Natural Gas (Tight Gas Sands & Coal Bed Methane)Natural Gas Natural gas is a combustible mixture of hydrocarbon gases. While natural gas is formed primarily of methane, it can also include ethane, propane, butane and pentane. Natural gas is combustible, and when burned it gives off a great deal of energy. Unlike other fossil fuels, however, natural gas is clean burning and emits lower levels of potentially harmful by products into the air. Tight Gas Sands (TGS) TGS, which consists primarily of methane, refers to gas that is stuck in a very tight formation underground, trapped in hard rock, or in a sandstone or limestone formation that is unusually impermeable and non-porous (tight sand). TGS is a gas resource in adjacent strata and underlying or overlying strata. Tight gas sandstones are gas bearing strata that are interbedded sandstone between, or overlying, the coal seams extensively found in Carboniferous and Permian aged rocks in the Ordos Basin. The figure below depicts a cross section of a conventional reservoir, and a tight gas reservoir, the dark areas indicating pore space that would contain natural gas in a producing gas field. In a conventional natural gas deposit, once drilled, the gas can usually be extracted quite readily, and easily. TGS reservoirs require special methods to stimulate the well and obtain gas flow. Several techniques exist that allow natural gas to be extracted, including fracturing and acidizing.
Cross section of a normal reservoir and a Tight Gas reservoir. Source: United States Geological Survey In SGE's well stimulations to date it has used hydraulic fracturing methods as depicted below.
Illustration of hydraulic fracturing: Source: SGE SGE's main target gas bearing targets are in the Permian and Upper Carboniferous systems and include the Shiqianfeng, Shihezi, Shanxi and the deeper Taiyuan and Benxi formations. The principal interbedded coal seams are located in the Shanxi and Taiyuan formations. Each of these various reservoirs has pay in the wells drilled on SGE's PSC's. Coal Bed Methane (CBM) Coal, a fossil fuel, is formed underground under similar geologic conditions as natural gas and oil. These coal deposits are commonly found as seams that run underground, and are mined by digging into the seam and removing the coal. Many coal seams also contain natural gas, either within the seam itself or the surrounding rock. The methane gas trapped underground is referred to as CBM. CBM is produced by drawing the water away from the coal seam and lowering the pressure so that the methane desorbs from the coal and flows to the well bore, as illustrated in below.
Diagram showing the production scheme of gas and water for a typical CBM well. Source: United States Geological Service Historically, coalbed methane was considered a nuisance and a safety threat in the coal mining industry. Today, however, coalbed methane has become a popular form of natural gas. SGE drilled 2 CBM wells during 2006, the LXSG-01 well on the Linxing PSC and the SJSG-01 well on the Sanjiaobei PSC, with the principal targets being the coal Seams 4/5 and 8/9. In the late 1990's, Phillips Petroleum conducted a 6 CBM exploration well and 5 well CBM production pilot on the Linxing PSC, which was ultimately terminated. Arco, and their predecessors, have also drilled many exploration and appraisal wells, and commenced a production pilot on the Sanjiaobei PSC. The data indicates the areal extent and approximate thickness of the Seams 4/5 and Seams 8/9, but also confirms the variability of key determinates to successful CBM developments: gas content and permeability across the various data points. Comparison of Coal Bed Methane (alternatively known as Coal Seam Gas) and Conventional gas reservoirs The figure below illustrates the performance of Coal Bed Methane and Conventional reservoirs over time. In this depiction, SGE's TGS reservoirs are analogous to Conventional reservoirs. TGS reservoirs have higher initial gas flow rates (immediately following stimulation) and decline over time. CBM reservoirs must frist be dewatered which has the effect of reducing pressure so that methane desorbs from the coal.
Source: Origin Energy Limited and Gaffney Cline Associates |
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