The process of geosteering, using real-time logging-while-drilling data to actively steer horizontal or highly deviated wellbores, has been in use for more than 20 years. Over this period, the demand for more sophisticated measurements has developed along with the need to access increasingly difficult reservoirs as the simpler drilling scenarios are being exhausted.Such a case is evident in the Yme Gamma development, located offshore south-western Norway, consisting of four horizontal producing wells and two vertical injectors. Abandoned by the original operator because of high water cut, to redevelop the field the current operator needed to drill subsequent wells in a narrow corridor up-dip between the existing producer wells and the bounding fault, in the thin YS 7 (4 to 8 m) inner-estuary sands or the YS 5/4 (2 to 6 m) inner/central-estuary sands.The Yme 9/2-C-2A well was successfully drilled as a replacement for the C-2 well, which had been redrilled because of the reservoir being swept in that area. Since the neighbouring well, C-3T3, showed encouraging results in the YS 5/4 reservoir, which was not targeted in that well, a horizontal well was planned to exploit this reservoir.There were several challenges that needed to be addressed in the geosteering of this well: the need to keep the wellbore within a narrow reservoir (between 2 and 6m in thickness); a very narrow lateral corridor between the main bounding fault and existing wells; observed sub-seismic faulting in neighbouring wells; and the possibility of a coal layer, which would necessitate a sidetrack if intersected.As with any geosteering operation, success is often the result of a combination of inter-department team work and communication as well as the use of appropriate technology. Integrated use of a bed boundary mapping device and real-time density images allowed the 9/2-C-2A well to be geosteered within these tight tolerances, skirting the bounding fault by less than 1 m lateral displacement, and allowing for the drilling of other tight tolerance wells.
The high structural uncertainty present in a field in Egypt required special logging-while-drilling (LWD) measurement specifications to achieve economically satisfying production results. East Zeit Oil Company (Zeitco) was targeting an oilbearing sandstone reservoir that is characterized by the presence of subseismic faults and high structural uncertainty. This uncertainty ranges from 6° to 23° in structure dip, which makes drilling successful horizontal wells in this reservoir challenging. A well with a horizontal penetration envelope from a minimum 500 ft measured depth (MD) to an optimum 1,600 ft MD was planned for this reservoir to make the well economic. Steering in such a challenging reservoir required having realtime measurements as close as possible to the bit to make the required real-time decisions. A real-time density image and with other azimuthal data were the primary measurements used in steering this horizontal well. Being used with a rotary steerable tool, the multifunction LWD tool provided density image at only 39 ft behind the bit, which was crucial for the success of the well. The well was successfully drilled with 100% exposure inside the sandstone reservoir with 1,532 ft MD horizontal penetration. In addition, well testing results indicated a natural flow of 2,000 BOPD, which made this well the highest oil producer in the field.
The coal-seam gas (CSG) industry has long been considered as a high volume, low cost market. As the industry has matured, the selective application of high-tier technologies has realized a step change in performance and real-time formation evaluation results. We investigated whether a high-tier LWD multi-function service could provide a suite of quantitative real-time measurements in several deviated wells. The key objective was to reduce the amount of non-productive rig time spent waiting for memory data in order to confirm the completion design. Significant savings in rig time could be realised if reliable, high-quality real-time data enabled the early identification of coal seams and permeable aquifers such that the swellable packer and slotted liner completion design could be completed without the need for final memory logs. The area of interest is characterized by thin Jurassic coal seams rather than thick Permian seams. It was critical to accurately identify thin coal beds in real-time whilst maintaining a high rate of penetration (ROP). Low-resolution data would result in poor completion design, underestimation of net coal reserves, and sub-optimal static models. Measuring coal thickness and properties can be difficult due to the fundamental differences between the formation evaluation measurements and their relative axial resolutions. The presence of thin coals can further complicate the interpretation. Another challenge was to optimize the real-time data transmission to prevent any limitation on the key directional drilling data parameters. Conventional LWD logs (gamma ray, nuclear, and resistivity measurements) provide formation evaluation information while drilling. The selection of a rotary steerable system (RSS) was critical as it ensured directional control and avoided any sliding intervals over key aquifers and coal zones, thereby ensuring optimal LWD acquisition. Advanced formation evaluation options of the LWD data also included using dual-pass resistivity inversions for Rt/Rxo to determine the invasion profile in a permeable aquifer zone above the main coal-producing reservoir. Having this information in real-time was critical in guiding well-specific competition decisions. Induction and laterolog-type resistivity tools were run on one well to quantify differences in the measurements and to determine the best resistivity acquisition tool for CSG wells drilled with saline muds in freshwater formations. The results showed that high-tier LWD technologies provide multiple benefits in CSG wells. The project was executed with all directional and logging objectives achieved. Quantitative real-time data was critical for completion decisions including ECP placement together with swellable packer and slotted liner designs. This resulted in significant cost savings which are important to major CSG developments operating within a low-cost operating model. LWD memory data provided a rich suite of additional measurements to complement the real-time data. Memory data was used for advanced reservoir analysis with industry-unique measurements.
The electromagnetic propagation (EMP) measurement frequently acquired with logging-while-drilling (LWD) tools in high-angle wells is sensitive to geometrical effects that can mask the true formation resistivity. Less commonly used, the LWD laterolog measurement is sometimes perceived as providing data too shallow to give a true formation resistivity (Rt). This paper presents modeling and actual examples to demonstrate that the laterolog can provide a superior resistivity measurement for formation evaluation than the does the EMP LWD tool. We examine the laterolog and EMP resistivities in several high-angle wells crossing carbonate formations in 8.5-in. and 6 1/8-in. hole sizes. In the 8.5-in. sections, producers and water injectors (high and low resistivity ranges) were evaluated. In the 6 1/8-in. sections, one reservoir sandwiched between two very high-resistivity layers and another borehole in a highly fractured reservoir were examined. The laterolog data were corrected for invasion using a 1D inversion of the memory data. Structure-based forward modeling was used to examine and explain the differences between the resistivity methods. In the first example, the laterolog data showed a clear conductive invasion profile. While the deepest laterolog real-time resistivity data indicated lower resistivity than the EMP resistivity, the true resistivity, Rt, from the 1D inversion matched the EMP resistivity. This result validated both measurements and emphasized that those differences were due to invasion. In the second example, a reservoir zone was initially drilled with resistivity measurements made only by the EMP tool. The LWD laterolog was run several days later, and the resistivity data were much lower in the relogged section compared with the EMP resistivity. The laterolog 1D inversion was unable to resolve Rt because of the excessively deep invasion that occurred over the course of several days. These two examples demonstrated that when acquired in normal drilling conditions, the laterolog measurements can provide the uninvaded formation resistivity even in the presence of invasion. A reservoir in another example was sandwiched between resistive layers that caused difficult-to-explain elevated EMP resistivity readings. Structural modeling reproduced the elevated behavior of the EMP data and explained the differences between resistivity measurements. This result showed that the laterolog is better suited to evaluate resistivity in thin reservoirs where there is a high-resistivity contrast to the adjacent layer. Finally, fractured reservoir examples are presented, which show that both the laterolog and EMP can be affected by the presence of fracture swarms. The examples presented in this paper demonstrate that in high-angle wells, under normal drilling conditions, invasion-corrected laterolog resistivity is nearer to Rt than is the EMP resistivity. Furthermore, the laterolog measurement provides data that are better input to water saturation calculations.
For most offshore and tidal zolone oil development in North China, one of the major challenges of the industry is high drilling uncertainty and low reservoir encountered rate at the braided river delta and fluvial deposition environment with the common characters of thin sand channels, severe lateral change, unstable sand structure and low sand connectivity. Optimizing the wellbore placement inside the complex reservoir and depicting the sand with detailed information are gradually being critical to real time geosteering in these areas. Over the last decades, the continuous improvement of distance-to-boundary logging while drilling workflows has dramatically enhance the drilling efficiency of horizontal well. However, relatively short depth of detection (DOD) and low sensitivity to multi-layer environment still cannot meet the requirement of drilling under these complicated geologies. To reduce the geosteering uncertainty and enhance formation evaluation in complex environment, a new advancement in mapping-while-drilling electromagnetic propagation resistivity method, with the industry's first combination of axial, tilted and transverse antennas and significant software enhancements, made a momentous progress for complex reservoir geosteering and characterization. Compared to the previous generation, this service could provide: Larger depth of detection which doubled the previous generation. For one hand, larger DOD means earlier proactive strategy for the well position optimization; For the other one, enlarged vision also helps achieve whole delineation of the target sand channel and thus much better geological understanding for the reservoir.More sensitivity for anisotropy and local sedimentary character. Improved measurements set and enhanced software algorithm can visualize the detailed characteristics inside the sand channel. With its up-to-eight-layer resistivity reconstruction, the refined inversion exceeds the existing propagation resistivity answer product. Outstanding performance was observed during the implementation. The target sand channel of 6-7m thickness could be delineated clearly by the refined inversion. It not only depicted the whole picture the sand body, but also provided an earlier sign of structural fluctuation, which ensured the success and high oil recovery rate of the horizontal section. For the well with higher anisotropy or more local sedimentary features, comparing to the blur reflection of the previous method, this ultra-high-definition technology could provide a sophisticated vision of the shape, thickness, direction and resistivity property of the local thin layers and shaly block. Reliable evidence of both outline and inside characteristics of the sand channels improved the further well path design and geological understanding. The ultra-high-definition mapping-while-drilling technology opened the market of complex deposition environment drilling. It remarkably increased the reservoir encountered rate and predictability of the environment, helping to reduce the budget and enhance drilling efficiency. The ultra-high-definition directional resistivity propagation method will surely lead the industry to the next level of the complex reservoir development.
The transgressive sandstones of the Badenian 16.TH reservoir have been on production for over 65 years. As part of a recent field re-development project the oil production has been accelerated with high-angle/horizontal wells. Targeting drainage areas to access attic oil with the accurate placement of these boreholes was deemed business critical. Previous mapping efforts did not capture the undulating structural nature of the top of the first sand layer. Since the "sweet spot" of the reservoir was assumed to have a gross thickness of 1 to 2 meters, the application of proactive geosteering with the latest Logging-While-Drilling (LWD) technology was viewed as essential. This paper describes the placement of two wells, which benefited from active geosteering based on the data transmitted and interpreted in real-time. A multilayer bed boundary detection service was the primary source of information to place the boreholes close to the target formation top and to map the presence of fluid transition zones. Deep azimuthal electromagnetic measurements enabled continuous real-time, 360 degree, mapping of the direction and distance to resistivity changes in the formation. Conventional LWD logs (gamma ray, nuclear, and resistivity measurements) provided formation evaluation and saturation estimation while drilling. The rotary steerable system completed the drill string and ensured directional control. Proactive decision-making used real-time inversions to optimize the landing and improve well placement, since critical data - distance to boundary, geometry of the remote reservoir top and fluid changes in transition zones - were available in real time. In all wells the trajectory was maintained within the zone of interest by taking the proactive, real-time decisions while drilling. The integration of multilayer inversion results with recorded borehole images enabled a comprehensive interpretation and detailed 3D structural modeling in the post-job phase. Sub-seismic faulting and local dip changes were revealed that were not predicted in the pre-drill geological model. Finally, structural information, formation evaluation results, and oil-water transition zone mapping were used to optimize completion design to delay the increase in water production. The production results confirmed the anticipated volumes, proving the advantages of the innovative LWD applications and their capability for optimized placement of such production wells. The use of the directional multilayer detection service aided structural interpretation, definition of the reservoir geometry and the position of the current fluid transition zones. This in turn led to improved accuracy in the description and understanding of the reservoir.
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