Philip Smith
Nexus Energy Limited

The major result for Nexus Energy Ltd (NXS) in the Gippsland Basin in 2006 was the successful sole-risk drilling, testing and completion of the Longtom-3H extended reach development well in VIC/P54 (Figure 1).

Following the discovery of the Longtom Field in 1995 and the encouraging results in the Longtom-2 well in 2004, Nexus devised an ambitious program to drill and complete an extended reach appraisal well in multiple sands of the Emperor Subgroup’s Admiral Formation.

While Longtom-2 had shown, with a drill stem test and analysis of core data, that some of these sands had the potential to produce gas at commercial rates, confirmation of a sufficient number and/or lateral extent of productive reservoirs was necessary to demonstrate the minimum gas volumes required to justify proceeding to development. The challenges involved optimising the positioning of a well path to maximise the net productive reservoir penetrated while keeping the drilling and completion risks to a minimum.

The initial task was to locate the best gas sand targets, and this was primarily addressed by analysis of elastic rock properties, which led to a seismic inversion processing technique able to differentiate between thicker gas- and water-filled sands. Mapping out the anomalies indicated by this technique provided a critically important guide for designing the optimal well path.

The task of then designing a well bore that could both reach and allow evaluation of these multiple zones was not a trivial one, especially considering that both of the previous (far less challenging) Longtom wells had encountered well bore problems that had slowed and/or compromised their execution. Extensive engineering studies were undertaken into all aspects of the operation, including well bore stability, drilling fluid, hole cleaning and monitoring and the running of casing and completions. The resulting recommendation was to drill a 9½” diameter well bore, with relatively heavy synthetic-based mud, and an operational emphasis on minimising torque and drag.

Finally, it was important to be able to reliably assess key reservoir parameters in the drilled section in near-to-real-time, so that once a sufficient pay interval (for the required production rates) was achieved a timely decision to stop drilling could be made. This was achieved with use of relatively new logging-while-drilling (LWD) tools that, in addition to the usual ability to calculate porosity and saturation, could also provide an estimate of permeability.

The Longtom-3 development program was conducted July-September 2006 using the Ocean Patriot semi-submersible drilling rig, and comprised a pilot hole (Longtom-3P), a short geological sidetrack (Longtom-3ST1) and an extended reach sinusoidal/horizontal production well bore (Longtom-3H) (Figure 2).

Longtom-3P, in addition to its primary purpose of providing depth control to assist with optimal placement of the extended reach well, enabled collection of wireline data, including pressures, that were not feasible to obtain in the main well. It also provided the first validation of the seismic inversion processing, which had correctly indicated that only the lowermost Admiral Formation sand would be gas-bearing along the planned pilot well path (Figure 3).

After Longtom-3ST1 (a short geological sidetrack to attempt to obtain additional wireline pressure data) the main appraisal/development well, Longtom-3H, was drilled down the flank of the structure and then across the Longtom Terrace. As planned, the well was gradually steered in and out of the two lowermost Admiral Formation sands to obtain lengthy penetrations through the zones with the strongest seismic inversion anomalies.

Longtom-3H was drilled to a total depth of 4674mMD and, after running 7” liner to 4190mMD, was completed with 1069 metres of perforations through four separate gas sands. During production testing the uppermost of these sands achieved a maximum flow rate of around 30 mmscfd and the lower three (combined) flowed at around 77 mmscfd (Figure 4). Longtom-3H has been suspended as a future gas producer.

The development of Longtom will comprise three sub-sea horizontal wells (including Longtom–3 H) producing via a new high pressure pipeline tied back to the existing, lower pressure Patricia-Baleen gas pipeline, and then onshore to the Santos gas plant near Orbost (Figure 5). The next production well, Longtom–4, is planned for drilling later in 2007 and the Longtom pipeline to Patricia-Baleen is expected to be laid in the first quarter of 2008.

For more information contact:
Nexus Energy Limited
Tel: +61 3 9660 2500
Fax: +61 3 9654 9303
Web: www.nexusenergy.com.au