To date, fossil fuels have been provided about 85 % of our energy demand in this world (Quintana, et al., 2011). These fossil fuels include oil, coal and also natural gas. Nowadays, fossil fuels are being declared as a finite resource and will be depleted in the near future. "Peak oil" production may have been reached as early as 2014 and will be followed by a rapid decline in our oil reserves. At the same time, demand for natural gas has increased approximately 30 % during the past 15 years, and today unconventional gas deposits, those found in coal-beds and shale, are receiving great attention from nations and major energy companies (IEA, 2012).
Indonesia is one of many nations that has very large conventional and unconventional gas resources. Natural gas extracted from shale formations, shale gas, will be a trend of exploration and production in this country. Shale gas has a significant advantage over to coal as the use of it for electricity production results in fewer carbon emissions, and a decrease in energy costs. Large-scale shale gas production would likely cause a significant decline in natural gas prices all while providing cleaner energy option (Haluzan, 2014). With lower costs and lower carbon, shale gas can become one of the most important energy sources in Indonesia. Microbial-enhanced shale gas production stands to fulfill this impending demand for unconventional gas.
Shale is a sedimentary rock that was once deposited as mud-combination of water, clay and silt-and is generally a combination of clay, silica (quartz), carbonate (calcite or dolomite), and organic material. Shale may also have thin beds or laminae of sandstone, limestone, or dolostone. The mud was deposited in deep, quiet water such as in large lakes or deep seas and oceans. The organic matter in the mud was algae, plant matter, or plankton that died and sank to the sea floor or lake bed before being buried (Blatt and Tracy, 2000).
In the shallow burial or a few hundred meters deep, mud will turn into shales where bacteria can feed on the available organic matter and release biogenic methane as a byproduct. Meanwhile, in deep burial or several kilometers deep, heat and pressure can crack the organic matter and turn it into thermogenic methane. The methane will escape and migrate into the more porous rock of conventional reservoirs, what we know as conventional gas, but some gas is usually still trapped in the micro pore spaces or attached to the organic matter within the shale known, this is where we find shale or unconventional gas. An individual gas shales resource can contain hundreds to thousands of billion cubic meters (tens to hundreds of Tcf) of gas, spread over hundreds to thousands of square kilometers . The problem in exploring shale gas lies in extracting these small ammounts of natural gas. The pore spaces in shale which the natural gas must move though and to flow into any well, are 1000 times smaller than pores in conventional reservoirs and the pore throats are only 20 times larger than a single methane molecule, giving shale a very low permeability (Bowker, 2007).
However, a fractures method is already available and can act as a conduits for the movements natural gas from shale and increase its permeability. This method was called hydraulic fracturing or fracking. Hydraulic fracturing is the process of transmitting pressure by fluid or gas to create cracks or to open existing cracks in hydrocarbon bearing rocks underground. Fracking is essentially consist of 4-step process :
- Pressure the reservoir rock using a fluid to create a fracture,
- Grow the fracture by continuing to pump fluids into the fractures,
- Pump proppant-non-compressible material, usually sand or ceramic beads- materials into the fracture in the form of a slurry, as part of the fracture fluid,
- Stop pumping and flow back to the well to recover the fracture fluids while leaving the proppant in place in reservoir.
The key element of successful fracking is constructs a proper well to ensure groundwater is isolated and minimalized damage to reservoir rocks (CSUG, 2014).
The fracking method in extracting shale gas can be improved by microbial-enhanced-shale gas generation. This is because of fact that about 20 % of natural gas is produced by microorganisms. The principal component in microbial natural gas generation is methane gas (CH4), and the overall biodegradation process called methanogenesis. Biodegradation pathways in methanogenesis are complex as can be seen in figure 1 and involve different microorganisms acting as a team or consortium. This bacterial consortium includes at least three groups of organisms:
- an acid former group for transforming complex organic molecules into 5 organic acids and alcohols by hydrolytic and fermentation
- an obligate, hydrogen producing acetonic group for converting organic acid and alcohol to hydrogen and single carbon compounds
- a group comprising of methanogens for converting hydrogen and single carbon compounds into methane.
The methods that can be used for enhancing microbial methane production from shale include :
- adding nutrients such as acetate, H2 or CO2 to enhance the activity of native populations of microorganisms (bio stimulation)
- adding microorganisms to enhance methane production (bio augmentation)
- altering the chemical and physical properties of the shale to increase its availability for microbial degradation
- using various combinations of these approaches [ Orem, 2013 ; Cokar, 2013 ].
By using microorganism to enhance methane production in shale gas reservoirs, natural gas resources from fractured shales will be secured to meet the energy demands, especially in Indonesia.
Figure 1. Microbial biodegradation pathway of coal geopolymers to methane gas. Source : Orem, 2013
The needs for developing shale gas exploration in Indonesia become greater as it has been identified in three basins in Sumatera (Baong shale, Telisa shale and Gumai shale), two basins in Jawa and two basins in Kalimantan and Classafet formation of potential shale gas resources. Total potential of all these shale gas reservoirs are about 574 tscf that makes Indonesia as one of the tenth largest nations of unconventional gas producers in the golden rules case, 2035 that can be seen in figure 2 (IEA, 2012).
Figure 2. Ten largest unconventional gas producers in the Golden Rules Case, 2035 Source : IEA, 2012
At last, shale gas when it used properly, is a safe and convenient energy source. Futhermore, the dominant fraction of methane in shale gas itself is a greenhouse gas and give lower carbon emissions, neither poisonous nor harmful if inhaled for short periods. The use of microorganisms for increasing methane generation in shale gas reservoirs could also be applied in the near future era of energy in Indonesia to help secure natural gas supply and resources.