Unconventional reservoir
- Also known as
- unconventional, shale reservoir, tight oil, shale oil
An unconventional reservoir is an oil or gas accumulation, such as shale, tight sandstone or coalbed methane, whose permeability is too low for wells to produce economically without special techniques, typically horizontal drilling and multistage hydraulic fracturing.
What is an unconventional reservoir?
In a conventional reservoir, oil or gas has migrated from the rock where it formed into a porous, permeable rock such as sandstone or limestone, where it is trapped and can flow to a well relatively freely. In an unconventional reservoir, the rock holding the oil or gas has such low permeability that it will not flow at economic rates without stimulation. Often the reservoir is the source rock itself, where the hydrocarbons formed.
Types of unconventional reservoirs
- Shale oil and tight oil: oil held in shale or in very low-permeability rocks associated with it.
- Shale gas: natural gas in shale.
- Tight gas: gas in low-permeability sandstones or carbonates.
- Coalbed methane: gas held in coal seams.
Heavy oil and oil sands are sometimes also called unconventional because they need special recovery methods, though for different reasons.
How they are developed
Unconventional development relies on long horizontal wells drilled through the target rock and on multistage hydraulic fracturing to create a large network of fractures. The combination of these technologies turned U.S. shale and tight oil into a major source of production, and tight oil now accounts for a large share of U.S. crude output, from plays such as the Permian Basin, the Bakken and the Eagle Ford.
Conventional vs. unconventional
| Conventional | Unconventional | |
|---|---|---|
| Rock permeability | Moderate to high | Very low |
| Hydrocarbon location | Migrated into a trap | Often in or near the source rock |
| Typical well | Vertical or deviated; stimulation optional | Horizontal with multistage fracturing |
| Production profile | Gradual decline | Steep early decline |
| Typical oil recovery factor | Tens of percent | Single digits |
Challenges
Unconventional wells decline steeply in their first years, so operators drill continually to sustain production, and much of the oil in place is never recovered. Improving recovery depends on how much rock the fractures contact and how well oil moves from the nanopore matrix into the fractures, which is influenced by capillary pressure and wettability.
Frequently asked questions
The terms are often used interchangeably. Strictly, shale oil is oil produced from shale, and tight oil is oil from any very low-permeability rock, including tight sandstones and carbonates.
Most early production comes from rock near the fractures, which drains quickly. Flow from the tight rock farther away is slow, so rates fall steeply before leveling off.
Sources
- unconventional resource, SLB Energy Glossary
- Tight oil and shale gas production, U.S. Energy Information Administration