Question

What is the basic difference between drilling a well to recover oil and gas from an anticline and drilling a well to recover natural gas from shale deposits?

Short answer

Anticlinal drilling targets natural reservoirs; shale gas requires hydraulic fracturing.

Step-by-step solution

Understanding Anticlinal Oil and Gas Recovery

In situations where oil and gas are recovered from an anticline, the drilling process involves targeting reservoir rocks that have naturally trapped hydrocarbons in an arch-like geological formation. The hydrocarbons migrate and accumulate at the crest of the anticline, making vertical drilling from the surface to the top of the anticline a common practice.

Understanding Recovery of Natural Gas from Shale Deposits

Natural gas in shale deposits is trapped within fine-grained sedimentary rock layers that have low permeability. This requires a different technique known as hydraulic fracturing, or 'fracking', where water, sand, and chemicals are injected at high pressure to create fractures in the rock, allowing the gas to flow out.

Comparing the Drilling Techniques

The basic difference between the two types of drilling lies in the formation type and the method required to access the hydrocarbons. Drilling into an anticline targets conventional reservoirs where hydrocarbons are concentrated and may flow naturally, while recovering gas from shale requires fracturing the rock to release gas trapped in low permeability formations.

Key concepts

Anticlinal Oil and Gas Recovery

Anticlinal oil and gas recovery involves drilling into special geological formations known as anticlines. An anticline is a type of fold that looks like an arch.
These folds form due to the pressure exerted on the Earth's crust, causing it to bend and create upward bulges. These bulges trap hydrocarbons, such as oil and gas, at their peaks.

• The process begins with identifying potential anticlines using geological mapping and seismic surveys.
• Once an anticline is identified, vertical drilling is typically performed to reach the reservoir rock where the hydrocarbons accumulate.
• Since the hydrocarbons are often under natural pressure, they can flow toward the wellbore without the need for enhanced recovery techniques.

Anticlinal traps are one of the most efficient and accessible sources of oil and gas. They are classed as conventional reservoirs due to their natural high permeability, which allows for easier extraction.

Hydraulic Fracturing

Hydraulic fracturing, commonly referred to as "fracking," is a technique used to extract natural gas and oil from rock formations that are typically impermeable or have very low permeability.
This technique is essential for extracting resources from unconventional reservoirs such as shale rock.
The process involves:

• Injecting a high-pressure fluid mixture composed of water, sand, and chemicals into the rock formation.
• The pressurized fluid creates fractures in the rock, allowing trapped gas or oil to flow to the wellbore.
• Fracturing fluid often contains additives that help maintain the fractures open, facilitating the resource flow.

The main aim is to increase the surface area within the rock, improving the flow of hydrocarbons. While providing access to previously inaccessible resources, hydraulic fracturing has raised environmental and regulatory concerns due to its potential impacts on groundwater and induced seismicity.

Shale Gas Extraction

Shale gas extraction focuses on recovering natural gas from fine-grained sedimentary rock layers called shale. Shale is recognized for its low permeability, which makes direct extraction challenging.
To overcome this barrier, innovative methods like horizontal drilling combined with hydraulic fracturing are employed.
Key aspects include:

• First, drilling is conducted horizontally within the shale layer to access a larger section of the gas-rich rock.
• Hydraulic fracturing is then applied to create pathways within the shale for gas to move through.
• As a result, natural gas can be retrieved more efficiently from the widespread shale plays.

Due to its abundant availability, shale gas has become a significant energy source. However, its extraction requires careful management to mitigate environmental impacts and ensure sustainable practices.