Jim Dodson
Ted Dodson
James K. Dodson Co.
Recent statistical studies of drilling performance of wellbores on the shelf of the Gulf of Mexico in water depths less than 601 ft show that it still takes about the same time to drill an average well as it did five years ago. The authors examined 804 shelf wellbores (excluding sidetracks) drilled on the shelf from 1998 through August 2002, which amounted to 32% of the total wellbores drilled during that period. The study focused on the drilling efficiency of ROP (rate of penetration – average feet drilled per day) from spud date to the date the final total depth was reached, in relation to the wells' Mechanical Risk Index.
The MRI is a complexity index that rates the difficulty of drilling a wellbore based on several key drilling metrics. These include the drilling depth, horizontal displacements, pressure, and other parameters. Once the metrics are weighted and compiled, the MRI value can be used to compare wells against one another based on their relative complexity.
In this study, the wells examined compared MRI against ROP values to test drilling efficiency (see table). By dividing the wells into MRI frequency groups, it is clear that the wellbores drilled have a near normal bell-curve distribution (see Totals, Wells segment) over a wide range of drilling complexity. These wells were used to analyze the GoM shelf drilling performance over the most recent five-year time period.
Drilling efficiency
When ROP and MRI are averaged for each frequency group and year, the performance metric, ROP/MRI, provides an aggregate ratio of drilling efficiency. A higher ROP/MRI ratio means faster drilling for wells of similar complexity. If this relationship rises over time, then drilling efficiency is improving. This is not the case for the Gulf of Mexico.
For the five-year period, average ROP/MRI has varied widely, ranging from 0.29 in 2002 to 0.45 in 2000 with average drilled well depths between 11,196 and 12,634 ft (see table). The ROP/MRI values show that drilling efficiency has not changed significantly over the period even with the application of better technology tools and equipment.
Recent drilling
GoM shelf drilling has been weighted more toward development drilling in recent years. Much of this effort is for multilateral sidetracks and horizontal drilling, using a single new well bore with multilateral holes or existing wellbores for development step-outs. Wellbore drilling activity has fallen due in large part to the increased use of this development strategy.
The average sidetrack drilled is around 6,000 ft total measured depth from kick-off point for GoM shelf wells. The more sidetracks that are used, the fewer vertical sections there are to be drilled for development wells. With the average wellbore drilled near 12,000 ft drilled depth, two average laterals can be drilled in the place of one vertical well. Thus, fewer rigs have been busy because the need to drill the vertical section has fallen. Exploration wells still require a large vertically drilled section, but the industry has not taken full advantage of the available rigs to drill new deeper wells.
Contracts
There is little incentive for the contract driller in the current economic environment. Turnkey contracts are available for oil companies to limit well costs, but only well known drilling environments are acceptable for drillers to assume the risk and cost of drilling a well. Even the turnkey cost has to be augmented slightly to offset the occasional troubled well or the driller will not be able to sustain his business.
Dayrate contracts provide no incentive to increase drilling efficiency. Drilling safety and optimal hole characteristics are the primary drivers, and the operator carries the risk. At low day rates and with the contractor's need to service both his crews and debt, there is little incentive to maximize drilling efficiency and lower the drilling time from the 31-32 days it takes to drill an average well on the GoM shelf.
Changes needed
For drilling efficiency to rise, a change in the business model is needed. As it presently stands, new improvements in drilling technology are doing little to improve ROP/MRI drilling efficiency.
Millions of dollars have been poured into new rigs, automated equipment, pipe, and bit technologies, but it still takes the same cycle time to get a well drilled. This has resulted in an over-capitalized position for the industry. A better approach to well optimization is needed for the whole industry, coupled with new business models to encourage drilling efficiency at all levels.
2003 drilling
The authors are enthusiastic for the coming 2003 drilling season. Over the next year, 955 wells are fore-cast (see table). Of these 705 are expected on the GoM shelf, while 250 wells are expected for the deepwater.
Strong oil and gas prices will support the increased level of drilling. But, even at this improved rate of drilling, the GoM shelf will be hard-put to replace reserves at current prod-uction levels without deep shelf exploration. With the MMS' shelf royalty incentive for deep shelf gas in place, independents are positioning themselves in 2003 for deep gas exploration.
