DRILLING TECHNOLOGY

Rotary steerable systems in the Gulf of Mexico gain acceptance

 RSS is now standard practice for deepwater GOM wells. 

Amanda Weber, Ivor Gray, Russ Neuschaefer, Dennis Franks, Goke Akinniranye, Schlumberger, and Ronald Thomas, PPI Managed Risk

Oilfield service companies have implied an order-of-magnitude improvement in drilling days with the use of Rotary Steerable Systems (RSS) in directional-hole sections. In a recent publication, Spears and Associates estimated that "the typical Gulf of Mexico well saw drilling days fall by 50% due to Rotary Steerable Technology (RST)," but qualified the statement by stating that the data to support it may not be available for several years.¹

Many complex factors contribute to a successful hole section, making it difficult to quantify RSS benefits. The overall impact that RSS has on the industry is even more involved. Service companies are forced to compare their results to mud-motor-derived drilling estimates, benchmarking their performance to wells drilled years earlier and under different environments. Often the mud-motor estimates for a section are either dramatically higher or lower than what would have occurred, giving a false impression of RSS performance.

This article reviews Gulf of Mexico (GOM) examples where mud motors and rotary steerable systems have been used under similar conditions (i.e. depth, rig and mud system). The RSS and bit are viewed as one system, because the success of one is directly affected by the other. This investigation compares Spears’ anecdotal statement with industry data to quantify RSS benefits in the GOM.

When rotary steerable services were first introduced to the market in 1996, they were heralded as the future’s enabling technology. This technology was readily applied to the Extended Reach Drilling (ERD) and ultra-deepwater markets, allowing drillers to overcome technical barriers that mud motors were not capable of breaking, Fig. 1.² However, due to high cost and low reliability, RSS was not viewed by the general community as a direct motor replacement technology.

Fig. 1. Rotary steerable systems were first used as an enabling technology for extended reach drilling (oval), modified after SPE 99124.

Improvements in tool reliability over the next eight years pushed RSS into other high-tier markets, areas where a few drilling days saved (faster RSS drilling versus motor drilling) and high spread rates could easily offset the new tool’s higher operational cost.³ RSS market share (directionally drilled footage) increased to almost 15% in 2005 from 2% in 2000.^{1, 4}

Today, RSS has captured more than 75% of the floating rig market, leading to a debate over RSS’ future growth. Some believe simple economics will slow RSS market expansion, since it is difficult to justify the high day rate on land or the GOM shelf where savings at lower spread rates will not always outweigh the tool cost.⁴

Quantifying RSS effects in the GOM is a difficult task. First, there is no comprehensive public database readily available to compare a large number of wells at the same time. Second, it is often difficult to make these comparisons because many variables change from well to well. The rig, the crew’s experience, the operator’s experience in the area, as well as environmental factors (formation lithology, mud type, mud weight etc.) all play an important role in drilling a successful hole section. In general, operators and service companies determine RSS success by comparing with AFE curves, which may not always be representative of what a motor would have actually done in the area.

The authors quantified and compared RSS and motor performance at three levels:

• GOM-wide analysis—using drilling statistics from one major service company for all GOM wells drilled with RSS and motors over the last three years
• Area-wide analysis—using data from one GOM operator who used motors and two major RSS suppliers to drill in a localized area
• Offset well analysis—which compared RSS and motors in two wells, where the tools were run in the same depth intervals under similar environmental conditions.

GOM-WIDE ANALYSIS

The largest RSS supplier in the Gulf has been tracking drilling statistics for wells they drilled in the GOM using motors and rotary steerables for the last three years. Using the total footage drilled and the total operating (circulating) times for each tool size, it is possible to develop an overview of RSS’ market impact.

In Table 1, the data was used to calculate an effective ROP, total footage divided by the circulating hours. A 6¾-in. tool is commonly used to drill hole sizes from 9 7/8–8½-in. for both motors and rotary steerables. The 3-yr average of this circulating-ROP shows a 38% improvement over a motor, when using a RSS. For the smaller hole size, 6¾–6-in. hole, which typically uses a 4¾-in. tool, the advantage increases to 85%. This is because RSS can apply more weight-on-bit at a higher flowrate and does not have the alignment troubles common with a motor in these hole sizes.

TABLE 1. Gulf of Mexico ROP comparison, based on footage per circulating hour.

Assuming that an operator drills a hypothetical well with an 81⁄2-in. section that is 5,000-ft in length and a 6½-in. section that is 2,500-ft in length, with a 5-day casing run in between, drilling days would be reduced over the hole by about six days or 28%; 27% in the upper section (9 7/8–8½-in. hole size) and 46% in the lower section (<6¾-in. hole size). Assuming a spread rate of $250,000/day this translates into more than $1.5 million saved.

AREA-WIDE ANALYSIS

In a recent study, a GOM operator asked for a review of RSS versus motor data for three areas, where the operator was planning to increase activity. The areas reviewed included: Grand Isle (GI), West Delta (WD), and Mississippi Canyon (MC), Fig. 2. In these areas, the operator had drilled 19 wells and information was available for 15 of these, Table 2.

Fig. 2. Well information was gathered for three areas offshore Louisiana�Grand Isle, West Delta and Mississippi Canyon.

TABLE 2. Summary of 15 well analyses performed in Grand Isle, West Delta and Mississippi Canyon areas.

Different tool sizes are capable of drilling a range of hole sizes. In this example, the 9-in. motors were used to drill a combination of 17½-in. and 14¾-in. hole. The 9-in. RSS was used to drill a 12¼-in. hole. As a result, no data is available to compare the larger hole sizes. Also, in this area most of the 6¾-in. tools were used to drill 9 7/8-in. hole. The runs from the 8½-in. section performed remarkably well, but do not follow the trends set by the other hole sizes.

Fig. 3 shows the average ROP for each hole size. In the 9 7/8-in. hole, the RSS had a relative ROP increase of 15%. The 8½-in. tool had an increase of nearly 160% improvement over a motor. This increase is significantly higher than expected and is caused by the few runs available in this hole size for this area. In the 6½-in. hole, the increase was closer to 31%.

Fig. 3. Averaged ROP for RSS and motors compared by hole size.

Using these ROP numbers and the same footage and casing assumptions used in the GOM-wide study, it is possible to construct a days versus depth curve, Fig. 4. The 9 7/8-in. and 8½-in. sections were combined and weighted, based on the runs performed.

Fig. 4. Days vs. depth for RSS and motors based on the area-wide analysis.

In Section I, which combines the data from the 9 7/8-in. and 8½-in. holes, the drilling-days reduction is about two days or 25% over the section. This is nearly the same result seen in the GOM-wide analysis. In the smaller hole size, Section II, the drilling-days reduction is over 74%. In this example, the drilling-days reduction over the well is 15 days, about 50%.

RSS reliability is often brought into question when making RSS and motor comparisons. Rotary steerables are newer, more complicated and contain more moving parts than a motor, so it stands to reason that it would have a higher failure rate. In Table 2, the motors had 31 runs and zero reported tool failures compared to the RSS, which had 37 runs and eight tool failures, a 21.6% failure rate. However, a hole-size analysis on the same data, Fig. 5, reveals that the RSS actually had a higher average footage per BHA than motors run in similar sections.

Fig. 5. Rotary steerables had a higher average footage per BHA than motors.

Ultimately, the decision of which method to use is a question of economics. In Fig. 6, the motor versus RSS comparison was done by comparing only the closest fields and blocks to one another. The RSS runs for the 97/8-in. hole in a well in the southwestern part of GI had many problems (including hole stability issues, reamer issues, and one RSS failure), making them a poorer choice for motor comparison. However, even including this well where the economics were not favorable for RSS, the operator saved more than $3 million by using RSS.

Fig. 6. Area financial analysis showed that using RSS saved money.

SIDETRACK ANALYSIS

The following example shows a series of sidetracks in Mississippi Canyon, which were drilled over five years, Fig. 7. Two of these sidetracks, ST03 and ST04, were drilled in succession, the first with a motor and the second with RSS. These wells were drilled from the same rig, using comparable mud systems and covering comparable MD/TVD depths. The logs indicated that the geology (based on TVD) did not change significantly between the two wells.

Fig. 7. Sidetracks were drilled over five years in Mississippi Canyon area using RSS and motors in water- and oil-based mud.

The first side track (yellow) from the original hole was drilled in 2001 with a motor and water based mud (WBM). It drilled 5,800 ft in 14 days. ST02 well (green) was also drilled in 2001 with a motor and WBM, completing 5,395 ft MD in 16 days. The final two sidetracks were drilled with oil based mud. ST03 (red) was drilled with a motor for a total footage of 5,374 ft in 16 days. ST04 (blue) used an RSS to drill 8,661 ft MD in 11 days.

Assuming that the other motor sidetracks would be able to maintain the ROP established from the earlier well section to the ST04 TD, the earliest a motor could have completed that sidetrack is almost 21 days (10 days longer than a RSS), based on ST01. ST03 would have taken more than 24 days to reach the same TD.

Using the same relative ROP comparison method, this translates to a 134% increase in ROP from the hole drilled with motors to the hole drilled with RSS. Computing the days saved over the drilling section reveals a 47% reduction in drilled days.

Using the operator spread rate for the well, the cost per foot of ST03 and ST04 can also be compared. In ST03, the cost per foot was $413.81, which is a significant increase over ST04 at $251.60 per foot.

RSS offers many improvements to the wellbore besides ROP enhancement, including an increased ability to steer toward the reservoir.⁴ Fig. 8 and 9 show the planned well path and the actual well path drilled for both the ST03 and ST04. The motor had trouble holding the azimuth tangent angle.

Fig. 8. ST03�s well path planned vs. actual shows that the motor had trouble holding the azimuth tangent angle.

Fig. 9. ST04�s well path planned versus actual shows how much closer the RSS was to the planned path.

In this example, the operator had trouble in the ST03 well and encountered many tight spots, which raised concerns about getting the casing smoothly to bottom. Using RSS, operators drill a smoother borehole, eliminating spiraling and reducing micro-doglegs. Operators are more likely to get casing to bottom on the first run, avoiding time delays and costly sidetracks if the casing gets stuck and cannot reach bottom.

9 7⁄8-IN. SECTION ANALYSIS

One GOM operator prefers to use the point-the-bit RSS systems, since they do not require a specific pressure drop at the bit, allowing them to increase horsepower at the bit. For the first run in this well the HSI was planned at over 6-hp/in2. The operator considered it a best case to be able to kick-off and drill this section at the same speed as the vertical offset well, Fig. 9.

After completing the build, the RSS entered unconsolidated sand. The high HSI caused the hole to washout, making it impossible for the RSS to maintain angle. After pulling out of the hole, the service company suggested that, if the client wanted to maintain HSI at the bit, a motor should be used to get past the unconsolidated sand.

Looking at the drill-time curves, the slope of the RSS and the motor curves showed no significant difference between the two tools’ ROP. Maintaining the tangent angle only required sliding 6% of the time (based on footage) and during the slides the ROP was not greatly affected.

At the end of the motor run there were many tight spots and the operator backreamed out of the hole. The RSS did not experience any of those hole issues, implying that hole quality was better in the RSS section.

ROP was not the only reason for this operator to run RSS. After the bit trip, the client elected to run RSS in the final run, citing that RSS directional control in the drop section and better hole quality was worth the additional cost.

ADDITIONAL RSS BENEFITS

RSS offers many benefits beyond ROP enhancement. One GOM operator claims that, since they began using RSS for their deepwater applications, they no longer have problems with casing runs. Another major operator uses RSS in tight hydraulic windows to ensure good hole cleaning and ECD management. While this does offer “time” savings by reducing non-productive time, it is impossible to quantify those benefits on a larger scale.

Some of the benefits from RSS include:

•Better hole cleaning, for safer operating in a tight hydraulic windows and less circulating time at the end of each run to clean the hole⁵

•Improved log quality, enabling operators to use LWD log data and eliminate wireline runs⁶

•Faster casing runs with fewer problems, eliminating ledges and reducing tortuosity for a smooth in-gauge wellbore⁵

•In-gauge hole allows for better quality cement jobs and casing shoes for safer and deeper drilling in the next section.

IDENTIFYING OPPORTUNITIES

While RSS may not always generate a higher ROP than a motor, overall, operators will see significant days and cost reduction by using RSS instead of a motor. To increase the chances of success there are several methods being used by service companies and operators to determine the steering method that best matches their well. The flowchart is a guide to make a steering selection that best fits each application, Fig. 10.

Fig. 10. The RSS vs. motor flowchart is a guide for a best-fit steering selection.

If ROP is the main driver for selecting RSS or a motor, then some service companies use previous field performance and a favorable bit recommendation, similar to the analysis being performed by bit companies today. If no information is available the motor-derived days estimate will be verified and then they will often use the “30% rule,” reducing the motor estimate 30%. Based on this analysis, 30% is optimistic. In general for 9 7/8–8½-in. hole, a 25% reduction would be a better assumption.

CONCLUSIONS

Spears & Associates suggested that the use of RSS had led to a 50% decrease in drilling days for the typical GOM well. The evidence presented here suggests that the typical GOM well saw drilling days fall by over 25% by the use of RSS in the 97/8–81⁄2-in. hole (where RSS is primarily applied today). There is not enough data in the larger hole sizes to quantify the total savings from RSS over motors, but data did indicate that savings increase significantly as hole size decreases. In the 63⁄4–6-in. hole size, evidence suggests that wells had at least a 46% reduction in drilling days over the section.

Several examples show that some wells will perform far better than the 25% and for some wells RSS may not be the right technological solution. The important thing is to properly analyze the planned wells and select the proper technology for each application.   

ACKNOWLEDGMENTS

The authors thank PPI Managed Risk L.P., Hall-Houston Exploration, Walter Oil & Gas Corp. and Schlumberger for allowing us to publish. This article was prepared from a paper presented at the 2007 AADE National Technical Conference and Exhibition held at the Wyndam Greenspoint Hotel in Houston, Texas, April 10–12, 2007.

LITERATURE CITED

¹ Spears, R., �Oilfield Market Report 2005�Covering the years 1999�2006,� September 2005.
² Musaeus, N., �Ringhorne development: Technologies applied in extended reach drilling�successes failures and communicating risks,� IADC/SPE 99124, presented at the 2006 IADC/SPE Drilling Conference, Miami, Florida, February 21�25, 2006.
³ Wand, P., M. Bible and I. Silvester, �Risk-based reliability engineering enables improved rotary steerable system performance and defines new industry performance metrics,� IADC/SPE 98150, presented at the 2006 IADC/SPE Drilling Conference, Miami, Florida, February 21�23, 2006.
⁴ Warren, T., �Steerable motors hold out against rotary steerables,� SPE 104268, presented at the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, September 24�27, 2006.
⁵ Johnstone, J. A. and D. Allan, �Realizing true value from rotary steerable drilling systems,� SPE 56958, presented at the 1999 Offshore Europe Conference, Aberdeen, Scotland, September 1999.
⁶ Neuschaefer, R., G. Sirkin and E. Tollefsen, �Realizing substantial rig time savings with next generation LWD and directional services,� presented at the Global Drilling Conference, Houston, Texas, April 19, 2005.

THE AUTHORS
	Amanda Weber earned a BS degree with double major in chemical engineering and chemistry from the University of Wyoming. She joined Schlumberger in 2003 as a drilling and measurements field engineer in the Gulf of Mexico. Weber is a member of SPE, AADE, and IADD and is a drilling engineer for Schlumberger in Houston, Texas.
	Ivor Gray earned a BS degree in production engineering, and an MS degree in information technology. He has over 25 years of oilfield experience and has worked extensively in Europe, Asia and North America in technical, sales and operation roles. Gray is presently sales manager for the northern Gulf Coast in Schlumberger Drilling and Measurements.
	Russ Neuschaefer earned a BS degree in aerospace engineering from the University of Oklahoma in 1990 and joined Schlumberger as a field engineer in 1991. He has worked as a field engineer in the Gulf of Mexico, an instructor for new hires and as a field service manager in Aberdeen, Scotland. Neuschaefer is active with SPE, is presently secretary for the Gulf Coast Section and is a sales engineer with Schlumberger in Houston, Texas.
	Dennis Franks earned a degree in electronics from Southern Technical College. He joined Schlumberger in 1996 in the MWD lab. He has held positions of rotary steerable lab supervisor, RSS service quality coach, and RSS domain champion GOM. In 2002, he was elected to the Applied Community Experts (ACE) for Rotary Steerable World Wide. Franks is a member of SPE and IADD and is a directional drilling coordinator for Schlumberger in Houston, Texas.
	Goke Akinniranye earned a BS degree in geophysics/geology from the University of Ife, Nigeria, in 1985. He joined Schlumberger in 1986 as a field engineer and has more than 20 years of oil and gas experience. Akinniranye has served as field service engineer, senior instructor, technical manager and InTouch engineer. He is a member of SPE, AADE and IADD and is presently drilling engineering manager for Schlumberger North and South America in Houston, Texas.
	Ronald D. Thomas earned a BS in petroleum engineering from Mississippi State University and an MBA from Houston Baptist University. He has been involved in the oil and gas industry in the Gulf of Mexico, inland waters and internationally since 1977. Thomas was a co-founder of PPI Technology Services, LP, has been a principle of the company and its predecessor since 1993 and now serves as PPI president.