Specialized Drilling Systems Set New World Records in High-Angle Holes
Authors: Gates Richard L. , Schwab Gary 
Published: December 2007

Summary
Sound well planning and carefully chosen drilling systems have proved successful in completing two high-angle holes through troublesome formations in the Gippsland basin offshore Victoria, Australia. Located about 60 miles [97 km] out in the Bass Strait, Mackerel Wells A-14 and A-16 set new records for being the longest horizontal displacement wells drilled to an angle of up to 72 degrees [1.26 rad] to the vertical. Both wells are producing oil from a previously fault-isolated reservoir southwest of the platform. The use of invert oil emulsion and aluminum drillpipe represented a significant change from normal Bass Strait drilling operations. This, in conjunction with tightly controlled angle buildup and directional control, ensured that the projected well paths were achieved. Completion procedures peculiar to highly deviated holes also were employed to bring the wells on stream successfully. A "controlled activity" oil-continuous mud system was chosen to alleviate problems associated with shale hydration and to increase wellbore lubricity. Rig modifications for cuttings disposal were essential before mud systems could be changed over. In combination with aluminum drillpipe for reduced string weight (Well A-16), the oil mud helped minimize torque, drag, and wall-sticking tendencies. With similar drilling systems and well-planned completion techniques, the drilling of high-angle holes will play a vital role in deepwater development.

Introduction
As the petroleum industry heads toward regions of extreme water depth, techniques are being introduced to allow for production of marginal reserves in the latter phase of basin development. Until recently, a conventional platform technique lent itself to initial development in a new area but was not economically suited to reach outlying incremental reserves. Offshore Victoria (Fig. 1), areal coverage in the Bass Strait has been extended significantly by two innovations, increasing the recovery of hydrocarbon accumulations otherwise considered uneconomical for platform development. In 1979, the use of subsea completions was introduced for development purposes. In addition, the drilling of very highly deviated, directional wells from existing platforms was initiated and proved effective in producing outlying reservoirs previously beyond the reach of conventional techniques. In the early 1970's, the location of platforms (Fig. 2) was determined by use of a 45 to 50 degrees [0.79- to 0.87-rad] deviation-angle limit for directional wells. With each platform, this limit was gradually extended until a very significant change was accomplished by drilling two wells with an average deviation of 69 degrees [1.20 rad]. The successful completions of Well A-14 and the follow-up, Well A-16, drilled off the Mackerel Platform A, achieved a combination of extreme high angle together with deep TVD that to date had never been obtained. As a result, two world records were set for horizontal displacement drilling with this high angle and, more important, new oil is being produced from this previously fault-isolated reservoir. The success of these wells can be attributed in part to special drilling techniques that represented notable changes in ordinary Bass Strait operations. To investigate the need for these specialized systems, a brief history of mackerel Well A-14 is beneficial. Well A-14 originally goes back to the exploratory well, Mackerel A-4, drilled to evaluate the southern fault block associated with the known Mackerel reservoir (Fig. 3). While the well did encounter a significant oil column, the true geological structure of the fault block was uncertain. It is currently believed that either minimal or no communication exists between oil trapped by the fault block and oil contained in the main reservoir being drained by Platform A. The estimated recoverable reserves contained in the fault block was 39 million bbl [6.2 × 10 m3 1. Note that this recovery will be helped by a prolific waterdrive mechanism (Fig. 4) and that an expected two-thirds of the oil in Bass Strait reservoirs could be recovered because of this very strong natural water drive. Roughly speaking, that is about two times the worldwide average for the amount of recoverable oil contained within a reservoir. This water drive originates from deep subterranean water flowing away from the rain that has fallen on the hills near Sale, Victoria, and exerts continuous pressure on the Bass Strait reservoirs. Feasibility studies indicated that two methods could be employed to produce oil economically from the South Mackerel fault:through subsea completion. similar to that successfully installed on Cobia-2, orby drilling a very high-angle well from Mackerel Platform A.
The directional well alternative was selected because of lower estimated initial and operating costs compared with the subsea completion. However, considerable risk of failure with the directional hole was recognized. The originally planned well path for Well A-14 represented the limits of directional drilling technology.
JPT
P. 241^