After the cement has hardened the permeability is very low (<0.1 millidarcy). This is much lower than most producing formations. However if the cement is disturbed during setting (e.g. by gas intrusion) higher permeability channels (5 – 10 darcies) may be created during the placement operation.
Formation water contains certain corrosive elements which may cause deterioration of the cement sheath. Two compounds which are commonly found in formation waters are sodium sulphate and magnesium sulphate. These will react with lime and C3S to form large crystals of calcium sulphoaluminate. These crystals expand and cause cracks to develop in the cement structure. Lowering the C3A content of the cement increases the sulphate resistance. For high sulphate resistant cement the C3A content should be 0 – 3%
The slurry setting process is the result of the cement powder being hydrated by the mixwater. If water is lost from the cement slurry before it reaches its intended position in the annulus its pumpability will decrease and water sensitive formations may be adversely affected. The amount of water loss that can be tolerated depends on the type of cement job and the cement slurry formulation.
Squeeze cementing requires a low water loss since the cement must be squeezed before the filter cake builds up and blocks the perforations. Primary cementing is not so critically dependent on fluid loss. The amount of fluid loss from a particular slurry should be determined from laboratory tests. Under standard laboratory conditions (1000 psi filter pressure, with a 325 mesh filter) a slurry for a squeeze job should give a fluid loss of 50 – 200 cc. For a primary cement job 250 – 400 cc is adequate.
The standard slurry densities shown in Table 2 may have to be altered to meet specific operational requirements (e.g. a low strength formation may not be able to support the hydrostatic pressure of a cement slurry whose density is around 15 ppg). The density can be altered by changing the amount of mixwater or using additives to the cement slurry. Most slurry densities vary between 11 – 18.5 ppg. It should be noted that these densities are relatively high when the normal formation pore pressure gradient is generally considered to be equivalent to 8.9 ppg. It is generally the case that cement slurries generally have a much higher density than the drilling fluids which are being used to drill the well. The high slurry densities are however unavoidable if a hardened cement with a high compressive streng this to be achieved.
The thickening time of a cement slurry is the time during which the cement slurry can be pumped and displaced into the annulus (i.e. the slurry is pumpable during this time). The slurry should have sufficient thickening time to allow it to be:
• Pumped into the casing
• Displaced by drilling fluid until it is in the required place
Generally 2 – 3 hours thickening time is enough to allow the above operations to be completed. This also allows enough time for any delays and interruptions in the cementing operation. The thickening time that is required for a particular operation will be carefully selected so that the following operational issues are satisfied:
• The cement slurry does not set whilst it is being pumped
• The cement slurry is not sitting in position as a slurry for long periods, potentially being contaminated by the formation fluids or other contaminants
• The rig is not waiting on cement for long periods.
Wellbore conditions have a significant effect on thickening time. An increase in temperature, pressure or fluid loss will each reduce the thickening time and these conditions will be simulated when the cement slurry is being formulated and tested in the laboratory before the operation is performed.
The casing shoe should not be drilled out until the cement sheath has reached a compressive strength of about 500 psi. This is generally considered to be enough to support a casing string and to allow drilling to proceed without the hardened cement sheath, disintegrating, due to vibration. If the operation is delayed whilst waiting on the cement to set and develop this compressive strength the drilling rig is said to be “waiting on cement” (WOC). The development of compressive strength is a function of several variables, such as: temperature; pressure; amount of mixwater added; and elapsed time since mixing.
The setting time of a cement slurry can be controlled with chemical additives, known as accelerators. Table 3 shows the compressive strengths for different cements under varying conditions.
The water which is used to make up the cement slurry is known as the mixwater. The amount of mixwater used to make up the cement slurry is shown in Table 2. These amounts are based on :
• The need to have a slurry that is easily pumped.
• The need to hydrate all of the cement powder so that a high quality hardened cement is produced.
• The need to ensure that all of the free water is used to hydrate the cement powder and that no free water is present in the hardened cement.
The amount of mixwater that is used to make up the cement slurry is carefully controlled. If too much mixwater is used the cement will not set into a strong, impermeable cement barrier. If not enough mixwater is used :
• The slurry density and viscosity will increase.
• The pumpability will decrease
• Less volume of slurry will be obtained from each sack of cement
The quantities of mixwater quoted in Table 2 are average values for the different classes of cement. Sometimes the amount of mixwater used will be changed to meet the specific temperature and pressure conditions which will be experienced during the cement job.
There are other, non-API, terms used to classify cement. These include the following:
• Pozmix cement – This is formed by mixing Portland cement with pozzolan (ground volcanic ash) and 2% bentonite. This is a very lightweight but durable cement. Pozmix cement is less expensive than most other types of cement and due to its light weight is often used for shallow well casing cementation operations.
• Gypsum Cement – This type of cement is formed by mixing Portland cement with gypsum. These cements develop a high early strength and can be used for remedial work. They expand on setting and deteriorate in the presence of water and are therefore useful for sealing off lost circulation zones.
• Diesel oil cement – This is a mixture of one of the basic cement classes (A, B, G, H ), diesel oil or kerosene and a surfactant. These cements have unlimited setting times and will only set in the presence of water. Consequently they are often used to seal off water producing zones, where they absorb and set to form a dense hard cement.
Institute of Petroleum Engineering, Heriot-Watt University
There are several classes of cement powder which are approved for oilwell drilling applications, by the American Petroleum Institute – API. Each of these cement powders have different properties when mixed with water. The difference in properties produced by the cement powders is caused by the differences in the distribution of the four basic compounds which are used to make cement powder; C3S, C2S, C3A, C4AF (Table 1).
Classes A and B – These cements are generally cheaper than other classes of cement and can only be used at shallow depths ,where there are no special requirements. Class B has a higher resistance to sulphate than Class A.
Class C – This cement has a high C3S content and therefore becomes hard relatively quickly.
Classes D,E and F – These are known as retarded cements since they take a much longer time to set hard than the other classes of cement powder. This retardation is due to a coarser grind. These cement powders are however more expensive than the other classes of cement and their increased cost must be justified by their ability to work satisfactorily in deep wells at higher temperatures and pressures.
Class G and H – These are general purpose cement powders which are compatible with most additives and can be used over a wide range of temperature and pressure. Class G is the most common type of cement and is used in most areas. Class H has a coarser grind than Class G and gives better retarding properties in deeper wells.
There are many reasons for using cement in oil and gaswell operations. As stated above, cement is most widely used as a seal between casing and the borehole, bonding the casing to the formation and providing a barrier to the flow of fluids from, or into, the formations behind the casing and from, and into, the subsequent hole section (Figure 1). However, when placed between the casing and borehole the cement may be required to perform some other tasks. The most important functions of a cement sheath between the casing and borehole are:
• To prevent the movement of fluids from one formation to another or from the formations to surface through the annulus between the casing and borehole.
• To support the casing string (specifically surface casing)
• To protect the casing from corrosive fluids in the formations.
However, the prevention of fluid migration is by far the most important function of the cement sheath between the casing and borehole. Cement is only required to support the casing in the case of the surface casing where the axial loads on the casing, due to the weight of the wellhead and BOP connected to the top of the casing string, are extremely high. The cement sheath in this case prevents the casing from buckling.