Tierra del Fuego drilling campaign overcoming hole stability concerns
Editor - Europe
- Extended reach wells drilled to date from the shore of Tierra del Fuego, in relation to the offshore Hidra platforms [129,222 bytes].
- ERD wells drilled during the 1995-96 offshore campaign [144,953 bytes].
- Wells drilled so far during campaign initiated one year ago [217,517 bytes].
- Forasol/Pride's 1625 land rig, used for both ERD offshore campaigns by Total and its partners [36,425 bytes].
The basement comprises a Jurassic volcano-sedimentary sequence - the Tuffaceous Series. Above lies the Springhill formation. The area's main reservoir is a combination of coastal-continental gas and oil-producing sandstones with excellent petrophysical characteristics, but with a wall thickness not exceeding 20 meters. The trapping geometry is a combined structural and stratigraphic feature due to the co-existence of faults and sedimentary traps generated by lateral and vertical facies variations.
The Cretaceous formation's Inferior Inoceramus (main source rock), Margas Verdes and Medium and Superior Inoceramus are a continuous succession of pebble deposits and marls that constitute an excellent seal for the underlying sandstones. The Tertiary sequence consists of alternated sandstones, glauconitic sandstones and shales.
Some of the offshore oil and gas pockets are within reach of the Hidra platforms and have been tapped accordingly in 1995-96 through a first series of extended wells. Others are closer to the shore, inviting access through long reach wells drilled directly from the coast. Since early this year, Total has become a practitioner of this technique under a "mix and match" strategy, designed to maximize production from this area.
Shore-based drilling is not new, having been deployed for years by BP from Wytch Farm on England's south coast, with increasingly lengthy step-outs - longer than those achieved so far by Total. But BP does not have to plot a course through the same awkward, unpredictable sands that make drilling off Tierra del Fuego so treacherous.
Gradually, Total is mastering the situation, based on lessons learned from its platform-based, long-reach campaign of 1996. Thin hydrocarbon layers are being targeted more successfully, thanks to improved geosteering modeling and practice. This has encouraged Total to attempt more complex step-outs from the shore during a new campaign initiated in May 1997.
Phased developmentTotal's acreage in southern Argentina covers 125,000 sq km offshore and onshore. The only offshore producing field is Hidra in the north of the Cuenca Marina Austral permit. Development of this field began in 1989 based on two normally unmanned wellhead platforms, Hidra Norte and Hidra Centro, 3.2 km apart and 13 km offshore.
In 1994-95, new 3D seismic was shot to assess various satellites around Hidra. By that time, more than 75% of Hidra's recoverable oil had been produced, leaving the prospect of spare capacity on the two platforms. However, the size of the satellites did not justify subsea tiebacks. Costs of securing a rig for subsea completions are fearsome enough in the South Atlantic, let alone mobilizing a barge for flowline installations.
The lower-cost alternative was to drill the wells directly from the platform using a jackup. Risks were great, however, in view of the minimal experience of extended reach wells in this part of South America. Because of the nature of the reservoir sands, there were worries over hole stability. On the other hand, Total had built up reasonable knowledge of deviated/long reach wells from its program on the Alwyn complex in the UK North sea, where it had rattled off 70! departures as standard.
Exploration historyIn 1989, one of the original Hidra production wells, HNP-10, had successfully reached a 2,700-meter departure with a 70! inclination. This was considered encouraging, as it was drilled with water-based mud. Then in 1995, a horizontal drain was drilled for the first tie on the Hidra Field on well HNI-3, also using water-based mud. This trial confirmed the feasibility of extended reach wells, but also highlighted the need for oil-based mud to obtain the low friction factors required by extended reach (ER) drilling.
The next step was to bring over the jackup Neddrill Trigon for the first extended reach drilling campaign in 1995, in 30 meters of water. Ara Sur was the initial target. The southern end of this compartment was tapped via two wells drilled from Hidra Norte, HNP-7 and HNP-12. A third well was drilled from Hidra Centor to Hidra Sur, a non-depleted structure located 5 km south east of the Hidra Centro platform.
For this first campaign, explains Andre Goffart, R & D head of Total's drilling and completion division, "we used state of the art geosteering equipment from Anadrill which allowed us to land in the Springhill sand reservoirs which are only 15-20 meters thick. However, things became even more complicated when trying to pilot the drain in the oil rim, clear of the gas or water zones and encountering minor faults which had not been picked up by the 3D seismic. To insure that only the oil intervals were produced, we were obliged to cement and then perforate a liner."
The first well, HNP-7, eventually reached its objective after 67 days, including completion, 6,300 meters from the Hidra Norte platform, at a T/D of 1,700 meters. It also flowed at 5,000 b/d, one of the highest rates derived at the time from an Argentinian well.
Each of these three wells had to be side-tracked once, following stuck pipe either in the 12 1/4-in. or 8 1/2-in. phases. But this was to be expected, claims Goffart, in view of the limited ERD experience at that time.
The next step was to try to produce other accumulations from the shore - Ara West and Kaus, both discovered in the1980s. This second long-reach project got under way in May 1997, and involves drilling six wells this year, up to 8.5 km long, from two onshore clusters using a land rig. These clusters are located in the vicinity of the Rio Cullen terminal which processes product piped from the Hidra Norte and Centro platforms via a subsea line.
Oil is stored in Rio Cullen and then exported to tankers via a loading buoy. Gas is pumped into the San Martin pipeline to Buenos Aires. The onshore drilling site Alfa Sur is located 13 km north of the Rio Cullen plant while the project team and the main service companies' support bases are located at Rio Grande, the nearest town which is 180 km to the south-west.
Total has maintained the collaboration from its previous campaign with French oilfield engineering specialists. Total's scientific and research center in France, CST, and IFP have concentrated on the improvement of hydraulic models used to predict mud pressure losses, particularly those experienced in the long eccentric annulus, and also on the required conditions for proper hole cleaning. IFP also worked on modeling of the behavior of a floated casing string, a technology which was needed due to the increased step-out of the onshore ERD wells, above all the AS-3 (7,974 meters) step-out.
Geoservices produced and installed a system for measuring the cuttings flow rate at the well outlet. "For the second campaign," says Goffart, we have installed a cuttings weighing machine under each of the five shale shakers on the onshore drilling rig, so we have a continuous measurement of the cuttings flow rate. After a short de-bugging period, the device appears reliable and the data useful - in real time for the site supervisor to monitor the cleanness of the well, and also for post-well analysis to determine when and how the well is best cleaned.
"For the current program, the idea is to acquire as much information as possible while drilling. That includes pressure at the surface and downhole using a specific sub provided by Geoservices, the cuttings rate and so on, in order to monitor the hole cleaning status and downhole circulating pressure. The equipment will also be used to improve the accuracy of our prediction software which remain insufficient, considering the small margins we have to work with."
Other contractors employed by Total include two stalwarts of the Wytch Farm development, Baroid (muds) and Schlumberger Anadrill (directional drilling and LWD). Forasol/Pride is supplying the 1625, 3,000 hp land rig. Wells are drilled from a six-slot cellar, 5 meters deep on the edge of a cliff. Due to local environmental sensitivities, oil-based cuttings are incinerated on the drilling location.
Four wells have been completed so far this year from the Alfa Sur cluster, the longest being AS-3 with a 7,974-meter step-out for a total depth of 8,530 meters and a sailing angle of 82!. This well was drilled and completed in 90 days at a cost of $11 million.
One more well (AS-5 - in progress) is scheduled from this cluster before moving the rig to the Cullen Sur location from which two oil wells are planned to tap the Kaus Field. Maximum step-out for these wells is around 7.5 km, but the reservoir is at a vertical depth of only 1,400 meters, giving a ratio of departure/vertical depth of almost 5:5 - at the limit of present technology, according to Goffart.
In addition to their extreme profile, all these wells have to be landed and navigated using state of the art geosteering tools in the Springhill sands which are only 10 meters thick. In some instances, the reservoir is lost temporarily due to the presence of a minor fault or an unexpected dip variation. The combined skills of geologists and drillers are then required to make the right connection in order to re-enter the pay zone as quickly as possible. "Navigation is even more difficult when trying to remain in the oil rim," says Goffart, "and the result is due mainly to the experience of the pilots."
The onshore ERD campaign may be extended further according to the results of current offshore appraisal work, which includes 3D seismic and delineation "needle" wells. New offshore wells may also be drilled from the Hidra platforms.
All Hidra area oil wells are gas-lifted. Produced gas from Ara West and Canadon A is exported overland to Buenos Aires via the San Martin pipeline. Last December, Total bought Argentinian LPG marketer Argon, which supplies one quarter of the nation's needs. From 1999, Total plans to have an LPG production facility operating in Tierra del Fuego outputting 250,000 tons annually.
To the north-east of the Hidra Centro platform, Total has identified one accumulation worthy of a subsea tieback. This is Argo, which features the same, complex geology. To the west of the permit is another oil discovery, Carina, which is farther out from the shore, but also in shallower sands. The water depth is 70 meters.
Here, Total is considering an extended well test via a jackup. One problem is that locally strong currents have left the seabed hard, making penetration difficult for a jackup. Noble Denton is helping Total seek a solution. The rig may also be used for exploration around Carina.
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