Abstract

OMV Pakistan applied an original design of Genesis XT fixed cutter bit with improved features for bit stability to drill the 8.5" section of Sawan-11 development gas well in southern Pakistan. Multiple vertical wells were drilled in this field with similar parameters and operating conditions. Latest proven technology was applied successfully to drill the 1760m of 8.5" section with record ROP of 25 m/hr which is more than twice as fast compared to previous wells records. Instantaneous rates in excess of 70 m/hr were also recorded while drilling this section. This section was previously drilled with multiple bit runs using multiple roller cone and PDC bits. Later, this was reduced to 2–3 PDC bits and now the complete section has been drilled with one PDC bit (HC 606Z). Dull grading comparison with bits used in the offset wells will also be discussed. The lithology of the drilled section consists of Ranikot formation (dominating claystone with inter bedded sandstone and siltstone). Throughout this zone bit stability and optimized drilling parameters of the bit were key factors affecting its performance. Below this zone Upper Goru (UG) and Lower Goru (LG) formations are found. UG Member mainly consists of marl with streaks of limestone having an UCS of > 35 kpsi in its contact with the Ranikot formation. This type of formation is generally the most difficult to drill. LG Member comprises of claystone and siltstone with occasional traces of pyrite having UCS of 20–25 kpsi. Towards the end of section, last 400m witness increased percentage of abrasive siltstone which affects cutter's sharpness and reduces the ROP. Offset data conforms that around this point bits quit drilling. However, at Sawan-11 bit maintained its sharpness and good ROP till the end of this section.

Introduction

Over the past few years, the degree of difficulty and cost of drilling an oil & gas well have increased globally. With the recent oil & gas price hike, exploration and development operators have expanded their operations substantially around the globe. With the help of advanced drilling technologies like RSS, MPD, etc. and high performance drilling fluids although sometime help and prove their worth. Nevertheless, if the drilling bit is not of appropriate design, then these advanced systems and technologies can't help drilling a well efficiently and economically.

OMV has been drilling gas wells in the Middle Indus basin of Pakistan since 1994. Drilling in this area is very challenging due to inter layered geological sequence that consists of clay, shale, limestone, with sand and siltstone (Figure 1). Wells are normally drilled vertically through these geological sequences which provide significant challenge to the bit performance. Previously OMV has attempted to drill these wells using roller cone, tungsten carbide inserts, and polycrystalline diamond compact (PDC) bits. But this required a lot of patience and extra rig cost due to low ROP. Many operators and bit companies have reported that PDC bit in general performs well, produces better ROP and usually drill more meters than tricone bits [1,2]. Yet the cutters on the PDC drill bits normally wear out quite easily due to excessive torque. This mostly occurs when the cutters encounter any hard and abrasive formation. This process makes the PDC cutters flat, dull and unable to penetrate the rock efficiently. Nevertheless, recent PDC drill bit cutter and design technology have made it possible to drill thru some of the very hard, abrasive and interbedded formations having UCS of >30 kpsi [3,4,5].

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