Shale gas is natural gas stored in rocks that are rich in organic material such as dark colored shale. Gas shales are often both the source rocks and the reservoir for the natural gas, which is stored in three ways:
- Adsorbed onto insoluble organic matter,kerogen that forms a molecular or atomic film
- Trapped in the pore spaces of the fine grained sediments interbedded with shale much like conventional reservoirs
Confined in fractures within the shale itself
Organic-rich shales, which traditionally have been viewed as source and cap rocks for hydrocarbon reservoirs, are also now viewed as reservoir rocks. Unlike a conventional oil and gas reservoir, in which the trapping mechanism limits the extent of an accumulation, shales can be a continuous layer of hydrocarbon bearing rock, often spread over a wide area. The shale gas success in the US has not only demonstrated the commercialization of shale gas, but has also laid the groundwork to fast-tracking its development in Canada. Shale gas development is emerging in Northeast BC and is now emerging in the St. Lawrence Lowlands of Québec.
Shale Gas Drilling
Although shale gas has been a known resource for a long period of time, production from these formations had only been marginally economic until relatively recent developments in horizontal drilling and completion techniques utilizing hydraulic fracturing.
Innovative horizontal drilling techniques have been key to developing unconventional reservoirs. Compared to vertical wells, horizontal drilling significantly increases the well bore exposure to the hydrocarbon-bearing formation. Not only are initial production rates higher, but more wells can be drilled from a single drilling pad, hence reducing drilling costs and surface impact. However, even with the increasing surface contact within the shale rock and fractures, horizontal wells do not necessarily yield commercial rates of production given the relative narrowness of natural fractures in shale.
Water-based hydraulic fracturing has been a key completion process to enhance production and economic returns. The process consists of pumping large volumes of fresh or slick water at high pressures into the formation. Slick water is a mixture of fresh water and light sand as a proppant. The proppant prevents newly formed fractures from closing. Low-permeability shale’s crack open, thereby creating higher-permeability channels through which gas can flow. The fractures nearest the well bore may be as large as .30 cm to .65 cm wide. Water based ‘fracs’ have proven to be compatible with low-permeability shale’s and typically cost less than gel-based fracs, while delivering comparable production rates and long-sustaining fractures.
Learn About Horizontal Drilling
Drilling and production of shale gas is very similar to that of conventional natural gas reservoirs; however, due to a lack of permeability, shale gas almost always require fracture stimulation and often require higher well densities for a given area.
One approach that is being widely applied in the development of shale gas is the Packers Plus StackFractm technology.
This unique technology facilitates multistage fracturing along the full extent of the horizontal wellbore. By employing isolated packers (tire-shaped rubber seals) that separate the wellbore into multiple segments or zones, each zone is fracture stimulated separately.
The process improves concentration and containment of a frac within the formation. Compared to a standard completion process where a repeated process of perforation and stimulation has to be run to achieve optimal fracs across the entire wellbore, the StackFractm process is less costly and provides equal or better rates of production relative to standard completion methods.
A StackFractm operation begins with the insertion of a steel liner into the horizontal well. The stimulation begins at the toe of the horizontal wellbore and stage fracs are shifted upward along the liner length.
The Utica Shale is a member of the Ordovician Trenton Group. This highly organic black shale is the source rock for oil and gas accumulation in the region. The Utica Shale was deposited on top of the Trenton Black River over steeply dipping faults which were created during the formation of the St. Lawrence rift valley. The Utica Shale was deposited extensively within the Appalachian Basin. After deposition, the Appalachian Mountain front encroached in a wide SW-NE arc that now forms the SE limit of the St. Lawrence Lowlands
The St. Lawrence Lowlands area is a relatively flat plain dotted with farms and towns along the St. Lawrence River between Québec City and Montréal. The large scale topographic features are the result of sub-aerial weathering and erosion by rivers of the nearly flat-lying Palaeozoic sedimentary rocks in the lowland, which includes sandstone, carbonates and shales.
The Utica shales are predominant throughout the area. The shales can be as thick as 200 meters in some areas and reach a maximum depth of 2800 meters. The Utica shale formation boundary to the north parallels the St. Lawrence River where the shales outcrop. To the south and southeast the shales become discontinuous with the Appalachian Mountains.