Well Cementing, Cement Slurry and Additives

Well Cementing

Well-cementing is a process that involves mixing dry cement and certain additives with water, to form slurry that is pumped into the well through the casing and place in the annular space between the hole and the outer diameter of the liner.

This operation requires adequate planning to select the systems of cement and washing fluids and spacers shall be used, as well as to define the displacement conditions of these systems to obtain good adhesion between the phases cement-forming pipe and securing a seal effective that isolates the different geological layers and to support the pipe.

Cementing has great importance in the life of the well since the works of a satisfactory completion depend directly on good cementation. The defective cementation operation would bring drastic consequences such as increased costs, risk of loss of the well, risks to the environment, and safety.

Cementing Function

So, what are well- cementing operations for? Well, it is carried out with several purposes, among which we can highlight:

• Protect and secure the casing pipe in the hole.
• Isolate areas of different fluids.
• Isolate areas of surface water and avoid contamination by drilling fluid or well fluids.
• Avoid or solve problems of loss of circulation and sticking of pipes.
• Repair wells due to fluid channeling problems.
• Repair leaks in the liner.
• Protect the hole from collapse.

Cementing Technologies

Primary cementing. It is done by cementing the well-casing during drilling. The main objectives of this cementation include: i.- adhering and fixing the coating strand, ii.- restrict the movement of fluids between the producing formations and the containment of the aquifer strata, iii.- protect the strand against corrosion, reinforcing the strand against crushing due to external forces, iv.- reinforcing the resistance of the strand to bursting pressures, v.-protecting the strand during shelling work (completion), vi.- seal the loss of circulation in "thief" areas.

Secondary or remedial cementing. Cement slurry is used to repairs/refurbishments or well-completion tasks. The main purposes of this cementation are: i.- repair deficient primary cementation works, ii.- reduce high water or gas production, iii.- repair leaks caused by coater failures, iv.-leave areas not producing or depleted, v.- sealing off circulation loss areas to protect fluid migration to production areas. Secondary cementing is defined as the processes of pumping cement slurry into the well under pressure, forcing it against a porous formation, either in the coater holes or directly into the open pit.

To perform cementation operations, some information is required:

1. Mechanical condition data. Bore diameter discovered, depth, deviation, diameter, and weight.
2. Field data. Static and circulating background temperature, type of formation, pore pressure, and fracture pressure.
3. Details of the fluids involved. Type, rheology, and density of drilling mud, cement slurry, and washing fluids and spacers. Compatibility tests on cement-sludge, sludge-spacer fluid and spacer-cement fluid are recommended to avoid undesirable reactions between fluids.

Classes of Cement – Nine API Classes

Cement is a mixture of limestone and clay, subjected to calcination and grinding, which has the property of hardening upon contact with water and used as a binder. Some of its most important features are:

• Raw material from calcareous and argillaceous rocks (limestone, clay, shale and slag)
• Chemical compositions based on lime, alumina, iron oxides and silica
• Calcination temperature of 2600-2800 °F

There are different types of cement, which are differentiated by their composition, strength, and durability properties, and therefore by destinations and uses. Below is shown the general classification:

Portland cement is used for oil wells. The API has defined nine (9) classes based on the use, the range of depth, pressures, and temperatures to withstand, and the ratio of the four fundamental chemical components.

The standard governing classification and implementation recommendations is API RP-13B-1, 1995. A summary of this classification is shown in the table below.

Cement Slurry

A cement slurry is a mixture of dry cement and water. In oil industry is used for the well-cementing process to creating fill the space between the casing and the hole, forming a solid barrier.

In primary cementations, cement slurry must have a viscosity or consistency that provides efficient sludge displacement and allows good adhesion of the cement to formation and casing. To achieve this, the slurry is mixed with a specific amount of water that prevents the separation of free water. Particle size, surface area, and additives all influence the amount of water required in the mixture to achieve a particular slurry viscosity.

Generally speaking, two types of slurry are currently used for well-cementing unless there are other hydrostatic requirements. The first type is a "net" or "tail" slurry with densities from 15.5 lb/gal to 16.5 lb/gal, depending on the type of cement and the temperature at the bottom of a well.

There are cases in which the pressure of the formations to cement is below normal, and well-cementing with conventional mixtures involves a high risk of fracture during pumping. A solution is a multi-stage cementation, which has as main objective the placement of cement in the annular, but this does not indicate necessarily achieve adequate isolation of the producing areas.

Cement Slurry Design

For cement slurry design, it is necessary to know the conditions of the well, as well as the hydraulic power required, flow rate, the volume of slurry, and the relationship between the diameter of the well and the casing. Cement strength data is based on the temperatures and pressures at which the slurry at the bottom of the well is exposed, and indicates the time required for the cement to be strong enough to withstand the casing. The main parameters to be considered are:

• Temperature is the variable that has a more significant influence on the cement slurry design since it affects the compressive strength that develops after setting. As the temperature increases, the slurry gets dehydrated more quickly, which also increases your resistance. If the temperature reaches values greater than230 °F a phenomenon called retrogradation of the cement causing the reduction of compressive stress. This phenomenon can be solving added silica flour to cement slurry.
• The pore-pressure gradient is a parameter that defines the minimum density value of the slurry in such a way that the cement can displace the mud. This is valid if the cement hydrostatic column extends from the surface to the settling depth of the pipe and into static conditions.
• Another design parameter is the equivalent circulation density (DEC), which must be greater than the maximum value of the pore gradient of the training to prevent it from manifesting when the washer and spacer potholes are in annular space.
• Time of cementation that is the minimum time required for the hardening of the slurry by the dehydration reaction of the cement, is usually 1.5 times longer than the duration of the cementation operation.