Unconventional hydrocarbons
Domenico Grigo 28 April, 2011
Unconventional hydrocarbons & Eni activity
From a geological point of view the Unconventional hydrocarbons are continuous accumulations not depending on structural trapping and buoyancy effects.
Unconventional hydrocarbons usually include the following sources:
Gas and liquids from very low permeability reservoirs (e.g. tight gas)
Oil and gas from shales (e.g. gas shale, shale oil, oil shales)
Gas from coal (coalbed methane)
Gas from hydrates
Unconventional hydrocarbons
Gas from hydrates
Heavy Oil/ Bitumen from oil/tar sands
From: Schenk and Pollastro, 2002
The lateral continuity of the accumulations make the deposits potentially very large
The low permeability of the reservoirs
makes development project extremely drilling intensive
Several factors contribute to determine the cost of the resources, their price on the market and the profit margin of the projects:
Good geological knowledge of the subsurface
Availability of adequate technology
Availability of infrastructures
Support from the local authorities and government
Favorable price long term prospects
Ability to keep profit margin on long term
Unconventional hydrocarbons - Success factors
Ability to keep profit margin on long term
From: NPC 2007, Topic Paper #29)
Unconventional gas around the world
3000 4000 5000 6000 7000 8000 9000
Technically Recoverable Resources (Tcf)
Tight Gas Shale Gas Coalbed Metane
0 1000 2000
N. America
Former Soviet U.
Central Asia
Latin America M. East &
N. Africa
W&E Europe
Technically Recoverable Resources (Tcf)
Gas Shales
Not all shales are “gas shales”
Shale gas is essentially natural gas contained within a sequence of predominantly fine grained rocks dominated by shale.
Shale traditionally has been regarded as a hydrocarbon source rock or seal. Shale gas boom in recent years has been due to modern technology in hydraulic fracturing as well as in horizontal drilling.
Gas Shales
Shale gas has become an increasingly more important source of natural gas in the United States over the past decade. It is expected that shale gas will greatly expand worldwide energy supply.
Shales that host economic quantities of gas have a number of common properties. They are rich in organic material and are usually mature petroleum source rocks in the thermogenic gas window.
Gas Shale: conventional source - unconventional reservoir
Gas accumulation is continuous and not related to buoyancy
The formation is simultaneously source rock and reservoir
Gas presence is not associated to geological traps:
the target is a portion of basin
Gas production is achieved only with fracture stimulation
Not all the shale gas plays can commercially produce gas Key geological factors are:
Key geological factors are:
High Organic Content and Maturity
Quality of organic matter: type II kerogene is the most favorable
Low volume of shaly mineral
Brittleness
Presence of natural fractures that can be reactivated
No producible water
Sealing layers at top and bottom
Limited Geohazards, like faults, karst areas and tectonic complexity
Adequate depth and thickness of the producing play: if over-pressured, depth >3500 m can be acceptable
CBM 28%
Tight Gas Sands
23%
Shale Gas 49%
Worldwide unconventional gas resources 30000+ TCF / 5000+ B boe*
Unconventional Gas
28%
Fredonia 1821
The year 1821 is regarded as the start of the commercial natural gas industry in the US.
The first commercial US natural gas production came from an organic-rich Devonian shale in the Appalachian basin.
an organic-rich Devonian shale in the Appalachian basin.
The gas was used to illuminate the town of Fredonia.
This discovery anticipated the more famous Drake oil well at OIL Creek, Pennsylvania, by more than 35 years.
Gas Shales: a new technology challenge
Besides a favorable combination of geologic factors, key success factors are:
Technology
Horizontal Drilling + Multi Frac techniques
Completion techniques
Optimal horizontal drain spacing
Logistics
Minimize environmental impact → cluster drilling
Easy Water supply and disposal
Value chain management
Value chain management
Low costs all along the exploitation chain (“manifacturing process”)
Location
Proximity to transportation and treatment facilities
Commercial
Competitive Gas Market
“ad hoc” contractual terms
*Gas shale projects are capital intensive and
INCREASING THERMAL MATURITYINCREASING THERMAL MATURITY
CRACKING CRACKING DEAD
CARBON DEAD CARBON
REACTIVE CARBON REACTIVE
CARBON
ORGANIC MATTER IN SHALES
OIL
OIL WET GAS WET
GAS
DRY GAS DRY
GAS
GENERATED HYDROCARBONS
MIGRATION TO SHALLOWER TRAPS
Gas Shale Geochemistry– Oil and Gas generation
More gas is generated at higher thermal maturity
CRACKING CRACKING
CRACKING CRACKING
INCREASING THERMAL MATURITYINCREASING THERMAL MATURITY
RETENTION IN SHALES
SHALE GAS SHALE GAS
Part of the generated gas is retained by the shales
INCRASING THERMAL MATURITY
DEAD CARBON
DEAD CARBON
REACTIVE CARBON REACTIVE
CARBON
ORGANIC MATTER IN SHALES
Gas Shale – Geochemical Characterisation
Total Organic Carbon % (TOC) Hydrogen Index (HI)
Original Quantity & Quality of the O.M.?
Reactivity of the O.M.?
Geochemical parameters related to the gas abundance
CRACKING
INCRASING THERMAL MATURITY
Kinetic of HC generation
Vitrinite Reflectance (Ro%)
Reactivity of the O.M.?
Pyrolisis RockEval (Tmax) Maturity Level of the O.M.?
Gas Shale – the Geochemical Parameters
Interpretative guidelines
for evaluating shale gas prospects
•TOC >1%
•HI <100 (but assuming HIoriginal >350)
•%Ro > 1.2 (1-1.2 “gray area”) Quantity & Quality
of the O.M.?
•%Ro > 1.2 (1-1.2 “gray area”)
•Tmax >455 °C
•Transformation ratio >80%
Reactivity of the O.M.?
Maturity Level?
Gas Shales - gas storage and production system
GAS in “3-porosity” system:
Free Gas in rock pores (Primary Porosity)
Free Gas in Natural Fractures (Micro- Fracture Porosity)
Gas Adsorbed into Organic Matter
GIIP = Gf + Gm + Gads
Network of natural fractures Gas desorption from organic matter
Matrix flow
Fracture system alimentation
Production Mechanism depends on Pressure Decline
Reservoir Volumes Splitting – example
HIGH depth – 8500 ft
0 20000 40000 60000 80000 100000
MMscf
30000 35000
Gas Shales - Gas Storage
LOW depth – 2500 ft
0 500 1000
1500 2000
2500 3000
3500 4000
4500 5000 pressure - Psi
Adsorbed Gas MMscf Free gas Matrix porosity MMscf Free gas in Microfracture MMscf TOTAL GIP MMscf
0 5000 10000 15000 20000 25000
0 500 1000 1500
pressure - Psi
MMscf
Adsorbed Gas MMscf Free gas Matrix porosity MMscf Free gas in Microfracture MMscf TOTAL GIP MMscf
