Deep-directional-resistivity (DDR) logging-while-drilling (LWD) technology has gained acceptance as an efficient method for horizontal well landing, geosteering, and reservoir and fluidcontact mapping. The largest part of the value lies in the automatic inversion of the measurements, improving the detection of geologic and fluid boundaries within a radius of investigation of more than 30 m around the wellbore in real time. The tool and its sensors were designed to acquire a data set with sensitivity to a number of formation properties ensuring the concurrent deployment of a robust inversion. Based on this, an inversion method implemented on computer clusters statistically samples (or explores) the multidimensional inversion space, returning the distribution of formation models that fit the available data. The most noticeable aspect to operators is that this is done without introducing bias. There is no need to commit to a number of layers or to the position, thickness, resistivity, or dip of the layers to generate the continuous images on which all DDR interpretation is based, which translates into several theoretical and practical advantages. In practice, this gives users a higher level of quality control and more confidence in interpreting the subsurface within a larger diameter around the wellbore, which is useful for integrating the information into a geomodel. This derisks current and future operation, improves real-time and planning decisions, and ultimately drives better well placement, completion, production, and return on investment.

Statoil has played a key role in testing and development of the new ultra-deep directional resistivity (DDR) logging while drilling (LWD) measurements for high angle and horizontal wells the last 4 years. Inverted resistivity images provide an overview of geological structures and fluid contacts tens of meters around the wellbore. The ultra-deep look around measurements, sensitive to resistivity contrasts up to 30 m away or even more in favorable conditions, are a step change, when it comes to possibility to position the wellbore strategically in the reservoir and to characterize reservoir structure and properties. This paper will present how the new DDR measurements have been applied with success in an operating license on the Norwegian Continental Shelf (NCS). Long horizontal wells in the reservoir sections have been identified as a key strategy to increase recovery. The main benefits from the DDR measurements in the license have been to maximize reservoir exposure by active geosteering, to optimize well placement above oil-water contact, and to increase subsurface understanding which is important input for future well plans.The DDR measurements are already a commercial service with regard to well placement and reservoir landing. Statoil is however also actively pushing for improved reservoir characterization, by coupling geomodels and DDR modeling and inversion software. This paper will also present how standard LWD logs and images can be combined with the DDR inversion results, to build a near-wellbore 3D structural model supporting all available data. This is an important step towards an extended use of the new data not only for well placement, but also for increased subsurface understanding and geomodel update.

Technology Update Safety, efficiency, and accuracy are fundamental goals of well construction. But as the search for new oil and gas resources pushes into deeper waters and increasingly complex reservoirs, meeting these goals has become more challenging. A better understanding of the subsurface is one of the most efficient ways to mitigate drilling risk and optimize operations’ performances. The ability to map the reservoir in real time, while drilling, contributes to step change, such as understanding sweep efficiency in horizontal wells, landing and maximizing the length of drain within the optimal zone of the reservoir, and avoiding timeconsuming sidetracks or pilot holes. While the industry has several bedboundary mapping tools and services to delineate the reservoir as a well is drilled, their depth of investigation is limited. The best of these systems can map to a distance of 15 to 20 ft (4.6 to 6.1 m) from the borehole. These limitations have made it difficult to improve directional drilling within narrow pay zones or complex reservoirs containing faults, unconformities, and injected or channel sands. As a result, wellbore positioning may be suboptimal and drilling may be less efficient. Real-Time Reservoir Mapping Schlumberger’s GeoSphere reservoir mapping-while-drilling technology was developed to improve the operator’s understanding of the reservoir beyond the first few feet from the wellbore. The system employs an array of multiple subs in the bottomhole assembly to transmit deep directional resistivity measurements that map multiple reservoir layers with resistivity contrasts in real time (Fig. 1). The multispaced receiver array extends the radial depth of investigation to 100 ft (30 m) from the tool, revealing subsurface bedding and fluid-contact details at a true reservoir scale. The significant improvement in depth of investigation provides a reservoir- scale view, enabling operators to optimize landing, reduce drilling risk, and maximize reservoir exposure. By integrating real-time reservoir maps with seismic surveys, the interpretation of reservoir structure and geometry can be confirmed and refined, enabling a step transformation of field development strategy. The system provides a wealth of realtime reservoir knowledge that improves well construction by helping the operator accurately land a well in its target zone and steer the drill bit to keep the wellbore away from reservoir boundaries, dips, and fluid contacts. The real-time mapping data obtained also helps the operator refine the structural and geological reservoir models, thus optimizing plans for recovery and improving development strategies for the entire field. A Smooth Landing The ability to land the wellbore accurately into the reservoir’s target zone of interest is a critical first step in delivering a well that achieves its long-term production potential. Although pilot holes may provide good local information about the reservoir geology, they are often ineffective in predicting lateral geological variability—a critical factor when drilling into long horizontal pay zones. Similarly, well-to-well correlation alone cannot accommodate various structural shifts inherent in many downhole environments. The reservoir mapping-while-drilling technology’s extended depth of investigation, which is enabled by deep directional electromagnetic measurements, helps mitigate the risk of shallow or deep landings. The system supports the identification of large- or local-scale depth shift of the reservoir, providing a precise localization in true vertical depth (TVD) of the top of the reservoir and thus eliminating the cost of drilling a pilot hole. By providing a clear, real-time view of formation boundaries and fluid contacts, the system also avoids the risk of losing lateral exposure and creating sumps.

The data delivered by a new reservoir mapping while drilling (RMWD) tool provides more geological information than that from any other logging-while-drilling (LWD) technology previously available in the oil field. Its answer product images the surrounding formation structure, and the resulting maps can be used by the geoscientists to improve their understanding of the subsurface, the well placement and the reservoir.To take advantage of the richness of the measurements and deep depth of investigation across multiple formation boundaries, an automatic stochastic inversion has been developed that combines approximately a hundred phase and attenuation measurements at various frequencies and transmitter-to-receiver distances. This efficient Bayesian model-based stochastic inversion runs in parallel with multiple independent search instances that randomly sample hundreds of thousands of formation models using a Markov chain Monte Carlo method. All samples above a quality threshold over the solution space are used to generate the distribution of formation models that intrinsically contain the information for model uncertainties.RMWD is a highly nonlinear problem; inverting for a unique solution is analytically difficult due to the well-known local minima issue. The stochastic method addresses that by sampling thousands of possible formation models and outputting a distribution of layered models that are consistent with the measurements. Statistical distributions are displayed for formation resistivity, anisotropy and dip at each logging point. Additionally, the median formation models for resistivity are shown along the well trajectory as a curtain section plot. This provides an intuitive interpretation for the entire reservoir formation around the tool. The inversion curtain section plot can be overlaid with the seismic formation model for combined interpretation. Furthermore, the curtain plot provides graphical information for dip and distance to boundary, which are critical for field applications such as landing, geosteering, remote fluid contact identification, etc. The stochastic-sampling-based answer product has been intensively field tested and has proven to provide reliable estimation of the formation geometries and fluid distributions in many locations and geological environments worldwide.Field applications and simulated examples of the stochastic inversion include remote detection of the reservoir to enable accurate landing, navigating multilayered reservoirs, remote identification of fluid contacts and reservoir characterization in the presence of faults. The stochastic inversion samples the formation properties randomly and provides the distribution of formation properties based on a large number of samples, instead of providing only the most likely solution as is typical for deterministic inversions. A statistical method of presenting inversion results in formation space provides an instant and intuitive understanding of the formation surrounding the tool. Quantifying the non-uniqueness of the inverted fo...

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractVeslefrikk is a North Sea oil field in its tail-end production period where optimal well placement is critical for the drainage of the remaining reserves. This paper presents two case studies representing different challenges with respect to geosteering. In both cases a newly developed Directional Electromagnetic logging while drilling tool (D-EM) was used together with a fully rotated point-the-bit 3D rotary steerable system (RSS) to achieve proactive geosteering. The LWD tool was able to detect resistivity contrasts in any direction up to 5 m from the wellbore. In the first case the objective was to position a 570 m long horizontal well section 1-3 m below the top of the reservoir sand, thereby attaining maximum distance from the water level and ensuring that no attic oil was left behind. In the second case the challenge was to optimize the amount of oil filled sand along the 1100 m horizontal trajectory, while drilling perpendicular to the depositional direction in a fluvial channel system.The early detection of the sand to shale boundaries resulted in an increase of 10-15 % in the recoverable reserves for each well compared with conventional geosteering.The workflow setup for both cases included the use of a Web-based system for communication and data transfer. This ensured efficient decision-making involving geosteering specialists, wellsite geologists, and onshore company personnel.