TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractReservoir navigation with LWD resistivity has traditionally relied on matching real time measurements with ideal logs. Reservoir navigation engineers initially build one or more resistivity models including all expected resistivity boundaries such as oil-water contact, reservoir to cap rock interface, faults and unconformities. Then, during drilling, they direct the well and update the earth model by matching actual measurements with forward response model data.Because common LWD resistivity sensors cannot differentiate between an oil-water contact approaching from below and a shale lens approaching from above or from the side, the reservoir navigation engineer fills in the missing information through expertise and local knowledge. In case of complex geology however, such as reservoirs with tilted or rotated fault blocks, multiple fluid contact levels, cross-stratification and shale intrusions, navigation becomes much more challenging and the risk of getting geologically lost is high. In recent years imaging LWD tools were introduced to help reduce the azimuthal uncertainty but they were limited to a few inches in lateral investigation.A new azimuthally sensitive propagation resistivity tool was recently tested for reservoir navigation and formation imaging in some of the more complex reservoirs of the North Sea. In cases where standard omni directional tool responses would lead to ambiguous interpretations, the azimuthally sensitive tool provided the basis for clear geosteering advice. A new imaging algorithm helped visualize approaching beds much like modern imaging devices, but with a depth of investigation reaching several feet into the formation. At fault crossings, the azimuthally sensitive signal helped recognize the relative movement of the formations on either side of the fault. In other instances where the well was run immediately below the cap rock, deep looking azimuthal propagation anticipated the intersection by several hundred feet. Also, analysis of the detailed deep electrical images brought a more complete understanding of the subsurface.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractReservoir navigation with LWD resistivity has traditionally relied on matching real time measurements with ideal logs. Reservoir navigation engineers initially build one or more resistivity models including all expected resistivity boundaries such as oil-water contact, reservoir to cap rock interface, faults and unconformities. Then, during drilling, they direct the well and update the earth model by matching actual measurements with forward response model data.Because common LWD resistivity sensors cannot differentiate between an oil-water contact approaching from below and a shale lens approaching from above or from the side, the reservoir navigation engineer fills in the missing information through expertise and local knowledge. In case of complex geology however, such as reservoirs with tilted or rotated fault blocks, multiple fluid contact levels, cross-stratification and shale intrusions, navigation becomes much more challenging and the risk of getting geologically lost is high. In recent years imaging LWD tools were introduced to help reduce the azimuthal uncertainty but they were limited to a few inches in lateral investigation.A new azimuthally sensitive propagation resistivity tool was recently tested for reservoir navigation and formation imaging in some of the more complex reservoirs of the North Sea. In cases where standard omni directional tool responses would lead to ambiguous interpretations, the azimuthally sensitive tool provided the basis for clear geosteering advice. A new imaging algorithm helped visualize approaching beds much like modern imaging devices, but with a depth of investigation reaching several feet into the formation. At fault crossings, the azimuthally sensitive signal helped recognize the relative movement of the formations on either side of the fault. In other instances where the well was run immediately below the cap rock, deep looking azimuthal propagation anticipated the intersection by several hundred feet. Also, analysis of the detailed deep electrical images brought a more complete understanding of the subsurface.
In order to maximize recoverable reserves in both new marginal satellite developments and bypassed oil in mature fields it is vital that a horizontal wellbore is optimally positioned within the reservoir. Recent innovations in drilling technology, three dimensional (3-D) visualization and logging while drilling (LWD) sensors have been integrated into a Reservoir Navigation Service focusing on maximizing the value recovered from every geosteered well. Introduction Standard geosteering techniques based on layer cake resistivity response modelling1,2,3 have proven to be inadequate for the complex geology of the North Sea. To effectively geosteer these fields a range of innovative techniques have been devised to achieve optimal wellbore placement. In this paper we review four recent successful geosteering projects illustrating the key benefits of developing project specific solutions as summarised below.Integration of the earth model into the wellplanning and wellsite geosteering process significantly reduced time and cost involved. The use of wellsite 3-D visualization assisted the entire asset team in realising the goals of the project.Near bit Formation Evaluation sensors4 were used to characterize formation dip and ensure effective navigation within the reservoir utilizing the improved steerability of Rotary Closed Loop Steerable (RCLS) drilling systems.5Advanced processing and unique interpretation of LWD propagation resistivity data confirmed fracture identification in a fractured chalk reservoir prior to completion6.Four horizontal wells were accurately placed within a 3 to 10ft thick zone for a carbonate reservoir gas storage project by geosteering on effective porosity. These case histories are used to demonstrate how geosteering in complex North Sea reservoirs requires field specific techniques to be developed for both the planning and successful execution of a project. Earth Model Integration into Planning and Drilling Wellplanning Conventional wellplanning techniques require the Geoscience team to select a number of targets defined by their location in 3-D space. The Drilling team then attempt to construct a wellplan satisfying these targets which also takes account of necessary engineering limitations such as torque and drag, dog leg severity (DLS), the practical limitations of proposed drilling tools and completion requirements. To achieve a wellplan that is acceptable to both Geoscience and Engineering needs, an iterative process requiring numerous adjustments is commonly used before a definitive wellplan is approved. To improve efficiency and reduce time and costs involved in generating an approved wellplan it is necessary to develop a service integrating wellplanning into the 3-D earth model. A recent proposal from a North Sea operator required the Directional Drilling / LWD contractor to construct a 3-D faulted geological model centred on a proposed horizontal well. Once an accurate model had been built and verified then the well was planned from the reservoir upwards based on the geological objectives supplied by the operator. The objectives were defined as stratigraphic well position within the Brent sequence for a specific fault block, e.g. in fault block 3 the well should remain in the Ranoch formation as opposed to conventional wellplanning where a geometric target is specified (Fig. 1).
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