Бурение грунтовых зондов, установка энергетических колодцев

The properties of a specific cement slurry will depend on the particular reason for using the cement, as discussed above. However, there are fundamental properties which must be considered when designing any cement slurry.

(a) Compressive strength

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.

(b) Thickening time (pumpability)

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:

• Mixed

• 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:

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• 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.

(c) Slurry density

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 strength is to be achieved.

(d) Water loss

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.

(e) Corrosion resistance

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%

(f) Permeability

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.

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