Further Advancements in Extended Reach Drilling in Bass Strait
Authors: Krepp A.N. , Mims M.G. , Santostefano V. 
Published: December 2007

Esso Australia Ltd. (EAL) has continued to make advancements in extended reach drilling (ERD) having recently broken two long-standing Bass Strait records for depth and reach. In breaking these records Esso has also set a number of performance benchmarks with the help of emerging ERD technologies.

These advancements in ERD performance have made previously unreachable targets viable and substantially reduced the overall cost of ER drilling for EAL. This paper will chronicle the engineering designs and operational factors behind these advancements, and discuss their applications in the field.

Introduction

EAL has been drilling directional wells in Bass Strait since 1968. See Figure 1 for a map showing the location of EAL's operations. In 1978 EAL drilled the Mackerel A-14 and A-l6 wells which set world ERD records that stood for a number of years. In 1992-93, EAL drilled several ER wells on West Kingfish platform that redefined ERD performance in Bass Strait. The longest well, W-32, was drilled and completed to 4975m in 63.0 days. In 1994 this benchmark was moved again by the successive Mackerel A-20, A-21 and A-22 infill wells (A-22 drilled & completed to 4,985m in 38 days). Each of these wells were drilled with a 4,500 HP platform rig, equipped with two 1600 HP pumps (with an effective maximum surface pressure limit of 3600 psi), a TDS-3 top drive, water based mud (WBM) and 5" drillpipe EAL is currently involved in an additional drilling program on Fortescue platform with this same rig originally designed to drill no deeper than 5,000m. This rig drilled beyond its perceived limits during the Mackerel Infill Drilling Program and has since been upgraded to make it better suited to ERD.

Three primary formations are penetrated at each of these platform locations (see Figure 2 for lithology cross-section). The "Gippsland" limestone/marl is a soft, sticky and dispersive formation extending down to 2000m TVD. Below this is the Lakes Entrance formation. The Lakes Entrance is a reactive & dispersive shale, and 300-400m TVD thick. Generally there are no over-pressured or permeable zones in either the Gippsland or Lakes Entrance formations (such zones have been encountered before, but the presence of such zones is extremely rare and isolated).

Underlying the Lakes Entrance is the reservoir formation, called the Latrobe. The Latrobe consists of highly permeable sandstones (1-10 darcies), with inter-bedded shales, siltstones and occasional coal and pyritic stringers. Generally the reservoir objectives are located at the top of the Latrobe, with about 200 – 300m TVD of Latrobe usually being drilled. Reservoir pressures within the Latrobe sands are slightly drawn down, due to depletion, to about 8.0 ppg EMW.

1995-96 Extended Reach Drilling Results

In 1995-96 EAL further improved ERD performance with the gradual implementation of new technologies. The Kingfish B-14B was drilled and completed to 4686m in 26.6 days using a Class A jackup rig and WBM. This broke a recently set performance record for wells of this depth, despite this being only the second well drilled in Bass Strait with this rig.

In late 1995, following upgrades to the previously mentioned platform rig, the Fortescue A-35 well was drilled and completed to 5561m in 37.6 days, using a pseudo oil based mud (POBM). This was EAL's first experience with POBM (EAL has not drilled with an OBM since 1980). A number of Exxon Corporation and Bass Strait performance and depth records were set on the A-35. This well also trial led several new technologies and practices aimed at further improving future ERD performance.

The Fortescue A-29 well, drilled in February 1996, again set new depth and performance milestones with its TD of 6,210m MD at 2,450m TVD. This well has only two casing strings (surface casing and production casing), and as such is considered to be a world class achievement for a producing well of this depth.

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