Shale is one of the most common sedimentary rocks in the world and it is primarily composed of clay and fragments of other minerals such as quartz and calcite. Shale can be the source, reservoir and the seal for natural gas. Shale formations normally have low permeability (limited ability for gas or fluids to flow easily through the shale formation) and normally require stimulation techniques (such as fracturing) to economically produce shale gas. Shale gas is natural gas that is attached to, or "adsorbed" onto, organic matter or is contained in thin, porous silt or sand beds interbedded in the shale.
Shale gas is unconventional gas
Shale gas is considered an unconventional source as the gas is contained in difficult-to-produce reservoirs, which require special completion, stimulation and/or production techniques to achieve economic production.
Industry is interested in developing Alberta’s shale gas
As conventional gas plays in Alberta continue to mature, industry is looking towards other potential gas sources for development. Shale has the potential to make a significant contribution to Alberta’s future natural gas supply as advancements in drilling and completion technologies allow the economic development of some shale gas resources.
Alberta’s shale gas and where it is found
The Alberta Energy Regulator (AER), is evaluating the shale gas resource potential of all prospective shale gas formations in Alberta. As many as 15 prospective shale gas formations have been identified, five of these formations (Duvernay, Muskwa, Basal Banff/Exshaw, North Nordegg, and the Wilrich) may contain up 1,291 trillion cubic feet (TcF) of gas-in-place. The Alberta Shale Data report contain estimates of the volumes. Given shale resources very early stages of development in Alberta, it is not yet known what portion of these resources can be economically produced.
Shale gas production techniques
The same techniques used for conventional shallow gas development may also be used with shale gas drilling. Shale gas can be produced from vertical and directional wells. However, due to the low permeability that is characteristic of shale formations, horizontal drilling and stimulation techniques such as multi-stage hydraulic fracturing are often required to achieve economic production. Vertical and directional shale wells are commonly commingled with other production zones to yield economic production. To date the majority of Alberta’s production has been achieved using vertical wells along with a few horizontal wells. In recent years, improvements in the use of horizontal drilling combined with multi-stage fracturing have resulted in some shale gas formations becoming some of the most attractive natural gas resources in North America.
Determining the number of wells per section
Shale gas can be developed using various drilling and completion methods depending on the reservoir’s characteristics but generally, due to its low permeability, 8 to 16 subsurface drainage wells per section are required to effectively recover the underground gas in a shale play.
Thermogenic shale gas, normally formed during deep burial where heat and pressure cook the organic matter releasing natural gas, is often developed using horizontal drilling and multi-stage hydraulic fracturing. These techniques can be combined with multi-well pad drilling to reduce the number of well sites per section that are needed. The production from one horizontal wellbore with 10 fracture stages may in some cases be equivalent to drilling 10 vertical wells with a single fracture each.
While early shale gas development in Alberta has occurred primarily through the use of vertical wells, horizontal drilling with multi-stage fracturing methodology is becoming more common.
The Alberta Energy Regulator (AER) regulates well spacing.
Regulating shale gas development in Alberta
Alberta has extensive experience in the development of energy resources and has a strong regulatory framework already in place. Shale gas is currently regulated under the same legislation, rules and policies as conventional natural gas. Although shale gas development in Alberta has not been using horizontal multi-stage fracturing extensively, Alberta does have considerable experience with hydraulic fracturing. More than 179,000 wells have been fractured in Alberta since the 1950s.
Most aspects of the oil and gas industry are regulated by the AER, the AER;
- sets requirements for drilling and production operations
- protects our fresh water aquifers (groundwater) with strict regulations that are designed to ensure that gas cannot migrate up a well bore to contaminate groundwater sources. Well bores are required to have cemented casings in place that meet stringent requirements and which are also set to depths far below any fresh water aquifers. This ensures there is an impenetrable barrier between the shale gas formation and the well bore so that gas and fluids are unable to use the well bore as a pathway to contaminate groundwater.
Alberta has strict requirements in place to manage the safe disposal of produced fluids (fluids that return to the well head as part of the hydraulic fracturing process) and does not allow produced fluids to be sent to municipal waste water treatment systems. Fluids that cannot be treated and recycled must be disposed of in approved disposal wells where the fluids are injected deep underground for permanent disposal.
U.S. Department of Energy: Modern Shale Gas Development in the U.S.