Month: March 2017

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UNDERBALANCED DRILLING SOLIDS-CONTROL POSSIBLE PROBLEMS

Shale

In general, thick shale sections cause problems with UBD. They slough or cave into the hole. This is probably due to thick shale sections having some elements of laminating, geo pressuring, or sensitivity to water. As a general rule, thick shale sections should not be drilled underbalanced. In the special case of air/gas drilling, shale usually remains stable as long as it is kept dry. Even the small amount of water in mist drilling will destabilize most shale. These formations need to be put behind casing within a few days. Watch out for excessive caving and especially long, thin cuttings. Once caving starts, it cannot be controlled with under-balance operations.

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SOLIDS-CONTROL SYSTEM FOR UBD

The destination, or method of disposal, is one of the major factors in designing the solids-control system for UBD. In the best of all cases, produced oil and water can be skimmed off, cleaned, and sent into the field production system. The solids can be partly dewatered for appropriate disposal. On the other hand, if annular cuttings injection is possible, the volume of waste generated will not be as critical. In this case the waste stream should be kept as wet as possible.

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UNDERBALANCED DRILLING WITH CONVENTIONAL DRILLING FLUIDS OR WEIGHTED DRILLING FLUIDS

UBD or ‘‘balanced’’ drilling with conventional drilling fluids is no different than conventional overbalanced drilling as far as basic solids control is concerned. Special pressure-control equipment is required to control well pressures as they occur. A rotating head, choke, and separator are basic to UBD systems (Figure 1).

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OIL SYSTEMS, NITROGEN/DIESEL OIL, NATURAL GAS/OIL

A special case of liquid/gas fluid is the diesel/nitrogen or synthetic-oil/ nitrogen mixture. These systems have low viscosities and are used primarily in reentries or horizontal slim holes. They tend to require more fluid than water-based fluid systems. Annular velocities of 150–200 ft/ min are common in the horizontal section of the hole. In the drilling of slim holes, hole volumes are less than they are in the drilling of conventional holes, but strong surging occurs. A closed pressurized separator system is used when drilling with an oil/gas system (Figure 19.1.).

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