Blast Furnace Slag
Definition - What does Blast Furnace Slag mean?
Blast furnace slag is a non-metallic residue usually obtained from steel plants or from the reduction of ores in a blast furnace. It is used as a mud cake modifier for the cementing of oil and gas wells.
It has been found that using a slag mix for cementing is beneficial economically and technically as well as better for the environment than Portland cement. Cement made with blast furnace slag has lower permeability than Portland cement and reduces the diffusion rate of ions through the hardened cement. This improves its durability in the presence of salts such as chloride and sulfate.
Blast furnace slag has a typical composition of:
- Silicon dioxide.
- Aluminum oxides.
- Iron oxides.
- Magnesium oxide.
- Sodium oxide.
- Calcium oxide.
The petroleum industry has a great interest in the use of blast furnace slag for converting drilling mud into suitable cement for well cementing operations for zonal isolation.
The use of blast furnace slag imparts cementing properties to the drilling fluid which converts to cement with the addition of slag. This also reduces the volume of mud that needs to be disposed of. On reaching the casing point, a mixture of drilling fluid, chemical activators, and high concentrations of hydraulic blast furnace slag are pumped in. This allows the drilling fluid to set, creating a complete seal for the annulus.
Usually, the annular space between the casing and the borehole is done using Portland cement. However, it has its own challenges such as effectively removing the drilling fluid from the annulus and its effect on the cementing properties of the cement slurry.
Trenchlesspedia explains Blast Furnace Slag
Blast furnace slag is durable and has long-term compressive strength due to its pozzolanic and hydraulic characteristics when finely ground. Since zonal isolation in the annular space between the casing and the borehole is very important, it has led to the development of a method to obtain improved zonal isolation using drilling fluid solidification technology.
The blast furnace slag is used to convert the water-based drilling fluid into cement. Where other solidification techniques are difficult to implement, blast furnace slag can be used for this purpose as it has a low impact on drilling fluids rheological and fluid loss properties. Due to its low impact, it can even be added during drilling operations at low concentrations.
Solidification of drilling fluid using this method has many benefits such as:
- Improved zonal isolation.
- A good combination of fluid and solid properties.
- Reduced burden of fluid disposal.
- Simple design.
- Easy application.
According to a paper published by the Society of Petroleum Engineers, the density of fluids that can be prepared using blast furnace slag is between 1198 kg/m3 to 2397 kg/m3. The mixture can be applied in wells with temperatures ranging between 40°F to 600°F.
Types of Blast Furnace Slag
Blast furnace slag primarily consists of silicates, alumino-silicates, and calcium alumina-silicates. Depending on the method used to cool the molten slag, different types of products are produced:
Air-cooled blast furnace slag
- Angular, roughly cubical, and rough, porous, or smooth textures when finely ground
- High water absorption up to 6%
Foamed blast furnace slag
- Angular and roughly cubical in shape, higher porosity and a rougher texture when ground than air-cooled slag
Pelletized blast furnace slag
- Smooth texture and rounded shape when ground, keeping the porosity and water absorption much lower than the air-cooled or foamed slags.
Granulated blast furnace slag
- Glassy granular material and varies in structure depending on the chemical composition and method of production.
- As fine as cement when ground and is used as supplementary cementitious material in Portland cement concrete.
- Conversion of Drilling Fluids to Cements With Blast Furnace Slag: Performance Properties and Applications for Well Cementing. (1992). P. SPE Annual Technical Conference and Exhibition.