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Abstract Wellbore instability remains a leading cause of drilling non-productive time (NPT), despite a considerable amount of work on the subject in recent years. For the most part this is not caused by a lack of understanding of the mechanics of wellbore instability. Rather, it is caused by the limitations of pre-drill subsurface predictions. In other words, it is the drilling surprises often encountered in reality, but not predicted prior to drilling, that give rise to wellbore instability problems. These surprises include formation tops and pressures occurring at different depths than predicted and the presence of unexpected faults or other fractured or fissile zones.
Typically, while drilling today, downhole data useful for understanding wellbore stability is scarce. Much of what we have to go on is indirect measurements; torque and drag observations, cavings, and annular pressure measurements. In order to unambiguously understand and therefore mitigate the wellbore instability problem, more information is needed.
Real-time wellbore stability modeling goes some way toward this goal. However, the missing piece in almost all cases is some direct observation of the state of the hole to determine where and how the wellbore is failing. LWD calipers and/or wellbore images can provide such information, and the option of transmitting the information to surface while drilling is now often possible.
This paper will present the results of a project conducted to evaluate the available real-time images, memory density images, and acoustic caliper information and their incorporation into an existing wellbore stability monitoring service.
Introduction A BP / Halliburton technical steering committee project was initiated as part of an active investigation by BP to assess the existing options in the market for real-time wellbore stability modeling and monitoring. The overall project's objectives included defining best practices, workflows, and limitations, recommending tool modifications, and actively using this type of technology to reduce drilling NPT.
The project was divided into two phases with the first phase being successfully concluded on the Aztec prospect in the Gulf of Mexico. This paper will discuss the second phase of the project covering the requirements to setup and configure real-time stability monitoring and real-time imaging using Sperry Drilling Service tools and software. The paper will present an outline of the results of the project.
Three main objectives were set for the phase II project which were to:Evaluate the existing LWD tools that can be used to determine the hole shape and mode of hole failure while drilling.
Evaluate existing LWD measurements and real-time software that can be used to predict and determine wellbore stability while drilling.
Utilize the existing services available to integrate the above and allow interpretation at the wellsite and in the office in a manner that will impact non-productive time.
A candidate well for the project had to be selected so that it met the following criteria: the well was to be drilled with Sperry Drilling Services LWD tools; there was an identified borehole stability risk. Wells in the UK North Sea, Holland, and the Caspian Sea were evaluated, and the Farragon prospect in the UK North Sea was chosen.
One benefit of this choice was that the Farragon prospect was to be developed by drilling two wells in batch mode, thus affording the opportunity to determine if the analysis of the first well's 12 ¼-in. or 8 ½-in. hole section could be used to modify the drilling of the second well's 12 ¼-in. or 8 ½-in. hole sections and impact any NPT that may have arisen.
The asset team had already performed risk assessments prior to the selection of Farragon as a candidate for the project, and further discussions were required in order to integrate the project LWD tool, software, and data access requirements with the existing drilling program.
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