You must log in to edit PetroWiki. Help with editing

Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information


Unconventional resources of oil and gas from a geologic perspective: Difference between revisions

PetroWiki
Jump to navigation Jump to search
No edit summary
No edit summary
Line 17: Line 17:


<span style="font-size:medium;"><span style="color:#008080;"><span dir="LTR">Formation of coal process</span></span></span>
<span style="font-size:medium;"><span style="color:#008080;"><span dir="LTR">Formation of coal process</span></span></span>
 
<p style="text-align: justify;"><span style="font-size:medium;"><span dir="LTR"><span style="font-size:small;"><span dir="LTR">Coal was formed about 400 million years ago from the remains of vegetations that grew at that time. Therefore,&nbsp;it’s called&nbsp;fossil fuel.</span></span></span></span></p><p style="text-align: justify;"><span style="font-size:medium;"><span style="color:#008080;">Formation of peat</span></span></p><p style="text-align: justify;"><span style="font-size:medium;"><span style="font-size:smaller;"><span dir="LTR">Peat is a soggy, dense material which is formed by accumulation of layers of sediments over the remains of dead plants and trees that sank to the bottom of the swampy areas,&nbsp;</span></span></span><span style="font-size:medium;"><span style="font-size:smaller;"><span dir="LTR">over long periods of time. The changes in the earth's surface caused deposits of sands, clays and other minerals to accumulate and&nbsp;bury&nbsp;the peat underneath. Then, s</span></span></span><span style="font-size:medium;"><span style="font-size:smaller;"><span dir="LTR"><span dir="LTR">andstones and other sedimentary rocks were formed, and the pressure caused by their weight squeezed water out from the peat.&nbsp;</span></span></span></span><span style="font-size:medium;"><span style="font-size:smaller;"><span dir="LTR"><span dir="LTR"><span dir="LTR">This depth associated with heat, gradually changed the material into coal. Scientists claim that&nbsp;3 to 7 feet of compacted plant matter is required to form 1 foot of bituminous coal <ref name="Rudy E. Rogers, Muthukumarappan Ramurthy, Gary Rodvelt, Mike Mullen, M.B. 2007. Coalbed Methane: Principles and Practices.Prentice-Hall.">Rudy E. Rogers, Muthukumarappan Ramurthy, Gary Rodvelt, Mike Mullen, M.B. 2007. Coalbed Methane: Principles and Practices.Prentice-Hall.</ref>&nbsp;This process is indicated in Fig. 1.</span></span></span></span></span></p><p style="text-align: center;"><span dir="LTR">[[File:2.jpg|2.jpg|link=]]</span></p>
<span style="font-size:medium;"><span dir="LTR"><span style="font-size:small;"><span dir="LTR">Coal was formed about 400 million years ago from the remains of vegetations that grew at that time, so it’s called a fossil fuel.</span></span></span></span>
 
<span style="font-size:medium;"><span style="color:#008080;">Formation of peat</span></span>
 
<span style="font-size:medium;"><span style="font-size:smaller;"><span dir="LTR">Peat is a soggy, dense material which is formed by accumulation of layers over the remains of dead plants and trees that sank to the bottom of the swampy areas,&nbsp;</span></span></span><span style="font-size:medium;"><span style="font-size:smaller;"><span dir="LTR">over long periods of time, the changes in the earth's surface caused deposits of sands, clays and other minerals to accumulate, burying the peat underneath.</span></span></span>
 
<span style="font-size:medium;"><span style="font-size:smaller;"><span dir="LTR"><span dir="LTR">Sandstone and other sedimentary rocks were formed, and the pressure caused by their weight squeezed water out from the peat.&nbsp;</span></span></span></span>
 
<span style="font-size:medium;"><span style="font-size:smaller;"><span dir="LTR"><span dir="LTR"><span dir="LTR">This depth associated with heat, gradually changed the material to coal. Scientists claim that, from 3 to 7 feet of compacted plant matter is required to form 1 foot of bituminous coal.<ref name="Rudy E. Rogers, Muthukumarappan Ramurthy, Gary Rodvelt, Mike Mullen, M.B. 2007. Coalbed Methane: Principles and Practices.Prentice-Hall.">Rudy E. Rogers, Muthukumarappan Ramurthy, Gary Rodvelt, Mike Mullen, M.B. 2007. Coalbed Methane: Principles and Practices.Prentice-Hall.</ref></span></span></span></span></span>
 
<span dir="LTR">[[File:2.jpg|2.jpg|link=]]</span>
 
<span style="font-size:medium;"><span style="color:#008080;"><span dir="LTR">Formation of&nbsp;coal bed methane</span></span></span>
<span style="font-size:medium;"><span style="color:#008080;"><span dir="LTR">Formation of&nbsp;coal bed methane</span></span></span>



Revision as of 16:27, 16 December 2015

Overview

For a clear knowing of the unconventional resources, you first have to know the difference between both the conventional and the unconventional reservoirs. The conventional reservoirs are those in which wells can be drilled so that oil and natural gas can be produced at economic flow rates without large stimulation treatments or any special recovery process. On the other hand, the unconventional reservoir is the one that cannot be produced at economic flow rates or that does not produce economic volumes of oil and gas without the assistance of massive stimulation treatments or special recovery processes and technologies.

Think of the difference between a sponge and a piece of clay! it’s easy to squeeze water out of a saturated sponge which represents a conventional oil and natural gas reservoir. However, squeezing water out of saturated clay is harder.The latter represents an unconventional reservoir. The steep decline in conventional oil and natural gas resources around the world resulted in a need to exploit the unconventional resources to cover the shortage in energy needs.

As the technology used in the petroleum industry is expanding and developing, introducing a new technology that exploits unconventional reservoirs has become a must to increase the world reserve by producing “unconventional” oil and natural gas resources that were previously out of the producibility scope. These unconventional reservoirs contain the future of our hydrocarbon supply.

Unconventional Resources Classification and Distribution

Classification of unconventional resources

The unconventional resources are classified into: shale gas, shale oil, tight gas, tight oil, coal seam gas/coal-bed methane and hydrates. Most of them will be covered in this article from a geologic perspective.

Distribution of the main unconventional resources

The distribution of the worldwide unconventional gas resources is indicated in the following table (after Rogner 1996 , taken from Kawata and Fujita 2001) [1]

1.jpg

 

Coal Bed Methane (CBM)

Formation of coal bed methane

In order to understand the formation of coal bed methane you have firstly to understand the formation of both coal and peat.

Formation of coal process

Coal was formed about 400 million years ago from the remains of vegetations that grew at that time. Therefore, it’s called fossil fuel.

Formation of peat

Peat is a soggy, dense material which is formed by accumulation of layers of sediments over the remains of dead plants and trees that sank to the bottom of the swampy areas, over long periods of time. The changes in the earth's surface caused deposits of sands, clays and other minerals to accumulate and bury the peat underneath. Then, sandstones and other sedimentary rocks were formed, and the pressure caused by their weight squeezed water out from the peat. This depth associated with heat, gradually changed the material into coal. Scientists claim that 3 to 7 feet of compacted plant matter is required to form 1 foot of bituminous coal [2] This process is indicated in Fig. 1.

2.jpg

Formation of coal bed methane

Biogenic methane is produced by anaerobic bacteria in the early stages of coalification.Thermogenic methane is mainly produced during coalification at temperatures ranging from 120 – 150 °C.Contrasting features between CBM and Conventional Gas Reservoirs.[2]

Gas Composition

Gas produced from coal beds may be initially higher in methane content than the gas produced from conventional reservoirs.

Methane is less adsorbed than ethane and other heavier saturated hydrocarbons; consequently, they may not be as readily desorbed at first.

Adsorption

The mechanism by which hydrocarbon gases are stored in the coal reservoir contrasts with the mechanism of the gas storage in conventional reservoirs.

Methane is held to the solid surface of coal by adsorption forces instead of occupying void spaces -as a free gas- between sand grains (only 1-2%).

The adsorption mechanism creates the paradox of high gas storage in a reservoir rock of porosity less than 2.5%.

A clear illustration of the enormous surface area in the micropores of the coal is that 1 lb of coal has a surface area of 55 football fields, or 1 billion sq ft per ton of coal. 

Water Production

In the early production life of a well, before methane can be desorbed, the water from natural fractures in the coal must be removed.

The large volumes of water in the first year or two of production, decrease thereafter to relatively small volumes for the remaining life of the well.

3.jpg

Rock Physical Properties

Conventional oil and gas formations are inorganic. Organic formations contain CBM; these formations may contain about 10–30% inorganic ash.

Gas Flow

For coals, an additional mechanism of gas diffusion through the micropores of the coal matrix is involved, where the mass transport depends upon a methane concentration gradient across the micropores as a driving force.

4.jpg

Distribution of coal bed methane

Distribution of coal bed methane is illustrated by the following table

5.jpg

Tight-Gas Reservoir

Definition of tight-gas reservoir 

In the 1970s, the U.S. government decided that the definition of a tight gas reservoir is “the one in which the expected value of permeability to gas flow would be less than 0.1 md.”

 However, that definition had some political aspects which were related to the recovery produced form tight reservoirs.

After that a better definition has been revealed which states as follows, a tight gas reservoir is “A reservoir that cannot be produced at economic flow rates nor recoverable economic volumes of natural gas unless the well is stimulated by a large hydraulic fracture treatment or produced by use of a horizontal wellbore or multilateral wellbores.”

Based on this definition, there are no “typical” tight gas reservoirs. A tight gas reservoir can be deep or shallow, high-pressure or low-pressure, high-temperature or low-temperature, blanket or lenticular, homogeneous or naturally fractured, and can contain a single layer or multiple layers.[1]

Formation of tight-gas reservoir

What makes a tight reservoir?

There are a number of reasons that can make a reservoir tight.

But we can say that the effective permeability of a reservoir is the main reason for making a tight reservoir, after that being stated, we can then include some of the important parameters controlling the effective permeability, which are effective porosity, viscosity, fluid saturation and capillary pressure.

In addition to the factors related to the fluid nature, the rock parameters are equally important; yet those are controlled by depositional and post depositional environments the reservoir is subjected to.

Geological characteristics of tight-gas reservoir

The analysis of a tight gas reservoir should always begin with a thorough understanding of the geologic characteristics of the formation and the interaction between Quartz Cementation and Fracturing in Sandstones.

Interaction between Quartz Cementation and Fracturing in Sandstones

Quartz cementation and fractures are complexly interrelated. Quartz cementation influences fracture systems by affecting the rock mechanical properties at the time of fracture formation, which in turn, influences fracture aperture distributions and clustering.

Additionally, cementation affects flow properties of fracture networks by partially or completely occluding fracture pores, due to extensive cementation by authigenic clays. The matrix permeability of these sandstones is extremely low, on the order of microdarcies.

The important geologic parameters for a trend or basin are the structural and tectonic regime, the regional thermal gradients, and the regional pressure gradients.

On the other hand the important geologic parameters that should be studied for each stratigraphic unit are the depositional system, the genetic facies, the textural maturity, mineralogy, the diagenetic processes, cements, the reservoir dimensions, and the presence of natural fractures.[1]

Distribution of tight-gas reservoirs  

Devonian

Jean Marie Member and related carbonates (NEBC)

Mississippian / Pennsylvanian / Permian

Mattson Formation (Liard Basin)

Stoddart Group (NEBC Foothills and Peace River Plains)

Triassic

Montney ±turbidite play (Peace River Plains)

Doig ±shoreface/channel sands ±Groundbirch play (NEBC) Halfway ±NEBC Foothills, Peace River Plains

Baldonnel / Pardonet ±(NEBC Foothills) 

Jurassic

Rock Creek (west-central Alberta)

Nikanassin ±Buick Creek (NEBC, West-central Alberta) Kootenay (southwestern Alberta)

Lower Cretaceous

Cadomin / Basal Quartz (Alberta / B.C. western Plains and Foothills)

Bluesky / Gething (Peace River Plains, west-central Alberta) Falher / Notikewin (NEBC and adjacent Alberta)

Notikewin / Upper Mannville channels (west-central Alberta) Cadotte (west-central Alberta and adjacent B.C.)

Viking ±(west-central Alberta)

Upper Cretaceous

Dunvegan (west-central Alberta and adjacent B.C.)

Cardium ±Kakwa shoreface (west-central Alberta and adjacent B.C.) Belly River (west-central Alberta)

Shale Gas

Formation of shale-gas

Natural gas that has no difference from what you currently use to heat your home, cook with, or use to generate electricity, which is naturally trapped in its original source rock; the organic-rich shale that formed from the sedimentary deposition of mud, silt, clay, and organic matter on the floors of shallow seas.[1]

Shale.1.jpg

Geological characteristics of shale-gas

Organic Material

They are rich in organic material (0.5% to 25%).

Thermal Maturity

Thermal Maturity is an indicator of how much pressure and temperature the rock has been subjected to.

The shale is usually more mature, has higher gas ratio and matured in the thermogenic gas window, where high heat and pressure have converted petroleum to natural gas.

Vitrinite reflectance (% Ro), where a value above approximately 1.0%–1.1% Ro indicates that the organic matter is sufficiently mature to generate gas.

Pore Space

Effective bulk permeability in shale gas is typically much less than 0.1 (md), although exceptions exist where the rock is naturally fractured (Antrim shale).

Here is a figure illustarating  shale gas compared to other types of gas deposits.[3]

Distribution of shale-gas

Here is a map of major shale gas basis all over the world from the EIA report World Shale Gas Resources: An Initial Assessment of 14 Regions Outside the United States.[3]

EIA World Shale Gas Map.png

 Methane Hydrates

Formation of methane hydrates 

Methane hydrates are crystalline solids occuring in sediments in arctic regions, below the floor of the deep ocean, and consists of methane molecule surrounded by a cage of interlocking water molecules, although they look like ice, where high temperature and pressure conditions can burn it.

Most methane hydrates deposits also contain small amounts of other hydrocarbon hydrates; these include propane hydrates and ethane hydrates. [4]

Why is it hard to be studied?

Methanehydrate occur naturally in subsurface deposits where temperature and pressure conditions favorable for its formation.

So removing it from these conditions makes it unstable; as they are brought to the surface, the pressure is reduced and the temperature rises. This causes the ice to melt and the methane to escape so they can’t be drilled or cored for any studying matter.[5]

4a.jpg

Deposits of methane hydrate 

As mentioned before, methane hydrates need proper pressure and temperature conditions for its formation and stability sake, and these conditions are found to be achieved at four Earth environments.

Sediment and sedimentary rock units below Arctic permafrost.

Sedimentary deposits along continental margins.

Deep-water sediments of inland lakes and seas.

Under Antarctic ice.

It's important to mention that except for the Antarctic ice deposits, methane hydrate exists near the earth's surface relatively to oil hydrocarbon; in most situations the methane hydrate is within a few hundred meters of the sediments’ surface.

7.jpg

Distribution of methane hydrates 

In these mentioned environments methane hydrates occur in the sediments as layers, nodules and intergranular cements.

The deposits are often so dense and laterally persistent that they create an impermeable layer that traps natural gas moving upwards from below. [4]

The most abundant unconventional natural gas source:

Methane hydrates are considered to be the most abundant unconventional natural gas source, yet they are the most difficult to extract. 

 It is conservatively estimated to be 4,000 times the amount of natural gas consumed in the United States in 2010. 

The following figure shows worldwide distribution of confirmed or inferred offshore gas hydrate-bearing sediments, 1996. 

8.jpg

References

  1. 1.0 1.1 1.2 1.3 Robert L. Kennedy, SPE, William N. Knecht, SPE, and Daniel T. Georgi, SPE, Baker Hughes,Comparisons and Contrasts of Shale Gas and Tight Gas Developments,North American Experience and Trends.Paper SPE –SAS-245 available from SPE, Richardson, Texas.
  2. 2.0 2.1 Rudy E. Rogers, Muthukumarappan Ramurthy, Gary Rodvelt, Mike Mullen, M.B. 2007. Coalbed Methane: Principles and Practices.Prentice-Hall.
  3. 3.0 3.1 the U.S. Energy Information Administration (EIA),P.A. 2015. U.S. Crude Oil and Natural Gas Proved Reserves.Reference (modified). the statistical and analytical agency within the U.S. Department of Energy. Eia.gov Online, https://www.eia.gov/naturalgas/crudeoilreserves/pdf/usreserves.pdf (accessed August 20, 2014).
  4. 4.0 4.1 United States Geological Survey . 2013.Global inventory of natural gas hydrate occurrence. http://www.usgs.gov/science/cite-view.php?cite=15
  5. United States Geological Survey. 2013. Gas Hydrates Primer. http://woodshole.er.usgs.gov/project-pages/hydrates/ (accessed August 20, 2014).

Category