Casing while drilling results in safer drilling, larger wellbores in Lake Maracaibo

Results show that nearly three days less drilling and more than $96,000 can be saved when drilling the surface section with casing while drilling in Bachaquero South West Field, Lake Maracaibo. 

José Francisco Gamarra Martínez, Hernán J. Díaz and Mariana Gamboa, PDVSA; Sael Orellan and André Van Balen, Tesco Corporation

Drilling in troublesome shallow gas areas represents one of the most risky activities in the oil industry. This is the case for Petróleos de Venezuela, S.A. (Pdvsa), in drilling Miocene and Eocene reservoirs in some areas in Lake Maracaibo in western Venezuela.

For years, drilling activity in Lake Maracaibo has been performed with cantilever barges using conventional drilling equipment and standard procedures, with a contingency plan to mitigate the risk of influx and subsequent well control events. Part of this contingency plan is to drill a pilot hole through the gas zone. The pilot hole then is under-reamed and cased. The time to construct the well is long and the number of trips required is high. Statistics indicate that most of the well control events occur during tripping operations.

Another area with shallow gas is Venezuela’s onshore La Vela and Cumarebo Fields, where drilling activity was deferred due to the high risk of drilling shallow gas sections in areas close to urban communities.

As an alternate technology to reduce the risk of influx and at the same time reduce costs and time, Pdvsa evaluated the Casing While Drilling (CWD) technique using Tesco’s Casing Drilling technology in Lake Maracaibo and the onshore La Vela and Cumarebo Fields, with very successful results. Drilling in the shallow gas areas of Bachaquero South West Field in Lake Maracaibo is discussed in this study.

The CWD operations in Lake Maracaibo resulted in decreased risk of influxes, allowing safer drilling in gas sands without trips, therefore avoiding swabbing effects without limitation to the hole diameter required by the well design.

Drilling time was reduced by 30%. CWD operations resulted in gauged holes, which reduced cement volumes by 53%. Better (thicker) filter-cake bonding reduced the risk of lost circulation as a result of the smearing, or plastering, effect achieved during CWD operations. The “plastering effect” (or “smear effect”) allows drilling through highly porous formations, stopping loss of circulation and reducing hole instability. This effect is achieved when the casing coupling is forced against the borehole wall as it advances; the filter cake and cuttings are plastered against the borehole wall by the coupling. Additionally, fewer crew members are required during CWD operations compared with casing-running procedures, reducing personnel risk exposure.

CASING WHILE DRILLING

CWD’s downhole and surface components enable conventional oilfield casing to be used as the drillstring, resulting in the well being drilled and cased simultaneously. The casing is rotated from the surface with a top drive via Tesco’s Casing Drive System (CDS). Drilling fluid is circulated down the casing ID and up the annulus between the casing and wellbore as with a conventional drillstring.

Casing used in CWD applications typically is the same size, weight and grade as would normally be used in a conventionally drilled well. The casing connections may be enhanced with accessories to provide adequate torsional strength, fatigue resistance and flow clearance.

One system available for drilling portions of straight-hole wells with casing uses a drillable shoe, or non-retrievable BHA, that is attached to the casing and can be drilled through to run subsequent casing strings. In other applications, a conventional bit may be run on the casing and left in the hole. Specific situations determine which of these techniques will provide the best solution. A non-retrievable BHA was used in Pdvsa’s CWD wells.

The retrievable CWD BHA typically consists of a pilot bit with an under-reamer located above it to open the hole to the final wellbore diameter. The BHA is temporarily locked, mechanically and hydraulically, into the bottom of the casing drillstring. Additionally, CWD tools are designed to be retrieved with a wireline to allow the BHA to be run under any normal well conditions while maintaining an ability to circulate and reciprocate the casing. Retrieving the BHA through the casing rather than through the open hole eliminates borehole damage due to tripping, providing a safer tripping process. Retrieving on drill pipe is also an option in certain applications.

If it is necessary to drill with a downhole motor to minimize casing rotation, or if it is determined that a section cannot be drilled with a single bit run, then a retrievable drilling assembly that can be re-run is required. Additionally, the use of a retrievable CWD system is the only practical choice for directional wells due to the need to recover the expensive directional drilling and guidance tools. A retrievable BHA provides a way of replacing failed equipment before reaching the casing point. It also satisfies the need for quick and cost-effective access to the formations below the casing shoe.

Casing connections are made in a manner similar to the way drill pipe is made up: A joint of casing is picked up, lowered into the casing stump and made up to the casing string. This process, using the CDS, is accomplished without a stabber in the derrick. The operation is safe and efficient. Connections can be made as quickly as with conventional drill pipe and safety is also improved as a result of minimal rig floor activity during the connection process.

SMEAR-EFFECT BENEFITS

Concerns regarding casing becoming stuck during CWD operations are minimized by the smear/plastering effect that occurs with CWD. This phenomenon results from a combination of circumstances. The smooth, continuous brushing of the casing as it rotates against the wellbore tends to pack drilled solids and/or thicker filter cake into the porous and permeable portions of the formation as the drilling fluid carries cuttings up the annulus.

Differential sticking is rare when using CWD and is predictable in most cases prior to drilling. Differential sticking can occur during connections when drilling through highly porous zones with slick casing strings that have little or no centralization or standoff from couplings. The casing can be freed quickly, however, by using tension and/or spotting an organic soak solution or lubricant. The company also has developed rotating and non-rotating centralizers to minimize sticking.

BACHAQUERO SOUTH WEST

Bachaquero Field in Lake Maracaibo was exploited about 80 years ago and now contains numerous depleted zones. The theory is that gas has migrated from lower formations, and from gas lift and gas injection through old wells, into permeable surface sands. Consequently, some gas traps can be encountered during drilling today.

Due to this phenomenon, there is no continuity of gas sands. The sands can be found between wells with 10-m or 100-km spacing, making it difficult to predict, so that once an area is classified as shallow gas, Pdvsa uses a contingency plan to mitigate the risk.

Since realizing this particular issue about 30 years ago, the company drilled with conventional rigs and drill pipe until the 1990s. At that time, the operator introduced coiled tubing drilling to drill the surface hole and then ran casing without the rig. Drilling with a conventional rig meant that there was essentially no limitation for hole size. However, there were some major logistical problems that resulted in increased drilling time, translating into higher costs.

There were advantages and disadvantages with coiled tubing as well. The company has extensive experience with the technique and has safely drilled many wells that are still producing, though the cost was relatively high. However, the major disadvantage was that the maximum hole size is 12¼ in., which can severely restrict production levels. In some cases, the operator was unable to reach the geological objective before running out of casing sizes if downhole problems existed.

BACHAQUERO CWD WELLS

The two Bachaquero South West Field wells were drilled in October and December 2007. The objective was to analyze the benefits of CWD surface-hole drilling through a shallow gas zone to improve and optimize drilling times and costs, reduce the risk of potential well control events, such as kicks and blowouts, and provide greater operational safety.

Using CWD technology, the company successfully drilled the surface holes through the shallow gas zone in both wells, reducing the number of days to drill the surface hole as well as saving more than $83,000 in rig costs in that well section.

Two wells, the BA-2645 and BA-2648, each have surface-hole depths of about 2,000 ft. The wells were drilled with 17-in. and 17½-in. drillable shoes and a casing size of 133/8 in. Compared with drilling the wells using conventional drilling equipment, CWD saved the operator 4.2 days when drilling the surface hole. More importantly, drilling with casing resulted in a thicker mud filter, eliminating lost circulation. Since the gas reservoir is low pressure, the thicker filter cake also helped prevent gas from entering the wellbore, reducing well control problems.

During conventional drilling operations, the operator was averaging seven days (168 hr) to drill and case the surface hole, Fig. 1. Tesco estimated that time could be reduced to 3.9 days (94.2 hr) with CWD. The actual time to drill the surface hole of BA-2645 was 5.6 days (135.5 hr), a reduction of 1.4 days compared with conventional drilling.

Fig. 1. Conventional drilling vs. Casing Drilling comparison shows a 30% average time savings for the surface section. PDVSA saved the times shown on the designated wells; the graph also shows the time breakdown.

For the second well, BA-2648, the drilling time was reduced even more, to only 4.2 days (101.5 hr), a savings of 2.8 days compared with conventional drilling operations. One reason the BA-2648 well resulted in faster drilling time was the improvement generated by using a drillable shoe that was better designed for the formation.

Another reason for the reduced drilling time using CWD is that the technique eliminates several operational steps. While the CWD method results in longer time to drill the hole, much of that time is made up by eliminating several steps to reach the surface hole depth. For example, using CWD means the operator can eliminate circulating and tripping time. Additionally, under-reaming the hole from 8½ in. to 17½ in. to run surface casing is eliminated. During conventional drilling operations, this can take up to an average of about two days.

Rigging up the casing running equipment and casing tongs is eliminated. And, of course, casing-running operations are no longer necessary, eliminating an average of nearly 12 hr for the surface section. Cementing operations take about the same amount of time with CWD as with conventional drilling and casing running, as does rigging down the system. However, with CWD, the cement volume was reduced more than 50%, Fig. 2.

Fig. 2. The narrower annulus with casing while drilling reduces the cement volume by more than 50%.

TIME IS MONEY

CWD technology achieved $100,000 in rig time and associated cost savings for Pdvsa compared with conventional drilling in Lake Maracaibo. Using CWD resulted in an operating time of 4.2 days, saving 30% during the drilling of surface sections on the BA-2645 and BA-2648 wells.

For conventional drilling operations, two tricone bits were required, an 8½-in. for the pilot hole and a 17½-in. for the final hole size. For a conventionally drilled well, there would be added costs for float equipment and centralizers, which are eliminated with CWD. Even though the cost of the drillable shoe is relatively high, the cost of CWD operations is still less than the additional expenses for a conventionally drilled surface hole section.

There are additional costs associated with conventional drilling operations, as well as CWD operations: personnel and equipment for extra contingency plans for each operation, including firemen, tug boats, paramedics and extra chemical and mud logging costs.

As a result of the Lake Maracaibo wells, Pdvsa wants to expand this successful experience to other fields and spread the use of the technology in shallow gas areas. Based on the company’s experience in the Lake Maracaibo fields, various joint ventures (between it and IOCs) have implemented CWD in their operations with the same scenario (shallow gas areas), such as EM Petrocumarebo Field in the Falcon area (La Vela and Cumarebo Fields). 

ACKNOWLEDGEMENTS

The authors acknowledge the following for their assistance in making this project successful: Ramon Carrasquero, Pdvsa drilling manager of the Distrito Social Lagunillas; Jose Rafael Perez, drilling major assessor of the Distrito Social Lagunillas; Alejandro Parra, drilling major assessor of the Pdvsa and EM Petrocumarebo; Victor Rosales, general manager of the EM Petrocumarebo; Alexander Benitez, Pdvsa wellsite supervisor; Jose Fuenmayor, Pdvsa wellsite supervisor; Hugo Moran, Tesco general manager, Latin America Business Unit; John Boyle, Tesco Casing Drilling vice president; Cristi Harrington, Tesco marketing manager; Jorge Castillo, Tesco tubular services coordinator; Jesus Nuñez, Tesco Casing Drilling engineer; Victor Benavides, Tesco casing driller; Anderson Olivero, Tesco casing driller; Carlo Julio, Tesco tubular services specialist; and Miguel Ángel Bautista, Tesco tubular services specialist.