Cementing and Dispersion Proppants
The Importance of Cementing in the Oil and Gas Industries
Hydraulic Fracturing, also known as “fracking” or “fracing”, is a drilling process that unearths natural gas or oil from deep within the earth. During this process, a fluid mixture (water and other proppants) is introduced into the existing cracks and fissures within the producing zone via hydraulic pressure, expanding existing, as well as creating additional, cracks and fissures in the earth to obtain the targeted oil or natural gas. Once the pressurized fluid is removed, the remaining proppants are left behind to keep the cracks and fissures open, which will allow the natural gas or oil to move more easily.
Before the removal of the oil and natural gas can begin, a vertical or horizontal well has to be drilled depending on where the producing zone is located. Drilling cannot be done as one continuous shaft, but rather a series of varying sized shafts starting as a larger diameter shaft that will telescope down to a smaller diameter shaft that will end in the producing zone. This process includes adding varying types of casing as well to provide strength to the sides of the drill shaft to eliminate cave-ins and other debris entering the drill shaft. The drill shaft begins at the surface starting with the conductor casing, followed by the surface casing, intermediate casing, and ends with the production casing.
Each casing provides a unique function and purpose. Surface casing, for example, protects concealed freshwater zones, controls gas and fluid movement, case off unconsolidated formations and lost circulation zones, provides a place to nipple up to the BOP’s and to attach surface equipment, and provides support for the deeper sections of the casing. The intermediate casing seals off large amounts of the open hole and protects it from increased mud weight, block off the gas, water, salt, shale, and lost circulation and troublesome zones, protects and controls pressure zones, and provides a place to hang a production liner. The final casing placed in the drill shaft is the production liner which cases off the gas, water, salt, and shale zones, provides control of the well at the wellhead for production and pressure controls, protects pay zones from unwanted fluids, and provides a place to change out drill fluids to completion fluids.
Silica fume is used in the oil and natural gas industry in a variety of ways. In the oil industry, silica fume can be used in primary applications such as grout in hydraulic sealing in the well bore. It can also be used in secondary applications in repairing leaks, splits, and the closing off depleted zones. When used in the natural gas industry, silica fume works as a blocking agent in oil well grout that prevents gas migration as well as increasing well productivity.
Cement is introduced when the well shaft is drilled to the casing point, and the casing has been set. This primary cement will be used to fill the annulus around the casing after the mud and dirt surrounding the casing has been pumped out. Primary cementing is completed to seal the wellbore from zones containing oil, gas, and water. Another purpose of primary cementing is to fix the casing securely to the side of the well shaft. Water, dry cement, and special additives are mixed to create the cement that will be used during this process. The cement is moved down and through the casing by a high-pressure cement pump. The top of the casing contains the cementing head, which allows the cement to be introduced into the pump and where the crew can control the volume and speed at which the cement is dispersed The cementing head also holds the bottom wiper plug that is used to wipe off the mud from the casing that separates the mud from the cement and the top plug that will clear the remaining cement from the displacement fluid cement. Displacement fluid is also dispersed through the cementing head which then will create pressure and position the cement into place.
The float collar is attached to the bottom of the casing where it will help hold the casing in place. When the cementing has begun, pressure from the cement pump sends the cement and the bottom wiper plug down through the casing, and once the bottom wiper plug is in place, pressure from the cement ruptures the bottom wiper plug allowing the cement to go out through the guide shoe and fill the annulus between the casing and the wall of the shaft. After the determined amount of cement has been dispersed, the top wiper plug is released. The top wiper plug is then released, and pressure from the displacement fluid pushes the top wiper plug to the bottom of the casing that will rest on top of the bottom plug creating a seal, and this will finish dispersing the cement into the annulus. The float valve then holds the cement in place and does not allow it to re-enter the casing. Depending on the type of cement and the mixture used, the typical time that it will take for the cement to harden is between 12 to 24 hours, which then completes the cementing process.