The Wellbore Quality Scorecard (WQS)
Abstract: TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractWellbore quality is a term commonly used in the drilling community; however its meaning is not consistently understood, its context sometimes confused and its value to the industry unknown. For many drilling engineers, a quality wellbore is most often associated with a directionally drilled borehole that is smooth, in-gauge, and has minimal spiralling. Some of the claimed benefits of such a wellbore include enhanced drilling performance, ease of running casing… Show more
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Cited by 11 publications
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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.
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.
TX 75083 -3836 U.S.A., fax 1 .972. 952.9435. AbstractTorque and drag (T&D) modeling is regarded as an invaluable process to assist in well planning and to predict and prevent drilling problems. Although T&D software has existed for over 20 years, some confusion still exists over the validity of the models used to characterize drilling and completion operations. This paper provides an assessment of current limitations of the various T&D models (soft-string and stiffstring) and appraises their validity. Field data from various operations is used to illustrate certain limitations. The paper defines future requirements for what is considered to be the next generation of T&D models . Probably the most important technical requirement is a more realistic stiff-string model to correctly account for the impact of tubular stiffness, hole clearance and tortuosity effects.
The operator and co-venturers are developing several Angola Block 15 fields in water depths of up to 1400m. To date, 83 development wells and 33 exploration / appraisal wells have been drilled in the Block. Most have experienced hole quality issues in the riserless interval that manifests themselves as tight hole while tripping the BHA and abnormal/excessive drag running casing. In a few cases, casing could not be run to total depth (TD) and in one unfortunate case, the casing buckled in open water.There has been uncertainty as to the root cause of the hole quality issues which have been attributable to various factors since drilling was initiated. A review of available literature revealed very little published information on riserless drilling and casing running practices. The operator undertook a study of its riserless drilling and casing running practices in order to look for trends that may suggest certain sources as the root cause and to allow a re-examination of long-accepted practices. The study, combined with some new perspectives on directional drilling mechanics, has identified potential root causes not previously considered. Changes to drilling practices have resulted in improved riserless hole quality and smoother casing running operations.
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