Abstract

The predominately vertical 8 1/2" and 8 3/8" sections of representative wells in northern Oman comprise lower Cretaceous through Permian formations with average unconfined compressive strengths (UCS) of 23,000 psi. Historically, drilling these medium to medium-hard intervals required as many as four IADC 437 to 517 roller cone bits. Despite the relatively short runs and low rates of penetration achieved with these designs, the operator previously did not employ PDC bits because of their relatively high base cost and unsuccessful attempts with conventional, older generation PDC bit technology. This paper describes the introduction of a new generation of PDC bits and its successful application in the limestones, dolomitic limestones and anhydrites intrinsic of these wells. The new line, comprising such features as highly polished cutters and carbide supported edge geometry, were run under a shared risk approach. The authors will describe the new technology and present case studies from Oman that shows average drilling cost savings achieved through a doubling of penetration rates in some instances and the elimination of up to three trips per interval.

Introduction

Since the 1977 discovery of hydrocarbons in the Khuff formation of the Yibal field, Petroleum Development Oman (PDO) has undertaken an aggressive evaluation of this complex carbonate structure and the underlying Gharif sandstones. The average thickness of the Permo-Triassic Khuff formation, which extends throughout northern Oman, is 865 m. Typically, wells evaluating the Khuff and underlying Gharif structures are completed in either 8 3/8" or 8 1/2" hole sizes.

Figure 1 presents the formations and general lithologies of the 8 3/8" and 8 1/2" intervals of representative Khuff wells. As shown, the Khuff comprises redbeds and carbonates, dominated by limestones, dolomites, diolomitic limestone with anhydrite, underlying the Lower Cretaceous Shuaiba limestone formation. The Khuff overlays the Gharif, which consists predominately of clastic deposits in coastal plain and alluvial facies that were deposited as a 200–250 m thick layer cake package over wide areas of Oman. The Gharif, which has been the target for much of the most recent drilling activity in northern Oman, is very heterogeneous in terms of sand quality and very variable in terms of geometries and connectivity.

With sonic travel time in the 43–94 s/ft range, the lower Cretaceous through Permian formations of the subject interval contain average unconfined compressive strength (UCS) of 23,0(X) psi, which is generally considered hard rock. Correspondingly, the internal angle of friction (IAF), which indicates the abrasiveness of a formation, was 400 or above for the three wells analyzed in this discussion.

Unconfined compressive strengths are estimated by manufacturing a shear travel time (ts) from the compressive travel time (tc) recorded on offset logs for each formation. With today's bit technology, UCS in the 20,000 – 23,000 psi is considered medium hard to hard. Conversely, earlier experience with conventional, older PDC bits showed that when average internal angles of friction stayed above 40 the formations were too abrasive for those earlier designs.

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