Бурение грунтовых зондов, установка энергетических колодцев

Drilling conditions that affect the selection of casing sizes are: bit size required to drill the next depth, borehole hydraulics and the requirements for cementing the casing.

Drift diameter of casing is used to select the bit size for the hole to be drilled below the casing shoe. Thus, the drift diameter or the bit size determines the maximal outside diameter of the successive casing strings to be run in the drilled hole. Bits from different manufacturers are available in certain standard sizes ac­cording to the IADC (International Association of Drilling Contractors). Almost all API casing can be placed safely without pipe sticking in holes drilled with these standard bits. Non-API casing, such as thick-wall casing is often required for completing deep holes. The drift diameter of thick-wall pipe may restrict the use of standard bit sizes though additional bit sizes are available from different manufacturers for use in such special circumstances.

The size of the annulus between the drillpipe and the drilled hole plays an im­portant role in cleaning the hole and maintaining a gauge hole. Hole cleaning is the ability of the drilling fluid to remove the cuttings from the annulus and depends mainly on the drilling fluid viscosity, annular fluid velocity, and cutting sizes and shapes. Annular velocity is reduced if the annulus is too large and as a consequence, hole cleaning becomes inadequate. Large hole sections occur in the

Table 3.2: Typical drilling and mud programs for a 19,000-ft well.

Drilling program:

0 — 350 ft —> 26-in. hole

350 — 5,000 ft -► 20-in. hole

5,0 — 11,100 ft 17.5-in. hole 11,100 — 14,000 ft -» 12.5-in. hole

14,0 — 19,000 ft -> 8.5-in. hole

Casing program

0 — 350 ft —> 20-in. conductor pipe

0 — 5,000 ft —*■ 16-in. surface casing

0 — 11,100 ft —> 13.375-in. intermediate casing

11,100 — 14,000 ft -► 9.625-in. liner

0 — 19,000 ft —+ 7-in. production casing

shallow portion of the well and obviously it is here that the rig pumps must de­liver the maximum flow rate. Most rig pumps are rated to 3,000 psi though they generally reach maximum flow rate before rated pressure even when operating two pumps together. Should the pumps be unable to clean the surface portion of the hole because they lack adequate capacity then a more viscous drilling fluid will need to be used to support the cuttings.

With increasing depth, the number of casing strings in the hole increases and the hole narrows as does the annular gap between the hole and the casing. Fluid flow in such narrow annular spaces is turbulent and tends to enlarge the hole sections which are sensitive to erosion. In an enlarged hole section, hole cleaning is very poor and a good cementing job becomes very difficult.

Annular space between the casing string and the drilled hole should be large

enough to accommodate casing appliances such as centralizers and scratchers, and to avoid premature hydration of cement. An annular clearance of 0.75 in. is sufficient for a cement slurry to hydrate and develop adequate strength. Similarly, a minimum clearance of 0.375 in. (0.750 in. is preferable) is required to reach the recommended strength of bonded cement (Adams. 1985).

In summary, the selection of casing sizes is a critical part of casing design and must begin with consideration of the production casing string. The pay zone can be analyzed with respect to the flow potential and the drilling problems which are expected to be encountered in reaching it. Assuming a production casing string of 7 in outside diameter, which satisfies both production and drilling requirements, a casing program for a typical 19.000-ft deep well is presented in Fig. 3.5. Table

3.1 presents the drilling fluid program, pore pressures, and fracture gradients encountered at the different setting depths.

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