The various forms of unconventional gas include tight gas, coal bed methane CB), shale gas, and gas hydrates. This paper will present a general overview of shale gas while providing Insight into specific shale gas plays located In Canada and the United States. Specifically.
It will discuss how the increase In shale gas production In the united States has validated the commercial viability of shale gas drilling through the use of new technological processes including hydraulic fractures in horizontal wells. This technology is then being applied to Canada where according to the N.B., over 1000 etc of shale gas has been discovered to date.
Introduction Shale rock is considered by the ERIC as "lithographically unit having less than 50% by weight organic matter, with: less than 10% of the sedimentary class having a grain size greater tha...
n 62. 5 micrometers; and more than 10% of the sedimentary class having a grain size less than 4 1. 020(2)(27. 1), Oil and Gas Conservation Regulations(OCCUR)). In conventional terms this means, a shale gas reservoir consists of very fine grained particles that are filled with organic-rich material.
Shale reservoirs are able to store gas in numerous ways. According to Centre for
Energy, natural gas can be adsorbed onto Insoluble organic matter called kerosene, trapped In pore spaces of fine grained sediments Interpreted with shale and has also been noted to store in hydraulic fracas and pore networks in the shale's organic matter as well (Agiler, 2012). A major difference with traditional reservoirs is that shale are often both the source rocks and reservoir rock. Gas in shale can be generated by either boogieing or thermometric. The cracking of organi
matter (kerosene broken into hydrocarbons) is what generates gas by thermometric means and the boogieing gas is generated from fresh water recharge ERIC, 2009).
Thermometric gas is generated in deep burial (several kilometers deep) where higher temperatures and pressures are present.
The high temperature and pressure causes the mature organic material (kerosene) to crack in to smaller hydrocarbons and into methane gas (N.B., 2012). Thermometric gas is often found with high natural gas liquids (ENG) yields which increases the value of the produced gas. Unfortunately there is also the potential of producing carbon dioxide (CA) which would need to be removed. Thermometric gas will generally flow at larger initial rates based on the larger reservoir pressure.
But to achieve higher rates, thermometric gases usually require horizontal drilling and extensive fracturing which is also much more expensive to reach these larger rates (egg. Musk's and Monotone formation in east central British Columbia and the Barnett Shale play in Texas). Boogieing gas is generated in shallow burial (Upton a kilometer deep) under lower temperatures and pressure. Boogieing gas is generated when anaerobic micro-organisms (bacteria) feed on organic matter (mature and non-mature) to release methane (N.B.
, 2012). Unlike thermometric gas, boogieing gas usually does not produce ENG and/or much CA.
Because boogieing gases are shallow and do not generally require horizontal wells (egg. Colorado Shale play in southern central Alberta and the Atria Shale play in the Michigan basin). Figure 1 - Conventional vs..
Unconventional Gas Reserves Source: Agiler et al, WAP, 2008. Horizontal Drilling Technology is fundamental to the success of production of unconventional gas. Without recent technologies, tight gas, shale gas, and coal bed methane would not be
commercially successful. Prior to horizontal drilling in shale, vertical wells were drilled with close spacing in the Barnett Shale play.
Horizontal drilling was first improvements and advancements in horizontal drilling over the last decade were able to increase production significantly (as shown in Figure 2). This includes a remarkable 2 year span from 2005 to 2007, in which all horizontal wells in the Ions, Hill, and Basque counties were a success (Kohl, 2007). Deckhands has also displayed the higher rates in the Denton and Tartan counties which the Barnett Shale produces from in Figures 4 and 5 below. The production ratio of horizontal wells and vertical wells is 3. 2:1, while the cost ratio of drilling the two types of well is only 2:1 (Naturals. Rig, 2004).
Although horizontal drilling has shown increased production in the Barnett Shale, it is not as beneficial in other United States shale plays based on the recovery per unit cost (ERIC, 2009). Horizontal wells can drill over mm laterally. The cost of one vertical well in the Monotone formation costs 2. 8 million CDC$ with 2.
0 MAC/D and lower, while horizontal wells cost about 5. 0 million CDC$ and produces at more than 4. 0 MAC/D (Urn, 2008). From the Monotone formation, horizontal wells with proper stimulation, are much more cost efficient than conventional vertical wells.
Figure 2 - Barnett Shale Gas production, 1997 Source: worldly.
Com, 2012. - 2010 Figure 3 - Vertical vs.. Horizontal drilling illustration Source: harmonistically.
Org, 2012. Thermal Maturity Thermal maturity is determined by the vitrified reflectance, Or. According to General Screening Criteria for Shale Gas Reservoirs and Production Data Analysis of Barnett Shale, "thermal maturity is
found to have more importance than thickness of the shale" (Deckhands, 2008). The thermal maturity determines whether hydrocarbons may be produced.
Deckhands describes thermal maturity to determine the oil window and gas window.
These windows determine whether gas ND/or oil could be produced within the shale. For the oil window, thermal maturity ranges from 0. 6 to 1. 1 percent. For the gas window, thermal maturity ranges from 1. 2 to 2.
0 percent (Deckhands, 2008). Thermal maturities below 0. 6 percent are Total Organic Carbon A key attribute to determine if a shale play is productive and economic is the total organic carbon, TCO. TCO is a measure of present-day organic richness (ERIC, 2008). The lower the TCO, the less gas is generated and vice versa.
Figure 4 -Denton County Production Source: Deckhands, 2008. Figure 5 - Tartan County Production Souse: Deckhands, 2008. Natural Fractures Natural fractures are discontinuities that results from stresses that exceed the rupture strength of the reservoir rock (Stearns, 1990). Natural fractures can enhance production but have some drawbacks as well. "Leakage can occur in natural fractures when they open up under fracture pressures and if early sendoff occurs during farcing and it intersects with the natural fracture, conductivity will not be high enough for economic gas production" (Hooch, 2012).
Natural fractures may also be connected to an aquifer and if opened, water may drown the hellebore and gas production may cease. According to Deckhands, shale with more silica are generally more productive due to being naturally fractured. Conventional sands usually require single TOT fractures and unconventional formations require multiple 50TH to 10TH fractures (Hooch, 2012). Hydraulic Fractures Hydraulic fracturing was first
developed in the sass and used in the Barnett shale in 1986 (Hayden and Purcell, 2005).
Hydraulic fractures create discontinuities in the reservoir rock so the well bore is in more contact with the gas in place.
In conventional terms, hydraulic fractures create a path for gas to flow from reservoir to the well bore (path of least resistance). Because shale gas has very low permeability, economical feasible. Likewise poor completions lead to poor production and wells that don't payoff. Horizontal wells may have up to 30 stage fracas. Clearwater fractures are composed of a large volume of water with some sand added in.
Other additives in Clearwater fractures can include friction reducers, biocide's, scale inhibitors, prospects, clay stabilizers, and surfactants.
Prospects are injected to keep the fractures propped open from closing when the fracture fluid is removed and the naturally occurring reservoir pressure is restored. Friction reducers are seed to reduce friction pressure so pumping pressure can be reached using less horsepower thereby saving money (Kaufman and Penny, 2008). But used improperly they can also cause damage to the formation. Clay stabilizers are used to reduce particle migration along with fluids.
Kaufman and Penny analyzed a north east US shale and proved that the permeability with clay stabilizers was 130 Darcie's compared to 70 Darcie's without.
Bacteria can cause acid production which biocide's are used to prevent (Kaufman and Penny, 2008). United States There are over 40 shale formations in the United States (Carport, 2008) of which hey are estimated to produce 2. 3 ETC by 2030 (United States Energy Information Administration, 2008). Figure 6 shows where these shale basins are located and table 1 outlines a
few specific examples and their respective properties.
Major shale production in the United States comes from the Fort Worth Basin, Fayetteville, Huntsville, Marvelous and Palo Dour.
Shale gas wells initially produce at about 4 CUFF/day in the United States and approximately 3 BCC/day from the Barnett Shale in Fort worth (CHUG, 2010). The Barnett Shale gas is the largest active gas play in the United States. It can be up to 8500 feet deep and has about 26 ETC of gas in place. Because the Barnett is a complex play, production may not be uniform throughout the basin.
The Barnett began development in 1981 (Booker, 2007). The Barnett began to take off in the last decade once the technological benefits of horizontal drilling and multi-stage fractures were evident.
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