Shale Gas 101
HOW IS SHALE GAS FORMED ?
There are two theories as to how natural gas is formed. The most widely accepted theory, the organic theory, maintains that natural gas formation begins with photosynthesis, where plants use energy from the sun to convert carbon dioxide and water into oxygen and carbohydrates. The remains of these plants and the animal forms that consume them are buried by sediment and as the sediment load increases, heat and pressure from burial converts the carbohydrates into hydrocarbons. Natural gas formation takes place in fine-grained, black, organic, shale source rocks. Continued pressure from burial forces most of the natural gas to migrate from the organic shales into more porous and permeable rock such as sandstone and limestone. The natural gas remaining in the shales is termed shale gas.
The other theory of natural gas formation is the inorganic theory which speculates that hydrocarbons did not originate from buried plant and animal material, but instead were trapped inside the Earth as it formed. This theory is most likely not applicable to shale gas.
HOW IS SHALE GAS FOUND ?
Exploration for gas shales is similar to exploration for conventional reservoirs which, for an unexplored basin, usually includes:
• review of existing information
• aerial surveys to gather data regarding magnetic fields, gravity and radiation
• seismic surveys to locate and define subsurface structures capable of trapping natural gas
• exploration drilling to test subsurface structures for the presence of hydrocarbons
• logging the wells to determine porosity, permeability and fluid composition
In the case of shale gas, the primary targets are shale formations with interbedded porous and permeable fine-grained sediments and natural fracture systems. Down-hole tools used to find fractures include density compensation, caliper and temperature logs, and formation microscanner imaging. Low-altitude, airborne multispectral imaging is a new tool used to locate subsurface microfractures and prospectivity of shale formations.
WHERE IS SHALE GAS FOUND ?
Shale gas in the United States
There are five major shale basins in the United States from which shale gas is produced. To date, more than 40,000 wells have been drilled. There are seven other shale basins from which there has been no production.
Shale gas in Canada
Currently, there is no commercial shale gas production in Canada. Canada has a number of prospective shale gas targets in various stages of exploration and exploitation in British Columbia, Alberta, Saskatchewan, Ontario, Quebec, and Nova Scotia.
HOW IS SHALE GAS PRODUCED ?
In order for a shale play to produce, the rock must have geochemical attributes that indicate that the organic content (kerogen) has been sufficiently heated to produce natural gas in the formation. If the shale does not reach a high enough temperature over the millions of years that it was buried, then it is not prospective for thermogenic gas generation. Conversely, if the rock reaches too high a temperature, the natural gas (methane) that is produced can break down into non-combustible gases such
as carbon dioxide.
To understand the geochemical (maturity, organic content), mechanical (stress, brittleness) and in-situ rock properties (permeability, porosity, water saturation, reservoir pressure, gas content), wells are drilled, and rock samples from core and cuttings are recovered and sent to specialized laboratories. Traditional logging tools do not provide sufficient information for evaluating shale prospectivity, and can be misleading unless calibrated to lab results from actual rock samples.
Because shale has such low permeability, gas will generally not flow unless the rock has been fractured. This involves injecting high volumes of water, mixed with sand, at high pressure into the targeted shales. This fractures and props open the rock, thus allowing the gas trapped in the reservoir rock to flow to the wellbore.
Generally, vertical wells are used to explore the shale resource and determine the best places for development, and then horizontal wells are drilled for commercial production.
Horizontal well drilling and multi-stage fracture stimulations have been the key to obtaining economic productivity in most shale reservoirs. Horizontal wells can be drilled over 1000 meters laterally through the productive shale and then 5-10 individual intervals are fracture stimulated. This technique connects larger volumes of the resource into a single wellbore, resulting in higher rates and reserves, and ultimately, a commercially viable project.
Shale plays can hold an enormous amount of natural gas and are capable of producing gas at a steady rate for decades.
C. Keddy
