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About Hydraulic Fracture Stimulation





What is hydraulic fracture stimulation, how is oil and gas extracted?


Hydraulic fracture stimulation (fracking) involves pumping fluids and ‘proppants’ (solid material such as fine sand grains or ceramic beads) into a low-permeability rock under high pressure to induce fractures. The fluid is typically 90 per cent water with 9.5 per cent proppant and 0.5 per cent chemical additives. Additives are used to suspend the proppant in the fluid, stop microbial growth, prevent corrosion and make it easier for the fluid to move through the fractures. When the pumps are turned off, the proppant contained in the fluid remains in place, holding the fractures open and allowing the hydraulic fluids and the oil and gas to flow out of the rock formation and up the wellbore (Figure 1)
Figure 1. Schematic diagram showing a horizontal well intersecting a shale formation, in a typical Western Australian setting. At the surface, hydraulic fracture stimulation equipment (including the frack tree) is attached to the Christmas tree on top of the well. The horizontal section has undergone hydraulic fracture stimulation. Lower inset shows induced fractures in the shale formation. Source: Department of Mines, Industry Regulation and Safety, WA.
 
Oil and gas is extracted from the low permeability rock in the immediate vicinity of the induced fractures. Initially oil and gas production rates are high as the oil and gas that is made available by the hydraulic fracture stimulation flows to surface and is produced. This initial high production rate will decrease as the amount of oil and gas left adjacent to the fractures diminishes. The effectiveness of hydraulic fracture stimulation can be enhanced in combination with horizontal drilling, which enables multiple fracture stages in a well.



What is the difference between conventional and unconventional petroleum resources?

Shale oil and gas, tight oil and gas and coal seam gas are ‘unconventional’ because the resource may not flow freely into a petroleum well. The pore spaces in these rocks are tiny and not connected so that it is difficult for the oil and gas to move through the rock. These types of rock are described as having low permeability, which is the low ability for oil and gas to flow (Figure 2).

Petroleum resources that flow freely are 'conventional'. It occurs in sandstones and carbonate rocks that have higher permeability, such as those found in the offshore sedimentary basins of Western Australia’s North West Shelf and many conventional gasfields in the northern Perth Basin.

Shale oil and gas is contained in a type of sedimentary rock formed of very fine-grained, or small, particles, such as clay, that have been compacted to form a layered rock. Tight oil and gas refers to petroleum resources found in low permeability reservoir rocks that are most often sandstone, but also include low permeability carbonate rocks.

Natural oil and gas from shale and tight rocks is generally the same as natural oil and gas from conventional sources. Natural gas is a mixture of combustible gases, mostly methane, with smaller amounts of ethane and propane.
Figure 2. A microscopic view of sandstones ranging from very low permeability rock to high permeability rock. Source: Department of Mines, Industry Regulation and Safety, WA.



How do WA’s shale and tight oil and gas resources differ from coal seam gas?

Coal seam gas (CSG), which is sometimes known as coal bed methane, is natural gas that is formed by, and found in association, with certain coal deposits. CSG typically lies at depths of 300 to 1000 metres. While the CSG industry is established in Queensland, to date, CSG has not been demonstrated as prospective in Western Australia.
In Western Australia, shale and tight oil and gas resources generally lie at depths between 2000 and 4000 metres. These depths usually lie significantly below groundwater resources and under multiple thick layers of low permeability rock that act as barriers between the formation and any water resources, and the land surface.

With CSG, gas is extracted by drilling wells into the coal seam. The goal is to decrease the water pressure by pumping groundwater from the well, a process known as dewatering. The decrease in pressure allows gas to be released from the coal and flow up the well to the surface. CSG extraction relies upon dewatering and only sometimes requires water for fracking the coal seam. In contrast, the production of shale and tight gas does not require the removal of groundwater to release gas, but will almost always require hydraulic fracture stimulation.

Hydraulic fracture stimulation for shale and tight oil and gas would require water. The amount of water required ranges from five to forty million litres, depending upon the type of well. It is estimated that a vertical well with three fracking stages would require seven million litres of water. This is the same as approximately three Olympic size swimming pools.

A horizontal well, as shown on Figure 1, (with a horizontal length of one kilometre) with ten fracture stages is likely to require 21 million litres of water per well. This is the same as approximately 8.5 Olympic size swimming pools.

Table 1. Typical differences between coal seam gas and shale and tight oil and gas. The data given in the table below will vary from case to case. Sources: CSIRO, Northern Territory Government.

  Coal seam gas Shale and tight oil and gas
Source Shallow coal seams Deeper shales and tight rocks
Depth 300m – 1000 m 2000m – 4000m
Drilling direction Mainly vertical Horizontal and vertical
Proximity to aquifers Shallow and therefore closer to potable water resources Deeper and therefore further away from potable water sources
Surface footprint Single exploration well per drill pad (a drill pad is the area disturbed by the installation of the drilling and extraction equipment) meaning a larger number of well heads in a given area. Multiple wells can be drilled from each well pad meaning a lower number of completed well heads in a given area than for coal seam gas.
Hydraulic fracturing Hydraulic fracturing in some coal seams with low permeability Always requires hydraulic fracturing
Hydraulic fracturing extent (length x height) 200 – 300 m x 5 – 30 m 200 – 6000 m x 30 – 300 m
Hydraulic fracturing pressure 35 MPa or 5,000 psi 35 – 70 MPa or 5,000 –10,000 psi
Water use Requires dewatering of coal No dewatering, but water is used for drilling and hydraulic fracturing
Hydraulic fracturing fluid per volume per well Approximately 1 ML (0.1 – 3 ML) Approximately 20 ML (5 – 40 ML)
Number of wells required Larger number of wells Fewer wells required than for coal seam gas
Productivity (over lifetime of well) Lower gas recovery
(0.5 – 2PJ per well)
Higher gas recovery
(2 - >10PJ per well)
 


What chemicals are used in hydraulic fracture stimulation?

Hydraulic fracture stimulation fluid is generally composed of around 90 per cent water. The next largest constituent is the proppant, which is usually sand or ceramic beads, which represent around 9.5 per cent of the fluid. Chemical additives represent approximately 0.5 per cent of the fluid (figure 3).

 


Figure 3. The make-up of typical hydraulic fracture stimulation fluid. Source Department of Mines, Industry Regulation and Safety, WA.

While a variety of chemicals may be used in fracking fluid, mixtures vary depending on locally specific needs. They are typically added to improve the transportation of the proppant, prevent the growth of bacteria, reduce mineral clogging and prevent well corrosion over time. In Western Australia, all chemicals to be used for any phase of a well’s life, including hydraulic fracture stimulation, need government approval prior to being used and are subject to full public disclosure. Specific chemicals used in fracture fluid depend on the properties of the rock being targeted. Types of chemicals and their use in hydraulic fracture stimulations are detailed below.
 

Table 2. Types of chemicals used in fracking fluids, their purpose and typical examples. Source: Department of Mines, Industry Regulation and Safety, WA.

Types of Chemicals

Purpose

Examples

Proppants

Proppant or tiny solids (e.g. sand) are used to physically hold open tiny rock fractures or cracks and to allow fluids and gas to move around them

Crystalline silica (quartz), crystalline silica (cristobalite), ceramic

Biocides

Limits the growth of bacteria in fluids which may reduce flow rates and contribute to well corrosion

Glutaraldehyde, sodium hypochlorite, quaternary ammonium chlorides

Oxygen scavengers and corrosion inhibitors

Removes or deactivates oxygen and other corrosive material in fluids which contribute to well corrosion

Zinc carbonate, isopropanol, methanol, formic acid, gelatine, sodium sulfite

pH control, buffers, salts, stabilisers, solvents

Adjusts the chemical and physical properties of the fluid to achieve optimal flow rates

Potassium carbonate, sodium acetate, sodium carbonate (soda ash), hydrochloric acid, acetic acid, ethylene glycol, magnesium silicate hydrate (talc), magnesium oxide

Friction reducer

Reduces the friction forces of fluids being pumped into the well to increase flow rates

Polyacrylamide, hydrotreated light petroleum distillate, methanol, ethylene glycol, sodium lignosulphate, glycerine

Clay inhibition, stabiliser

Counters clay swelling in the well when drilling and in the rocks being fractured to optimisedrilling and flow rates

Sodium chloride, isopropanol, tetramethyl ammonium chloride, potassium chloride, magnesium nitrate, silica gel

Gelling agents, binders, cross linker

Increases the thickness of fluids which allows more proppant to be carried into rock fractures

Bentonite, boric acid, triethanolamine, sodium chloride, hydrogen peroxide, sodium sulphate, guar gum, xanthan gum

Breakers

Breaks down the gelling agents and releases the proppant into rock fractures

Sodium persulfate, hemicellulase enzyme, ammonium persulphate, magnesium oxide

Surfactants

Reduces the stickiness of fluids to improve flow rates

2-Butoxyethanol, ethanol, lauryl sulphate