Horizontal-departure-to-true-vertical depth ratio decreasing in US Gulf

Maximum closure distances increasing

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Continued ERD in existing marginal fields will yield a few points in the Dep/TVD range of 4-6. Exploration in ultra-deepwater will drive the data distribution, down and to the right, with Dep/TVD ratios of 0.5-2.0.
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Drilling trends in the 1990s resulted in a majority of extended reach (ERD) wells with horizontal departures in the range of 10,000-15,000 ft. The second and third highest number of wells, were in the 5,000-10,000 ft and 15,000-20,000 ft ranges, respectively (see Mason). These trends were further fueled by the 1997 Royalty Relief Act, which encouraged deepwater drilling by designating geographic areas and allowing deepwater lease-holders to apply for royalty suspensions in these areas.

Looking at horizontal departure (Dep) and true vertical depth (TVD) of wells drilled through the mid-1990s, a general "shallowing" trend is evident. The ratio of Dep and TVD increased over this same time interval. This increase in the Dep/TVD ratio was due to the increased efficiency of directional steering systems in horizontal and multi-lateral drilling applications. The trend first appeared onshore early in the decade, and moved into the offshore sector in the mid-to-late 1990s.

Dep/TVD ratio

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Maximum closure distance and maximum hole inclination of US Gulf wells.
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Prior to the 1990s, deeper wells with Dep/TVD < 2 were the industry standard. The years following this period up until 1998, revealed a gradual trend to more shallow wells (shorter TVD), but with Dep/TVD ratios in the 3-5 range. Global offshore activity increased during this period, with extended reach wells dominated by straight and low-to-moderate slant exploratory holes. This pushed the TVD values higher, and the Dep/TVD ratio even lower.

A misinterpretation can easily be made at this point, due to the below-rotary-table (bRT) scale, and greatly increased water depth of these wells. Data points of these wells plot more to the lower center of the graph. The cluster of data points in the 10,000-20,000 ft range and along a Dep/TVD = 1.5 line, are representative of this trend.

If you assume a linear progression of future activity into deeper waters, one would anticipate a trend downward, and to the right of the graph. (see highlighted area). A few shallow water wells in the future will continue to be drilled, extending horizontal wellbores and pushing the Dep/TVD ratio greater than 5. But, expect the majority of ERD wells in the early 21st Century to be drilled with ratios less than 2.

An abrupt stop in horizontal departure at approximately 27,000 ft was observed at the end of 1995 (see Mason). Two important events occurring in the 1996-1998 time frame contributed to this anomaly. Existing drilling technologies were approaching a critical limit, and a sharper increase in deeper water activity occurred.

The industry, in healthy shape at this time, saw a considerable number of offshore, extended reach projects being drilled. Conventional steerable drilling assemblies were the dominant drilling technique of choice. The limitations of these conventional assemblies were finally limiting the drilling process, instead of contributing to it. At this point, rotary steerable technologies began to emerge as a possible solution for this problem, extending reach capability even further.

Directional closure

Horizontal and extended reach wells drilled in the US Gulf of Mexico pushed maximum closure distances in 1996 and 1997 to even greater lengths - the last years for which there is recorded data available. The US Minerals Management Service maintains a two-year grace period for operators before releasing directional well information. None of the directional wells drilled in the Gulf of Mexico during the 1996-1997 period reached as far as the 18,275 ft record set by British Petroleum (1997). That extended-reach well reached a maximum inclination angle of 73° 32 min.

An interesting anomaly in maximum inclination can be seen in the 85-100° range. The downward trend in number of wells with increasing maximum-well-inclination reversed itself and increased slightly over this inclination range.

In the early to mid-1990s, there was a trend of drilling into shallow producing reservoirs with horizontal wellbores, then continuing to build angle at a gradual rate to 100-105° of inclination. This improved reservoir drainage substantially in some geological settings and became a fad overnight. The technique is still used today in similar forms and added innovations.


Mason, C., Judzis, A., BP Exploration; SPE Paper No. 48942.

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