TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe analysis of deep-reading electromagnetic measurements is critical to the evaluation of hydrocarbon reserves. However, in thin bed formations, poor tool vertical resolution and corresponding low sensitivity to hydrocarbon presence make interpretation in the virgin zone difficult. A priori knowledge such as the formation geometry or auxiliary petrophysical information is necessary to overcome these difficulties. This paper presents a prototype code developed by Schlumberger S-RPC in collaboration with AGIP. Using this code, wireline or LWD, laterolog and induction measurements can be more correctly analyzed in thinly bedded environments (2-D geometry, fluid invaded layers perpendicular to the borehole).This code has been implemented in a software framework that provides a common environment specifically designed for electrical tool interpretation. Processing modules have a common interface and share common functionalities. Their organization reflects an implicit processing methodology, with progressive refinements that provides the interpreter with a robust and simple to use product, to better quantify reserves.A preliminary step is to determine the formation geometry, which is carried out by detecting bed boundaries and representing the formation as a vertical sequence of layers. Petrophysical analysis can be invoked to characterize certain formation properties such as shale volume and porosity. These steps are performed prior to resistivity log measurement analysis and serve as a form of a priori knowledge.Once the formation is described as a sequence of layers, wireline l ogging or logging while drilling (LWD) tool response can be computed using fast 2D simulators. The estimation of resistivity and the subsequent estimation of saturation will correspond to the minimization of a cost function, defined as the weighted squared difference between the measurement and the simulated response. Confidence outputs can be related to the local shape of the cost function at the end of the processing.Two important advantages of the new code must be emphasized: (1) the possibility to choose among several petrophysical models to better describe the environment and determine directly parameters such as hydrocarbon saturation, and (2) the possibility to group together beds which are too thin or too close to each other to be analyzed independently, into a so called single optimization interval described by a reduced set of parameters.This paper presents results obtained on selected benchmarks extracted from real data and compares them with those obtained through more traditional approaches.
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...
Advanced logging-while-drilling (LWD) ultrasonic images provide 0.2 in. resolution, the highest possible resolution for well logs, enabling detailed reservoir characterization at the borehole wall of key geological events. In this paper, we demonstrate how this plethora of information provided by such imagers can be used to identify borehole wall features, but to also specify drilling dynamics. Optimal drilling can be considered as being the efficient transfer of available energy from the surface to the drill bit. When this energy transfer is compromised, drilling rates decrease, hole condition might suffer, and drilling accessories can fail. These problems often lead to poor directional control and compromise the drilling objective itself. The instantaneous kinetics of the drilling bottomhole-assembly (BHA) transparently reveals how efficiently mechanical energy is used to drill a well. This paper presents an application able to understand high-resolution BHA motion from the ultrasonic sensors in an LWD imager. Essentially, information from four pulse-echo ultrasonic sensors is combined using an inversion method to resolve the instantaneous position of the center of the tool, the hole shape as well as the mud slowness. The inversion algorithm was validated by experimental measurements where a tool was immersed in drilling mud and the transit-time data were extracted using a peak detection method applied to the received ultrasonic waveforms. The inversion showed that the computed caliper image and mud slowness were matching the parameters of the experimental setup, and that the tool center of mass moved in x and y directions, as the tool entered into lateral resonance. The inversion was applied on field data to retrieve the history of the tool position in wells after drilling completion. The BHA kinetics from these borehole images highlighted the type and number of lateral movements compared with the hole size and helped to identify improvements in future drilling operations. A comparison of the BHA vibrations simulations during the pre-job planning with the post-drilling dynamics has also provided insights for subsequent wells. The real-time definition of a more optimal workflow was made possible by streaming computed results to the surface. Using the LWD ultrasonic imager as a tool for high-resolution BHA dynamics analysis is an innovative approach with a potential to identify multiple drilling-related issues downhole, in addition to the existing applications of high-resolution borehole imaging. The LWD ultrasonic imager appears to bridge the gap between geologists and drillers with both providing meaningful inputs to each other.
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