Supersize Hole Creates Drilling Opportunity

Abstract

Abstract Traditional well designs for high angle and extended-reach wells often mandate long 12¼" hole sections through the overburden. Effective hole cleaning and wellbore instability management are key technical challenges commonly associated with delivery of this section. However, despite the use of rotary steerable systems, which help promote good hole cleaning and minimise open hole exposure times, problematical trips out of hole are commonplace. The symptoms of tight hole are often associated with significant cuttings bed accumulations, use of packed assemblies or creep arising from swelling formations. In an attempt to improve overall drilling performance, a supersize hole strategy has been implemented to intentionally drill an enlarged hole through the overburden. At face value, this tactic seems to have as many disadvantages as advantages. However, practical experience of simultaneously drilling and underreaming 12¼"×13½" hole sections over a number of wells has resulted in overall efficiency improvements. The primary benefits associated with drilling an enlarged hole are ease of tripping-out drilling assemblies and more straightforward casing runs. Other advantages relate to lower downhole pressures or equivalent circulation density (ECD), decreased swab and surge pressures, and less string contact area. Disadvantages relate to increased hole cleaning requirements, additional tool failures as a result of higher vibration and a decreased rate of penetration (ROP). The paper discuss es various aspects of drilling and underreaming and contrasts results between enlarged and conventional hole sizes. Results from drilling both 12¼" and 12¼"×13½" hole sections in the Azeri-Chirag-Guneshli (ACG) fields in the Caspian Sea are reported. ROP (Rate of Penetration) and a new metric TTSW (Time To Secure Wellbore) are used to provide meaningful comparisons. Introduction The Caspian Sea includes an area of world class hydrocarbon accumulations. Reserve estimates for the ACG development are in excess of 5.4 billion barrels. Anticipated production rates are expected to exceed one million barrels per day by 2010. Hundreds of wells will be drilled from various platforms and semi-submersibles to exploit reserves across the structure. Having a cost effective well design strategy for the various well types will enhance both drilling performance and ultimate recovery of reserves. A significant number of these wells will be highly deviated or extended-reach in nature. Experience to date has shown that drilling and casing running through the overburden at high angle is particularly challenging. This is primarily due to interdependent problems arising from wellbore instability, hole cleaning and casing running. The paper will first provide an overview of operations on the first extended-reach wells drilled from the Chirag Platform. The discussion will then be extended to show how improvements to the basic well and casing design have helped to overcome problems in these early wells. The paper will then focus on more recent developments where field trials have explored the impact of purposefully drilling an enlarged hole section through the overburden. A comparison of the advantages and disadvantages of drilling an underreamed hole section will be given. The main differences hinge on managing hole cleaning, downhole vibration and performance. Each of these topics is examined in more detail where results from various field operations are used to illustrate various points. Finally, the paper addresses the issue of selecting the right hole size for underreaming and discusses options for future well designs. Chirag Field History The Chirag platform is located 120 km East of Baku in a water depth of 120m. The 24 slot jacket was installed in 1993 and first oil achieved in 1997. The initial drilling programme comprised 15 wells which were in close proximity to the platform. Early drilling operations were considered relatively trouble free; however in higher step-out wells just beyond 4 km, higher levels of non-productive time (NPT) were experienced.