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Tight Gas Reservoirs Professor Ole Torsæter

11/11/11

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Unconventional natural gas resources

What is really considered unconventional natural gas changes with time. However, there are six main categories of unconventional natural gas:

• Deep gas • Tight gas • Gas-containing shales • Coalbed methane • Geopressurized zones • Hydrates

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Resource Pyramid

Small Volumes

Large Volumes

Incr

ease

d D

eman

d

Bet

ter T

echn

olog

y High Quality

Medium Quality

Low Quality

Coalbed Methane

Gas Shales

Tight Gas

1000 md

100 md 1 md

0.1 md

0.001 md

0.0001 md Con

tinue

d D

rillin

g an

d D

evel

opm

ent

4

5

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Tight natural gas

• Permeability less than 0.1md (1*10-16 m2)

• Porosity approx. 5-15% • Limestone or sandstone

formations • Not readily extractable. We must

put more effort into the extraction process than for conventional gas reservoirs; like fracturing, acidizing etc.

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Shale gas

• In Devonia shales (formed in shallow seas more than 350 million years ago).

• Permeability in the nanoDarcy range (10-9D or 10-21

m2 ) • Difficult to produce due to the properties of shale and

therefore the expected recovery of the gas is low. • Shale gas is found all over the world.

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Shale gas

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Statoil

From Statoil.com 1,8million acres=7300km2

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The Marcellus Shale

• Organic rich shale at a depth of approx. 1000m • The gas occurs in three ways:

– Within the pore space of the shale – Within fractures (vertical) and joints in the shale – Adsorbed on mineral grains and organic materials

• Most of the recoverable gas is contained in the pore space.

• Natural fracture system is important for well productivity.

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Coalbed methane

• Many coal formations (seams) contain natural gas, either within the seams itself or the surrounding rock.

• What was once a by-product of the coal industry is becoming an increasingly important source of methane and natural gas.

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Productivity of tight gas reservoirs

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Presentation Summary

• Some additional information about tight gas

reservoirs • Conditions generally required for economic tight gas

reservoir production • Common formation damage types occurring in tight

gas reservoirs

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‘Conventional’ vs 'Unconventional' Accumulations • Characteristics of ‘Conventional’ Accumulations

– Relatively high matrix permeability – Obvious seals and traps – High recovery factors

• Characteristics of ‘Unconventional’ Accumulations – Regional in extent – Diffuse boundaries – Low matrix permeabilities – Low recovery factors – Productivity of a tight gas reservoir is only 25% of gas in place in the best case

(average 10-15%). Gas connectivity only in 4% of porosity.

From John Lee (Texas A&M University) Advances in Unconventional Resources Technology: Assessment Methodology

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What controls the ability to economically produce tight gas reserves?

• Effective permeability to gas • Initial saturation conditions • Size of effective sand face drainage area accessed by

the completion • Reservoir pressure • Degree of liquid dropout from gas (rich vs. dry gas)

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Capillary equilibrium in gas reservoirs – High Perm

Water Saturation Water Saturation

Cap

illary

Pre

ssur

e - P

si

Rel

ativ

e P

erm

eabi

lity

FWC

Brant Bennion, presentation at Canadian Well Logging Society

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Capillary equilibrium in gas reservoirs – LOW Perm

Water Saturation Water Saturation

Cap

illary

Pre

ssur

e - P

si

Rel

ativ

e P

erm

eabi

lity

FWC

Brant Bennion, presentation at Canadian Well Logging Society June 9, 2004

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Generally if a tight gas matrix is in equilibrium with a free water contact, → equilibrium water saturation reduces reserves and effective permeability to gas below the economic limit for production

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Non - capillary equilibrium in gas reservoirs – LOW Perm

Water Saturation Water Saturation

Cap

illary

Pre

ssur

e - P

si

Rel

ativ

e P

erm

eabi

lity

NO FWC

Brant Bennion, presentation at Canadian Well Logging Society

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For significant reserves and mobile gas production in very low perm gas reservoirs, a CAPILLARY SUBNORMAL water saturation condition usually must exist

Water Gauge

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Subnormally water saturated tight gas reservoirs:

• A gas reservoir in which the initial water saturation is less than that which would be achieved on a conventional drainage capillary pressure curve at the effective capillary gradient of the reservoir

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Some interesting aspects of tight gas relative permeability.

23 Relative permeability of tight gas reservoirs

Demonstration cross-plot of drainage and imbibition gas-water relative permeability.

24 Comparison of relative permeability between conventional and unconventional gas reservoir

Conventional reservoir Unconventional reservoir

Conventional reservoir: wide range of water saturation where both water and gas can flow. Unconventional reservoir: broad range of water saturation in which neither gas nor water can flow.

Shanley et al. AAPG Bulletin 2004

25 Numerical study

Note crossover, where krg=krw is approximately 67% for all permeabilities but krg value at crossover decreases with decreasing permeability. Dark black horizontal line marks the krg = 2% (0.02). The Sw region where both gas and water have kr < 0.02 broadens as kik decreases.

26 Capillary pressure of tight gas reservoir

Comparison of capillary pressure between conventional and unconventional gas reservoir

Shanley et al. AAPG Bulletin 2004

27 Relationship between capillary pressure, relative permeability and position within a trap.

Shanley et al. AAPG Bulletin 2004

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Shanley et al. AAPG Bulletin 2004

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Fracturing of tight gas reservoirs.

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Performance of fractured wells in tight gas reservoirs.

Frac technique: horizontal well with multiple fractures Test rate: 14,000 m3/h vs 4,000 m3/h without technology

From Gaz de France

Seven Fracture system ready to open sliding sleeves

3.5 increase in well productivity

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Causes of productivity loss of fractured wells in tight gas formation

• Rheological fluid properties (affect fracture length)

• Polymer residus in the fracture (affect fracture conductivity)

– with cross-linked fluids – up to 90% of conductivity loss – optimization of cleaning fluids like

"breakers" • Filtration (affects reservoir properties)

– fracturing fluid invasion into the reservoir (decrease of the fracture length, reservoir damage around the fracture)

• Water blocks (affect gas back flow) – decrease the gas permeability

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Water blocks investigation. Aim: • Simulation of the filtration of the fracturing

fluid and evaluation of damage. Method • Cores at Swi (gas) • Fracturing fluid filtration with 200 bars on

core face • Gas backflow at various pressures • Water blocks identified by XRay

acquisition Results • Water saturation profiles • Gas return permeability Conditions of the equipment • Direct measurement of water blocks by

XRay • Temperature: 130°C, Pressure 330 bars

Gas Back flow 5, 10, 15 bars

Fracturing fluid filtration 200 bars

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x-ray

core holder

fluids

Tmax =130°C (210°F) Pmax=330bar (4700psi)

Flow rig at Institut Francais du Petrol.

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Workflow for improved predictions

Fracturing fluid selection- Lab study filtration, absolute permeability damage, measurement of in situ saturation (water blocks), petrophysics (capillary curves)

Modeling at laboratory scale

Near Wellbore Simulator

Well Productivity Evaluation

Modelling parameters cake properties, absolute permeability reduction, relative permeability curves

permeability hysteresis

Reservoir and fracture description

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Conclusions

•Tight gas reservoirs have a huge future potential for production.

•Generally to be economic tight gas reservoirs are normally in a subnormal water saturation condition.

•Fluid trapping (water blocks) tends to be a dominant damage mechanism for tight gas reservoirs.

•Productivity prediction is almost impossible without detailed petrophysics data measured in representative conditions (stress, hysteresis, capillary pressure...)