Giving tophole top priority

Deepwater wells continue a seemingly endless march toward greater complexity.

Deepwater wells continue a seemingly endless march toward greater complexity. All types of specialized materials and technologies have been developed to allow these wells to succeed. Everything from synthetic muds to expandable tubulars have been applied in an effort to probe the deepest sections of the deepest wells in deepwater.

There is no denying that environmental conditions this far down are extreme. High temperatures and pressure put great demands on equipment and fluids. The extreme length of the well bore makes it difficult to regulate the effective circulating density and receive accurate mud-pulse data. Reaching these deep targets requires a judicial use of casing strings. A target reached with casing too small to produce through is of little benefit.

The risks are great, and of course, so are the rewards, so operators and service companies continue to improve their odds with offset data, real-time data, and increasingly accurate earth models. Real conditions in the wellbore and the formation surrounding it are matched to these models to correct any variation. The final sections of a deepwater well are very much the most technically advanced the industry has to offer.

Synthetic muds that won't break down in the extreme temperatures of these wellbores are used, then special spacer trains are run to remove the muds ahead of specially formulated cements. Every stage of the process is closely monitored by the driller, operator, and specialists from the service provider.

All of this technology and attention has dramatically improved the discovery rate for such wells, and the number of successful completions. While these achievements are good news for the operators and the industry in general, they may represent too narrow of a focus.

In a recent survey of well control incidents in the Gulf of Mexico, it was found that the vast majority of problems do not occur in the highly technical sections of a well bore, but in the well's shallow sections. The majority of problems occur after the surface casing has been set, when simple muds and cements are being used, and relatively little attention is being paid to down-hole conditions.

Operators have access to offset data from nearby wells, which feed into a model of the subsurface. This can be used to identify potential trouble zones deep below the mudline. However, such data are notoriously weak at identifying shallow hazards. In designing a well it would make sense to hold the specially formulated fluids, cements, and data acquisition equipment until deeper, more risky sections of the well. That is where this equipment and technology pay big dividends.

Nine times out of ten the top-hole sections of the wells are drilled simply as a matter of course with no incident. The trouble occurs on that tenth well, when a shallow gas pocket is encountered. The simple muds and cements used in these sections are not very effective in combating annular flow. In addition, it is not common for these sections of a well to be as closely monitored as the deeper zones. Often, there are early indicators of the presence of gas. If, for example, the crew has to up the mud weight even slightly to hold back the formation, this could be a sign of trouble. The hydrostatic pressure created by the mud is sufficient to control the influx of fluids into the wellbore. As the cement sets, however, it becomes self-supporting. Its hydrostatic properties erode, allowing the gas to move in.

It is not simply a case of using less expensive materials in shallow sections of a well. Traditionally, these sections have been drilled quickly with little trouble. The crews are not looking for specific problems, and so they are not identified. Once a problem does occur, the fluids in the hole are not those designed to handle it.

The solution, as is often the case, is one of communication. Prior to spudding the well, the driller, the operator, and the service companies must communicate about any variance in the well parameters that might indicate the presence of gas. The American Petroleum Institute is in the process of developing guidelines that will assist in this effort, but if the attention that is paid to the lower section of such wells were also given to the top-hole section, that would go a long way toward eliminating many well control events.

William Furlow

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