|Figure 1: Shale sample in water- methane gas bubbles|
The existence of oil and natural gas in shale formations across the world has long been known by experts in the oil and gas industry. Shale is one of the earth’s most common sedimentary rocks. It is a fine-grain rock composed mainly of clay flakes and tiny fragments of other minerals. Shale can be a gas reservoir, but only formations with certain characteristics are viable for exploration. Thermogenic (from the Greek word meaning ‘formed by heat’) gas forms when organic matter in shale is broken down at high temperatures, often created by burial deep underground. The gas is then reabsorbed by organic material to trap the gas within the shale.
Geologists have understood for decades that shale formations are the source of oil and natural gas from “conventional” production extracted from sand and carbonate rock formations. However, the extraction of these energy resources was considered technically impossible to recover because the shale formations lack the permeability (interconnected spaces between the rocks) that would allow the oil or natural gas to flow to a well.
How is Shale Gas Recovered?
Horizontal drilling involves drilling a well from the surface downward to a point where the borehole is turned and the well is drilled along a horizontal plane. The figure below illustrates a cross sectional view of a horizontal well showing how the well is drilled downward to a point just above the target formation and then drilled horizontally into the shale. A horizontally drilled well exposes a greater area of the shale reservoir, which allows a greater volume of oil or natural gas to migrate into the wellbore. The greater exposure to the shale reservoir provided by a horizontal wellbore is necessary because of the low permeability of a shale formation. To recover this volume of gas in the past many vertical wells would have been drilled from the surface requiring the use of more surface land. In order to access the natural gas trapped within the shale, the formation is hydraulically fractured (see our hydraulic stimulation section).
Figure 3: schematic of horizontal well undergoing hydraulic stimulation