Shale Gas Experience from a Global Gas Company Perspective 25 - - PowerPoint PPT Presentation

Shale Gas Experience from a Global Gas Company Perspective

25 October 2011

Alex Gabb

Agenda

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• BG Group business and Shale Gas.
• Overview of Shale.
• How is shale gas appraised and developed?
• Technological challenges that BG considers need to be addressed in order

to put more science into what has been a largely empirical understanding to date.

Global LNG: growing a global business

*exclusive right to supply 3

Global assets, supply and markets

Potential liquefaction Existing long term supply source Liquefaction under construction Equity position Existing import capacity Long term customer Potential import capacity UK USA Nigeria Egypt Trinidad & Tobago EG Chile Singapore* Australia Tanzania Brazil China Japan Italy

Shale Gas Basins of the World

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Shale – An Outcrop

What makes a good shale?

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Gas In Place

Free and Adsorbed ‘Resource Density’

Matrix Porosity/Permeability

Porosity >4% & Micro/Nanodarcies

Organic Richness

High TOC >2% and Adsorbed Gas Content

Thermal Maturity

Degree of ‘Cooking’

Containment

Frac’ Containment

Reservoir Pressure

Typically Overpressured; 0.6 psi/ft upwards.

‘Fracability’

Able to Initiate and Propagate and Complex Fracture Network

Unlikely that you can get your shale to work if you don’t have all of these!

Shale Formations – Finding the right sort!!

0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000 10000 100000 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 Permeability (mD) Porosity (Frac.) Conventional Oil Shale

Matrix Permeability & Porosity

8 Conventional Gas ~100 – 0.1 mD Conventional Oil ~100 mD - 10D Shale Gas ~0.001 – 0.0001 mD = Microdarcies – Nanodarcies! Typical Limit of ‘Standard Core’ Measurements.

Shale in Microscopic Detail

500 nm

Quartz + Other Minerals Phyllosilicates Pore Kerogen

Connected Pores (Blue); Kerogen (Green); Isolated Pores (Red)

Gas within complex pore system with even more complex flow physics.

• Pore structure has similar dimensions to the gas molecules themselves.
• Darcy Flow versus Diffusive Flow.

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Adsorption/Desorption Mechanisms

Adsorption

–

Adhesion of a single layer of gas molecules to the internal surface of the coal or shale matrix.

–

Physical Process versus Chemical Desorption

–

The process whereby adsorbed gas molecules become detached from the pore surfaces and take

• n the kinetic properties of free gas.

• Determined from Canister Tests.
• At Reservoir Temperature? Possibly not!
• Lost Gas + Measured Gas + Crushed Gas.
• In shales this is a combination of free and

adsorbed gas.

• Varies according to TOC%; so need to

take enough samples to characterise the reservoir interval. This is more than one sample!

How do we measure Gas Content?

Data should be considered qualitative

Methane Isotherm Results

10 20 30 40 50 60 70 500 1,000 1,500 2,000 2,500 3,000

Pressure, psia Methane Storage Capacity, scf/ton

48.5 scf/ton 18.9 scf/ton TOC = 4.97 wt. % TOC = 1.98 wt. % 50 100 150 200 250 300 1,000 2,000 3,000 4,000 5,000

Pressure, psia Gas Storage Capacity, scf/ton

Methane Carbon Dioxide Ethane Mixture 200

40 80 120 160 200 2 4 6 8 10 12 14 16 18 20

Gas Content (scf/ton) TOC (Wt. %)

Antrim Shale New Albany Shale

Gas Transport in Shale : An Analogy

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Shale Gas Log Responses

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USA Haynesville

Caliper/PEF GR 0-200 NPHI-RHOB DT RT-RXO

• Borehole conditions are typically good; low clay content = hole stability.
• GR is very high; Marcellus shales > 800 API
• Density – Neutron

–

Less Shale separation; Cross-over due to gas kerogen and lack of water.

• Resisitivity; Usually quite high > 20 ohmm

–

Low Rt caused by water-wet shales or graphite (in some over-mature areas)

• PEF; Often > 30 due to heavy minerals

Rock (Density Log) Total Porosity Total Porosity (GRI Method) Matrix V Clay

Bulk Minerals Clay Layers

Kerogen

Clay Surfaces & Interfaces Small Pores Large Pores

Hydration

• r

Bound Water Irreducible or Immobile Water Hydrocarbon Pore Volume

Total Saturation (GRI Method)

Modified from Hill et al, 1969

Capillary Water Structural Water (OH)

• Kerogen

Wellsite Canister Homogeneous Wholecore sections

(0.3 to 0.5m, canisters at reservoir temp.)

Measure Gas Composition during Desorption (90 days approx.) Halve and quarter sample using diamond saw Langmuir Isotherm Analysis

Half canister sample Quarter canister sample Quarter canister sample

Grain Density, Porosity, Total Organic Content, Rock Eval Pyrolysis, Vitrinite Reflectance, XRD & XRF Mineralogy Residual/Crushed Gas Analysis

Select Fresh State Core Samples (approx 500g) Fresh State Bulk Density Matrix Permeability Crush Sample

Shale Gas Reservoir Core Analysis

Wireline or fast retrieval of core recommended to minimised lost gas

Dean-Stark Analysis for Sw, So & Sg Grain Volume and Grain Density High Pressure Mercury Injection SEM (Argon/iron beam milling of surface) Select Fresh State Wholecore Samples for Rock Mechanics Tests (Triaxial Static Tests, Vp & Vs) Fluid Sensitivity Tests (Clay swelling & fracture flow tests)

Core Description

Gas Shale Core Analysis

• Standard or conventional methods of core analysis for porosity,

saturations, and permeability are unsuitable for Gas Shales

• Porosity requires sample cleaning of a plug and a Boyle’s Law porosity

using Helium

– Difficult to take plugs in many shales due to bedding plane partings – Measurement requires equilibrium to be obtained which requires a

long time in nano-darcy permeability and diffusion rates

• Permeability measurements on plugs have the same problem and must

use pressure decay techniques for the low permeability ranges

Objective: Create a high conductivity crack within the reservoir

• Rock is split using liquid that is pumped

under high pressure

• Tiny split or fracture held open using

proppant

• Gas flows from the fracture

Principals of Hydraulic Fracturing

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It is also imperative that the fracture system stays open.

What System Do We End Up with?

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Complex Pore System

Complex Fracture System Complex Well Geometry in a Tight Reservoir

Key Differences : Conventional and Shale Gas

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Characteristic Conventional Gas Shale Gas Generation Gas is generated in the source rock and then migrates into the reservoir. Gas is generated and trapped within the source rock. Gas Storage Mechanism Compression. Compression and adsorption. Gas Produced Free gas only. Free and adsorbed gas. Production Performance

• Minimal transient period followed by a long boundary-

dominated flow period.

• Production rates are mainly relatable to permeability

and declining reservoir pressure..

• Very long transient (linear) flow period that can extend many
• years. In some cases, it is debatable if boundary-dominated

flow will ever be fully realized.

• Production rates are mainly relatable to the success of

creating a large fracture network around a long horizontal wellbore and to the matrix permeability. Recovery Factors

• Recovery factor = 50% – 90%
• Recovery factor = 15% – 40%

Conventional Gas Shale Gas

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What did Arps intend? And what do we do?

• Most decline curve analysis is based on the Arps Equation (or set of equations!)

which was presented in 1945.

b = 0; Exponential Decline 0 < b < 1; Hyperbolic Decline b = 1; Harmonic Decline

• Supposed to be a constant pressure steady-state solution; in a shale gas well typically we

would not have this condition.

• b = 1 intended as a special case since implies infinite recovery at infinite time; this implies an

unbounded system; the use of b>1 is common place in the production analysis of shale gas wells.

• Shale Gas well is almost always in TRANSIENT FLOW .. Arps is intended for a STABILISED

FLOW scenario.

5000 10000 15000 20000 25000 5 10 15 20 25 30 Gas Rate (mscf/d) Year Haynesville Marcellus

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Typical gas shale production profiles

Gas Desorption and Diffusive Flow leads to long production ‘tail’

Field Haynesville Marcellus IP (mscf/d) 20000 5000 Di (%) 80 68 B 1.1 1.3 Dt (%) 6 6 EUR30 (bscf) 7.380 4.301 IP30 (mscf/d) 17676 4537 Typical Duration of Production Dataset Typical Duration of Production Forecast (with minimal understanding of reservoir physics!!) Risks

• Water and/or Condensate Hold-Up.
• Well Integrity (or Lack of)
• Reservoir Compaction

1 4 3 2

500 1000 1500 2000 2500 3000 3500 4000 4500 5000 1 2 3 4 5 6 7 8 9 10 Gas Rate (mscf/d) Year

Rate Transient Analysis

Characterized by infinite-acting linear flow into exposed fracture surface area. Characterized by quasi-steady depletion of SRV Characterized by transient infinite-acting linear flow into external faces of SRV. Boundary-dominated flow characterized by quasi- steady flow from the depletion volume into external faces of SRV

1 2 3 4 10 days 100s days 1000s days 10000s days

Analysis Options

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• Stretched Exponental Production Decline (SEPD)

– Avoids the requirement for key parameters to vary with time but may require a

large population of wells in order to constrain parameters effectively.

– Ref. John Lee, Valko, Ilk et al.

• Root Time Methodologies

– OK if the well is in transient linear flow but the deviation from linear trend

indicates transition to boundary dominated flow.

• Simulation

– A large number of input parameters; which are poorly defined and constrained,

not least the characteristics of the Stimulated Rock Volume (SRV).

– BUT also in-situ matrix permeability and desorption-diffusion coefficients.

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Shale Gas Development

• Gas shale developments utilise

horizontal wells and multi-stage fracs.

• This is repeated many times over a

large area.

• These developments are CAPEX and

human resource intensive.

• For success, the process needs to be

efficient.

• Well planned appraisal and pilot

production stages are essential.

• Success is NOT guaranteed; so ‘off-

ramps’ need to be clearly defined before moving to a development.

Technical Challenges – Five Key Ways to Improve

• Prediction of SRV permeability prior to treatment.

– Combination of Geomechanics and flowing well behaviour.

• Better understanding of transport physics.

– Prediction of Diffusivity and Darcy flow in shales.

• Understanding of Transient Behaviour of a producing well.

– Can the geometric attributes of the SRV be defined from flowing rate and

pressure data?

• Impact of Liquids on Shale Gas Well Deliverability.

– Liquids rich plays are becoming more attractive (as a hedge against low gas

prices); how does the physics differ from a pure gas play?

• Water Management.

– Shale Gas is a BIG water consumer; and water is becoming a scarce

commodity.

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Questions?