Advanced levels of depletion cause unexpected reduced pore pressures and thus reduced reservoir fracture gradients, presenting considerable drilling challenges in the Burgan Field in Kuwait. This can lead to matrix damage due to mud losses and result in borehole collapse due to the relative increase of effective stress concentration in the vicinity of the borehole. The area of the study showed high levels of non-productive time (NPT) as well as increased costs due to the drilling of unplanned sidetracks in highly deviated wells. LWD Penta-combo measurements including azimuthal sonic and formation pressure have been utilized to model fracture gradient and borehole collapse gradient in real-time, and proved effective at reducing risks and rig time by allowing proactive management of these challenges. Borehole collapse in offset wells was analyzed to predict and simulate the wellbore stability of a planned well via a pre-drill geomechanics model prior to drilling the well. The well was planned with a high deviation of 56° and oil based mud. The salinity of the water phase was recognized as an essential factor in assessing wellbore stability risk with respect to shale-dependent time failure. The integration of real-time LWD sonic and density data with formation pressure testing data in the geomechanics model for the first time in the 12 ¼-in. section showed excellent correlation and confirmed different levels of depletion in the reservoir. Uncertainty in the modeled fracture gradient was significantly reduced and effectively eliminated with the inclusion of real-time formation pore pressure testing data. This successful combination of modeled pore-pressure curves from the real-time LWD sonic data with the real-time formation pressure test data acquired while drilling in the same run is a first in Kuwait. A total of 15 pressure points were sampled in real time while drilling, allowing for proactive mud window optimization and borehole stability geomechanical analysis. This real-time wellbore stability technique based on advanced LWD azimuthal acoustic technology in conjunction with real-time formation pressure testing while drilling has been developed with a unique process and workflow allowing the operator to drill the well with no significant wellbore stability issues and with the optimized shape of the borehole for a safe casing run. In addition, NPT was reduced to 0 hours and overall days per well were reduced significantly.
Pulsed-neutron capture (PNC) logs are commonly used to determine formation water saturation in cased-hole environments, often for time-lapse monitoring purposes. This paper describes a new diffusion-corrected sigma algorithm developed for a pulse neutron logging tool. In southeast Kuwait, diffusion-corrected sigma log data was recorded in three wells using an array of four optimally spaced gamma ray detectors above a neutron generator. To calculate a diffusion-corrected sigma, an algorithm based on a dual exponential fit was applied to the time-decay spectrum of the near and far detectors. This calculation separates the formation and borehole decays. This approach provides an apparent formation sigma for the near and far detectors. The algorithm uses the near detector for final sigma, and a diffusion correction to the near sigma is determined by a function of a near-far sigma difference. The diffusion-corrected sigma matched the expected results and provided a good statistical quality—even at high sigma values—because it is based on the near detector with its higher count rate, as demonstrated in the examples presented. Also, the formation sigma was independent of different borehole conditions in which the data was recorded. The final formation sigma results were compared to volumetric results from open-hole data (volume of shale, effective porosity and water saturation) and sigma calculated from open-hole volumetric using material balance. The PNC data recorded in the three wells allowed determination of the most recent oil-water contact (OWC) and update of water encroachment maps from the time-lapse monitoring. Comparing with previous sigma data recorded in these wells, it was concluded a normalization transform is not needed because R2 value of the linear regression is close to 1. The diffusion-corrected sigma algorithm using dual exponential fit showed that this technique was able to extract independent values for borehole sigma and formation sigma for each detector and to perform an accurate diffusion correction. This algorithm will provide reliable sigma values regardless of the borehole conditions in which the data was recorded.
Well control situation in high angle wells put challenging environment for the acquisition and interpretation of formation evaluation data on wireline. This case study puts a highlight on a new technology, which allows mitigating the well deviation and trajectory-related complications in addition to wellbore stability to acquire accurate wireline quality data, resulting in an improved completion design and an increase in production. The discussed well was drilled and completed as a horizontal open hole producer in 1994. It showed unstable production and was put on schedule for re-completion to increase production. The planned fishbone well completion design implied high-resolution caliper data acquisition to ascertain the borehole geometry in detail. Moreover, a time-lapse water saturation analysis was mandatory. On the other hand, a considerable operational risk was associated with conventional wireline tools deployment, pushing the toolstring downhole. However, this risk was minimized using the new compact well shuttle system (CWS) which has all the logging sensors housed inside the drill pipe before they can exit it to the open hole at the start of the operations. This conveyance method gave the drillers full well control, having the ability to rotate the pipe and even circulate mud in this 95-degree inclined, unstable well. Using open-end bit allowed reaming during RIH and POOH. Also, memory data acquisition allowed getting the required well log data in one run. Consequently, the well objectives were achieved, and operational risks were minimized. Also, the total well cost was optimized without sacrificing data quality, which supported formation evaluation of Mauddud reservoir drain. Finally, this led to an efficient completion and production decision-making. The advanced conveyance method discussed in this case study has been deployed in a Horizontal well within Mauddud reservoir (onshore Kuwait) for the first time, reduces the risk and cost of accessing high angle wells and wells with well control situation.
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