The Drilling and Casing Running Enigma

Abstract

Running casing in high angle and extended-reach wells is often recognized as one of the most demanding of all well construction operations. In such wells, frictional drag and unexpected downhole mechanical resistance can exceed available surface slack-off weight. The ability to reliably predict casing running performance will significantly help to reduce costs in challenging wells. Interestingly, a complex and often subtle relationship exists between a drilled wellbore and the subsequent casing run. This relationship is sometimes predictable and at other times surprisingly different. A key to resolving this enigma lies in understanding the many factors, such as wellbore quality, that influence the outcome of casing running operations. The paper reviews a number of topics that are considered key to understanding the relationship between drilling and casing running. Various results are illustrated using case studies from field operations. Introduction The requirement to access hydrocarbon reserves in environments that require ultra-extended reach wells will continue to increase. Typical scenarios involve accessing offshore reserves from nearby shore locations or satellite fields considered just out of reach of existing platform structures. Step-out records for extended-reach wells currently exceed 10 km. Such noteworthy achievements are very likely to be surpassed by some considerable margin during the next 5 to 10 years. Recent examples of ultra-reach wells include several 10 km wells drilled off Sakhalin Island in Russia [1], 10 km record wells at the Wytch Farm oil field development in Southern England [2] and also at the Austral field off Tierra del Fuego in Argentina [3]. In the future, extended-reach developments will drive the need for robust well designs and highly reliable technology to ensure cost effectiveness. A particular feature of such well designs will be the requirement to drill and case very long hole sections. In this respect, one of the drivers for success will be to understand the relationship between a drilled wellbore and its companion casing run. Some aspects of this topic have previously been investigated, where various guidelines and methodologies have been developed [4], [5], and [6]. The disparity between a drilled wellbore considered fit-for-purpose and a subsequent casing running failure is the main theme of this paper. This so called enigma is explored by dividing the paper into a number of focus areas. These broadly encompass the topics entitled wellbore quality, hole geometry and torque and drag (T&D) modelling. Each of these is assessed for evidence that may help with understanding factors that contribute to the enigma. What is the Enigma? During planning of an extended-reach or other challenging well type, drilling and casing running operations are usually reviewed using a wide ranging set of well design assumptions. These may be based on offset well experience or might be based on industry experience using publications such as SPE papers. The validity of these assumptions should always be closely examined, as they often play a fundamental role in determining the outcome of a drilling or casing running operation. The drilling and casing running enigma arises from time to time in varying circumstances. This is sometimes as a result of using invalid assumptions, sometimes as a result of poor knowledge transfer and occasionally as an event that has not been observed before. The enigma considered here is that a drilled wellbore is considered fit-for-purpose for running casing. However for some apparently unknown reason, casing fails to get to depth. Usually casing running problems are attributed to a geometrical anomaly such as a fault or ledge, wellbore instability, poorly cleaned hole, time dependent effects or mechanical equipment failure such as centralizers.

The Effect of Different Drilling Fluids on Mechanical Friction

“…Wellbore tortuosity is also one of the sources for excessive torque and drag. As stated by Mason [7] borehole spiraling and micro tortuosity may cause excessive contact forces on the sidewalls and may cause increase in the friction forces. Effective borehole drift diameter is a critical issue in the BHA section with bigger diameter than drill pipes.…”