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 than 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 organic 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..
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
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