Abstract

Technology that permits simultaneous drilling and reaming was introduced in a high-pressure exploration well in the operator's Nile Delta program. This paper describes the development of ream while drilling (RWD) technology and its application in one well in the Mediterranean Sea offshore Egypt. In combination with an anti-whirl PDC bit and oil-based mud, the tool was first used to drill a 13 3/4" hole below the 13 3/8" caing. Afterwards, the RWD system successfully drilled a 12 1/4" hole below the 11 3/4" casing. For both sections, the operations were accomplished in a total of three runs at average penetration rates as much as 152% higher than those recorded in the best available offsets. Total savings resulted from simultaneous drilling and reaming was US $1 million.

Introduction

To date, Amoco Egypt Oil Company has drilled four wells as operator of the Ras El Barr Concession, located offshore the Mediterranean Nile Delta area, some 56 km northeast of Damietta (Fig. 1). The first two wells - Ha'py-l and Seth-l evaluated upper Pliocene sands (1800-2000 m). The Jj 70- 1 (Akhen-l) exploration well represented the operator's first attempt to evaluate the Middle Miocene Serravallian sands (3564 - 4200 m), in which a sizable gas accumulation had been discovered in an adjoining concession. Concurrent with the drilling of Akhen-1, the operator prepared the well design for Ji 70–2 (Osiris East - 1), which was intended to be a third shallow gas test. Located 6 km east of Akhen-1, the Osiris East-l was originally being designed as a further evaluation of the Ha'py-l (A20 sand sequence). However, with the unexpected discovery of gas in the deeper Pliocene sands of Akhen-1, the Osiris- 1 was re-designed as a second Serravallian evaluation. Figure 2 illustrates the structural cross-section of the Ras El Barr Concession, highlighting the mapped horizon of the A70 gas sands (Kafr El Sheikh) discovered in Akhen-1 and their correlation to the subsequent Osiris E-l.

Stratigraphically, the Middle Miocene Serravallian section consists primarily of shales with streaky sandstones in the upper section and blocky sandstone bodies near the base of the section. These two sections represented the main producing sands in the adjoining Serravallian producer (Offset 1). The Serravallian section underlies the Kafr El Sheikh formation, which consists primarily of highly reactive shales with thin sandstone and siltstone streaks. Figure 3 illustrates the lithology representative of the two Ras El Barr Serravallian tests wells, along with the corresponding rock strength and abrasive characteristics.

As shown in the figure, with the exception of an anhydrite stringer at 2850 m, the Serravallian and the wet Pliocene sands of the overlying Kafr El Sheikh formation ((1292- 3564 m) are not particularly hard or abrasive.

Conversely, abnormal and variable pore pressures and resultant hole instability problems - swelling shales, caving, tight holes and eventual loss of circulation - encountered in Nile Delta wells have been well-documented in the literature. More specifically, the overpressured zones originate through the 2272-m thick Kafr El Sheikh formation, primarily because of overburden and tectonic effects. The pressure gradually increases with depth and shale percent, reaching its maximum value near the bottom of the Kafr El Sheikh shale. Thus, completing a trouble-free well through the Kafr El Sheikh requires the optimum in mud type/weight.

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