While conventional natural gas streams from the earth relatively easily, unconventional gas finds are more difficult to develop and more costly to produce. As technologies and skills improve, unconventional gas is a variable concept because some finds may become more easily or economically produced over time, no longer making them unconventional. Right now, there are six main types of unconventional gas, including deep gas, gas-containing shales, coalbed methane, geopressurized zones, Arctic and subsea hydrates, and tight gas.
Unconventional natural gas deposits are likely to account for much of the world's remaining reserves. According to the EIA, there is more than 309 Tcf of recoverable tight natural gas deposits in the US, which represents some 17% of the total natural gas reserves in the country.
Helping to boost interest in developing technologies that can overcome the challenges of producing unconventional gas resources in the United States, the Natural Gas Policy Act offers incentives to companies exploring for and producing unconventional gas plays.
What Is Tight Gas?
Tight gas refers to natural gas reservoirs locked in extraordinarily impermeable, hard rock, making the underground formation extremely "tight." Tight gas can also be trapped in sandstone or limestone formations that are atypically impermeable or nonporous, also known as tight sand.
While a conventional gas formation can be relatively easily drilled and extracted from the ground unassisted, tight gas requires more effort to pull it from the ground because of the extremely tight formation in which it is located. In other words, the pores in the rock formation in which the gas is trapped are either irregularly distributed or badly connected with overly narrow capillaries, lessening permeability -- or the ability of the gas to travel through the rock. Without secondary production methods, gas from a tight formation would flow at very slow rates, making production uneconomical.
While conventional gas formations tend to be found in the younger Tertiary basins, tight gas formations are much older. Deposited some 248 million years ago, tight gas formations are typically found in Palaeozoic formations. Over time, the rock formations have been compacted and have undergone cementation and recrystallisation, which all reduce the level of permeability in the rock.
Typical conventional natural gas deposits boast a permeability level of .01 to .5 darcy, but the formations trapping tight gas reserves portray permeability levels of merely a fraction of that, measuring in the millidarcy or microdarcy range.
In order to overcome the challenges that the tight formation presents, there are a number of additional procedures that can be enacted to help produce tight gas. Deviating drilling practices and more specific seismic data can help in tapping tight gas, as well as artificial stimulation, such as fracturing and acidizing.
Developing Tight Gas
One of the most important aspects of drilling for any petroleum is predetermining the success rate of the operation. Operators do not just drill anywhere. Extensive seismic data is gathered and analyzed to determine where to drill and just what might be located below the earth's surface.
These seismic surveys can help to pinpoint the best areas to tap tight gas reserves. A survey might be able to locate an area that portrays an improved porosity or permeability in the rock in which the gas is located. Should wells directly hit the best area to develop the reserve, costs of development can be minimized.
Most tight gas formations are found onshore, and land seismic techniques are undergoing transformations to better map out where drilling and development of these unconventional plays. Typical land seismic techniques include exploding dynamite and vibroseis, or measuring vibrations produced by purpose-built trucks. While these techniques can produce informational surveys, advancements in marine seismic technologies are now being applied to land seismic surveys, enhancing the information available about the world below.
Not only providing operators with the best locations for drilling wells into tight gas formations, extensive seismic surveys can help drilling engineers determine where and to what extent drilling directions should be deviated.
While vertical wells may be easier and less expensive to drill, they are not the most conducive to developing tight gas. In a tight gas formation, it is important to expose as much of the reservoir as possible, making horizontal and directional drilling a must. Here, the well can run along the formation, opening up more opportunities for the natural gas to enter the wellbore.
A common technique for developing tight gas reserves includes drilling more wells. The more the formation is tapped, the more the gas will be able to escape the formation. This can be achieved through drilling myriad directional wells from one location, lessening the operator's footprint and lowering costs.
After seismic data has illuminated the best well locations, and the wells have been drilled, production stimulation is employed on tight gas reservoirs to promote a greater rate of flow. Production stimulation can be achieved on tight gas reservoirs through both fracturing and acidizing the wells.
Fracturing, also known as "fracing," a well involves breaking the rocks in the formation apart. Performed after the well has been drilled and completed, hydraulic fracturing is achieved by pumping the well full of frac fluids under high pressure to break the rocks in the reservoir apart and improve permeability, or the ability of the gas to flow through the formation.
Additionally, acidizing the well is employed to improve permeability and production rates of tight gas formations. Acidation involves pumping the well with acids that dissolve the limestone, dolomite and calcite cement between the sediment grains of the reservoir rocks. This form of production stimulation helps to reinvigorate permeability by reestablishing the natural fissures that were present in the formation before compaction and cementation.
Furthermore, deliquification of the tight gas wells can help to overcome some production challenges. In many tight gas formations, the reservoirs also contain small amounts of water. This water can collect and undermine production processes. Deliquification is achieved in this instance through artificial lift techniques, such as using a beam pumping system to remove the water from the reservoir, although this has not proven the most effective way to overcome this challenge.
Engineers continue to develop new techniques and technologies to better produce tight gas. Through their efforts, maybe one day, tight gas will no longer be considered an unconventional play.