In the deepwater Gulf of Mexico (GOM), an operating company planned to drill and log a challenging wellbore in a mature field within the Upper Tertiary set of target sands. High levels of depletion as well as extremely tight pore pressure margins were anticipated. The operator and the service company's drilling and evaluation (D&E) integrated teams developed a highly collaborative environment from the early planning stage of the project, aligning people and processes and enabling applications. Regional knowledge from an archived geomechanical model was updated during collaborative planning sessions, enabling both parties to have a consistent understanding of the subsurface challenges to correctly drill and log each interval. Potential wellbore instability issues were mitigated using a proactive geomechanics analysis and hydraulics management from an integrated real-time operations center (iROC). Formation compressional slowness from a logging-while-drilling (LWD) sonic system was used, updating the geomechanical model for accurate real-time pore pressure and wellbore stability analysis. Additionally, the sonic system was used for top-of-cement (TOC) evaluation behind the intermediate casing to satisfy the Bureau of Safety and Environmental Enforcement (BSEE) requirements to differentiate fully bonded pipe from free pipe. Geosteering services from real-time log response correlations and at-bit geological predictions were used to correctly geostop for an intermediate casing point before pressure regression. An LWD formation pressure system provided pressure tests over various depth intervals, providing excellent fluid gradient determination for the primary target sand package. An LWD azimuthal density system delivered high-quality borehole images within the 16 ½-in. borehole section, providing dip information for geological correlation to seismic. Further, the azimuthal density image system resolved the interbedded shale/sand sequences, allowing dip analysis for geological model correlation within the reservoir. Challenges for this wellbore included shallow water hazards, wellbore instability, setting intermediate casing above the sand targets, and the depleted reservoir section. The deployment of specific technologies with associated unique applications discussed in detail within this paper led to superior well construction execution under time (8 days) and under budget (USD 4 million).
Evaluation of cement placement is an important part of the majority of deepwater wells. Cement placement confirmation is an important step following a cementing operation. More than one technique can be used to provide information about the top of cement (TOC) and about the depth interval of a good bond between the formation and the casing. Determining the length of annular cement coverage, which is an indication of correct cement placement, is useful knowledge before drilling and/or completion operations can proceed. The requirement for additional and improved cement evaluation techniques is greater now than ever before. A variety of methods can be used to evaluate cement placement. The routine approach after a casing or liner cement job uses a job chart to calculate lift pressure and actual vs. predicted system pressure. These data enable an estimate of cement height in the annulus to be made, but they do not confirm the TOC. These methods vary in accuracy and difficulty, depending on well conditions. Common TOC evaluation methods in specific wellbore casing/liner sections typically require running a temperature survey or cement bond log (CBL) sensors/systems on a wireline. These operations use the rig's critical path time for each wireline run, which can add risk or difficulties, depending on the well trajectory. In addition, cement bond evaluation for large diameter casing can be technically challenging because it can reach the upper threshold measurement limitations for conventional wireline-conveyed CBL tools. Many operators now use logging-while-drilling (LWD) sonic sensors for compressional and shear data acquisition in openhole environments. Using the same sonic systems, with minimal additional rig time, logging data acquired through the casing/liner strings while running a drilling or clean out assembly can be evaluated to confirm the TOC. This paper demonstrates how LWD sonic technology can provide confirmation of the TOC, saving a considerable amount of rig time, as compared to performing a dedicated wireline evaluation run or potentially unnecessary cement squeeze operations. The paper presents and discusses Gulf of Mexico (GOM) case studies. Based on various specific challenges, through correct data analysis, TOC evaluation best practices are implemented to optimize the LWD acoustic data acquisition inside the casing/liner. New data examination techniques are reviewed that can be applied to different scenarios, such as TOC evaluation behind dual pipes and real-time assessment for quick data analysis turn-around. In conjunction with the case studies, the paper also provides information about the LWD cased-hole logging techniques, analysis, and results of the data application.
In the current market, operational geology and geoscience asset teams have clear and aggressive financial reduction targets that need to be met without compromising the formation evaluation (FE) requirements of a well construction project. Advances in drilling and completion technologies and practices for deep-water wells commonly require operators to drill larger borehole sizes throughout the well construction process. For deep-water subsalt wellbores, this often implies exiting a thick salt layer with borehole deviation in borehole sizes ranging from 14.5 to 17.5 in. This paper introduces a unique 9.5-in. nominal collar size logging-while-drilling (LWD) density tool that makes it possible to address the FE challenges encountered in large borehole sizes. Any LWD method that can provide crucial cost-effective and accurate FE data can add value to well drilling and logging programs. The new tool provides density and photoelectric measurements in large-diameter boreholes. It also contains an ultrasonic sensor that can provide accurate borehole geometry information, which is useful for identifying stress-related breakout and providing accurate estimates of borehole volume for later placement of cement for zonal isolation. In such settings, formation density measurements are crucial for determining key evaluation parameters, such as porosity and rock mechanical properties, but acquisition of these measurements can be challenging using existing LWD technologies. In addition, real-time structural dip information for subsalt environments provides insight for the interpretation of the geological structure of the field but is often difficult to obtain in large-diameter boreholes. Several case studies demonstrate the value added by the new tool and its breadth of application, as well as the implications for pre-job analysis, bottom-hole assembly (BHA) modeling, data-acquisition procedures, sensor response analysis, and economic benefits to the operator. The capability of acquiring logging data for interpretation purposes and to fulfill specific regulatory requirements without negatively affecting the drilling program provides a desirable cost-management opportunity. The results presented here provide a reference for appropriate business cases to help justify the use of this unique LWD technology in drilling and logging projects involving large-diameter boreholes.
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