Mercury Pore size radius: Mercury Intrusion DataIntrusion Data

Top sample is a MUDDY SILTSTONE from about 4850 m below mudline, porosity 17%.Dominant pores are 2 – 20 nm. A very good seal.Algeria sample for Krechba– has a dominant pore throat of <10 nm. An excellent sealShallower – we see a sample indicative of a mixedlithology. A weak seal.

BP Algeria KB502, 1520m

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

1 10 100 1000 10000 100000

Pore Radius (nm)

Incr

emen

tal/C

umul

ativ

e P

oros

ity

0

0.05

0.1

0.15

0.2

0.25

1 10 100 1000 10000 100000

Pore radius nm

Cum

ulat

ive/

incr

eam

enta

l po

rosi

ty

BP Algeria KB502, 780m

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 10 100 1000 10000 100000

Pore Radius (nm)

Incr

emen

tal/C

umul

ativ

e P

oros

ity

Kv = 9.5E-21 m2

1 Darcy ~ 1e-12 m210 nanoDarcies ~ 1e-21m2

Capillary Seal SummaryCapillary Seal Summary

Fluid density, pore throat and interfacial tension all play a role in capillary sealsGas fields are good sites for storage, as the seals are proven.IFT effects reduces the column potential for CO2 in a gas fieldOilfields cannot hold as much (column) CO2 as gas fieldsThis is a density and IFT effect.

Pressure SealPressure Seal

A pressure seal is where the buoyant fluid is held back by the wall of water flowing downwards through the pores due to a pressure potential – which can be considered to be an equivalent column of water – or “head”.

Fluid Flow Goes From High HeadFluid Flow Goes From High HeadTo Low HeadTo Low Head

1000

2000

3000

4000

5000

6000

7000

8000

0 5000 10000 15000 20000 25000

Pressure (psi)

Dep

th (m

)

Hydrostatic Pressure (psi)Lithostatic Pressure (psi)Pressure (psi)

Flow occurs from high head to low head—this can be vertically upward as well as down or lateral!

HydrostaticLithostatic (Overburden)

Fluid pressure

Overpressure

Lithostatic = combined water/rock pressure

Pressure SealPressure Seal

Capillary pressure alone is not enough to hold back the column of CO2Combined with a downward head of water CO2 is trapped.Capillary

Head

h

p

Capillary

Permeability SealPermeability SealAfter the capillary pressure of a rock has been reached, flow becomes dominated by the relative permeability of the system.Darcys Law shows that flow will always take place – but at different rates depending on:-– Pressure differential– Relative Permeability (and saturation)– Fluid viscosity

EG. In the shale sample above with an intrinsic (rock only) permeability of 9.85e-21m2 (100nd), in a 20m thick bed holding back a pressure of 5MPa will flow at a velocity of <1 m per 1000 years or less. So for millenia storage this rock is a very effective layer. It would take at least 20,000 years for the flow to reach the next bed (20m). A silt with a permeability of 1 e-16 m2 (0.1md) will flow at 10 m/year – so it would take 2 years to get across.

Trap GeometryTrap Geometry

Number seals to define the trap. Identifying the weakest seal relative to the strongest upward force.The sealing process.– Fill or spill –maximum column

Column Height ControlsColumn Height Controls

First Question is …What is controlling my column? or …How do I want to control my column?Is it leaking or spilling?

Spilling

Give a minimum seal effectiveness

Leaking – through caprock

Provides a “true” seal effectivess for the fluid

Know it leaks because we detect hydrocarbons in the mudrocks above

Seal presence and continuity Seal presence and continuity Stratigraphic Trap with 5 seals

Dip-parallel channel pinching out (shale-out) on monocline. How many capillary seals?

5 : top, bottom and three lateral

13

2

+

_

3

4 5

Map (left): sub-crop to, and contours on, base top-seal. Section (right): north-south transect Which is going to be the weakest seal – depends on the lithology

Faults: And their role is…Faults: And their role is…Types of rocks affect faulting.– Soft, fine grained and unconsolidated– Deform easily and are ductile– Likely to smearThe more clay smeared in a fault the more capillary pressure the fault will have to a lateral sand across the fault. Harder, more brittle rocks will not smear as readily and faults likely will be more permeable.Faults may be propped open by pore pressure but even this permeability is unlikely to provide efficient vertical conduits.Volumes and rates will be quite low and on geological scales.

Fault Seal ExampleFault Seal Example

Shale Gauge Ratio and Cross Shale Gauge Ratio and Cross fault flowfault flow

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000 21000 22000 23000 24000 25000 26000 27000 280Arbitrary distance (metres)

View: arbitrary (Isometric)

Project: emsealUser: zrgg08

Date: Wed Aug 28 08:18:57 1996

Gouge ratio (average)

Z.119

em25

em25

0.000

25.0

50.0

75.0

100

25 ss HW HWC

25 ss FW HWC

spill point from HW to FW whereself-juxtaposed with low gouge ratio

Gouge Ratio

Shale Gauge Ratio values calculated using the local stratigraphyindicate that the fault-zone is unlikely to prevent the lateral movement of fluid.

Characteristics of a Good Characteristics of a Good SealSeal

Petrophysically has small pore throats without large connected poresHomogenous both vertically and laterallyLaterally continuousThick to reduce the number of pathways Has no “bypass” systems (sand injection features, faults, etc.)Water wet – to increase the capillary effects

Poor SealsPoor Seals

Larger pore throatsLithologically variableDiscontinuous layeringThin bedsFractured and faultedHydrocarbon wet Can be good for stacked reservoir systems to get high CO2 storage density – just as we do for hydrocarbons –provided a shallower super seal exists.

SummarySummaryAll seals have a threshold above which fluids will leakFine grained lithologies are essential for good seals.CO2 is lighter than oil so imparts more buoyancy pressure per unit. CO2 has lower interfacial tension than gas, so leaks easier than gas.The low permeability of fine grained rocks means that flow rates are extremely slow.Fluid flow UP faults may occur but at slow rates and at low volumes, flow ACROSS faults is much more likely.