Use of cost-effective technologies needed
Over the past 10 years, 1993 through 2002, "trouble time" caused by hole problem incidents has consumed 24-27% of Gulf of Mexico operator's drilling budgets. This proportion brackets the general "authorization for expenditure fudge factor" of 25% many drilling engineers use when preparing well plans. Because most wells will have some trouble time, this flat-line time on the drill-time curve must be estimated, and the 25% figure covers trouble time on most wells. The 25% proportion has become the industry's "best practice" because companies have trained the cost-cutting focus on unit prices rather than total project cost effectiveness.
Many new technologies have come into play over the last 10 years, but the proportion of trouble time has remained relatively constant. The industry has not managed to address certain basic drilling issues that create routine problems. Rig equipment failure, lost circulation, weather waits, and stuck pipe are the main issues that form trouble time, as defined by James K. Dodson Co.
Chart of gas wellbore problem incidents drilled in 600 ft or less of water depth with 15,000 ft or less TVD from 1993 through 2002 showing common drilling problems for shallow shelf wells.
Problem incidents are consistent regardless of the depth of the hole being drilled. The average cost per foot rises as the complexity of the well increases, shown by the increasing Mechanical Risk Index. Only recently have operators shown that they are willing to climb the learning curve and spend more money on cost-effective technologies that work in combination to lower the overall cost of drilling a well into the deeper reaches of the GoM Shelf.
The largest proportion of hole problem incidents relates to rig equipment failure, which averages over 20% of the incidents over the past 10 years. The MRI frequency groups have increased for shallow shelf wells, showing more complex drilling of horizontal wells on the shallow shelf. Deep shelf well records show more vertical wells than deviated wells.
Shallow shelf gas wellbores
There were 2,520 gas wellbores drilled in the years 1993-2002. Of this number, 549 were benchmarked in a "problemistic" study of trouble-time incidents in drilling the wells. This amounts to 1,383 incidents in the aggregate, for a total of 4,264 trouble-time days, or 24% of 17,641 days drilling from spud-date to the date the final total depth (FTD) was reached.
The first pie chart depicts the percentage of total trouble incidents attributed by cause for gas wellbores drilled in water depths of 600 ft or less with 15,000 ft or less TVD. Rig equipment failure amounted to 21.9%, weather waits 14.4%, lost circulation 12.7%, and stuck pipe 11.5%.
The first table analyzes 510 gas wellbores in an MRI frequency group well count that expresses averages of rate of penetration (ROP) and MRI, or drilling complexity, as a ratio of ROP to MRI (ROP/MRI). This ratio expresses the units of ROP drilled versus units of the MRI in that frequency group from 1 through 9.
As the complexity of the well increases, with higher MRI values, the ROP/MRI ratio declines. Cost per foot increases. Average days drilling, hole problem incidents, and problem days all increase, as do average bottom hole pressures.
Deep shelf gas wellbores
There were 249 gas wellbores drilled in 1993-2002, of which 102 were analyzed. These wells had 359 trouble-time incidents, accounting for 1,703 problem days. These days were 22% of the 7,680 total days drilling counted from spud-date to the date FTD was reached.
The second pie chart depicts the percentage of total trouble incidents attributed by cause for gas wellbores drilled in water depths 600 ft or less with greater than 15,000 ft TVD. Rig equipment failure incidents amounted to 20.9%, followed by lost circulation 12.8%, waiting on weather at 12.3%, and stuck pipe at 11.1%.
Chart of gas wellbore problem incidents drilled in 600 ft or less of water depth with greater than 15,000 ft TVD from 1993 through 2002 showing common drilling problems for deep shelf wells.
The second table analyzes the 102 gas wellbores in a well count frequency array by ranges of MRI that forms a bell curve. In column E, the ROP in column C is divided by the MRI in column D to compute the ROP/MRI ratio. As this ratio decreases, average cost per foot increases, average days drilling, and problem days all increase.
Trouble time – cost
For wells drilled with 15,000 ft or less TVD, the average hole problem cost computed in the 10-year period amounted to $71/ft of the $291 total average dry-hole cost per foot for 549 gas wellbores (32.2% of total) drilled in 1993-2002. For gas wellbores drilled with greater than 15,000 ft TVD, the average dry-hole cost computed for 102 gas wellbores (45.8% of total) drilled in 1993-2002 amounted to $444/ft. Hole problem days had an average dry-hole impact of $98/ft.
Trouble time – ROP
For 549 shallow shelf gas wellbores analyzed with hole-problems, the average TVD was 10,226 ft. The average FTD amounted to 11,668 ft. The average ROP was 363 ft/day. The impact of trouble time decreased ROP an average of 116 ft/d.
For 102 deep shelf gas wellbores analyzed with hole problems, the average TVD was 17,326 ft and the average FTD was 17,982 ft. The average ROP was 236 ft/d. The impact of trouble time decreased ROP an average of 68 ft/d.
Trouble time – drilling days
Trouble time days drilling for shallow shelf gas wellbores from spud-date to the date FTD was reached amounted to 32 days on average, including eight days lost to trouble time. For deep shelf gas wellbores, the average days drilling to FTD amounted to 81 days, including an average of 22 days lost to trouble time.
For most shallow shelf gas wells, downhole equipment is over capitalized because some equipment installed may last 10-15 years, but may be pulled in one-three years after the well has depleted. A better use of company's drilling and completing dollar would be to spend the money on improving drilling performance by finding new combinations of drilling fluids and tools to reduce trouble time. With trouble time costing $71/ft for shallow shelf wells and $98/ft for deep shelf wells, there could be significant savings by drilling wells with cost effective technologies.
Significant savings await companies willing to challenge the current "best practice" by focusing on total project costs and placing additional money on products and tools that help the driller avoid trouble time.
For information, contact James Dodson, James K. Dodson Co., Tel: 800.275.0439, firstname.lastname@example.org, www.infogulf.com.
Editor's Note: Mechanical Risk Index and MRI are trademarks, and the MRI Calculator is a registered trademark of James K. Dodson Co.