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...
An experimental pulsed-neutron logging-while-drilling (LWD) tool is currently under field test. The tool provides a suite of nuclear measurements that include neutron porosity, thermal neutron capture cross section (i.e. sigma), pulsed-neutron density and the relative abundance of certain elements (e.g., calcium, silicon, iron, sulfur, etc.) that are used to calculate mineralogy. The tool provides azimuthal measurements in real time that are useful for geosteering applications. Use of a pulsed-neutron source eliminates the need for radioactive-chemical sources that are used in conventional nuclear LWD tools. This results in increased wellsite safety and efficiency. Procedures and equipment required for radioactive-source handling, storage and retrieval are also eliminated. The experimental LWD tool is the result of a joint collaboration that began in 1995 between the Japan National Oil Corporation and Schlumberger. The primary goal of the tool is to demonstrate feasibility of pulsed-neutron measurements in the hostile LWD environment. Introduction An experimental pulsed-neutron LWD tool has been developed that has successfully logged several test wells. The prototype tool represents a merging of pulsed-neutron wireline measurements with LWD measurement technology. The applications of pulsed-neutron measurements for formation evaluation are well established. Pulsed-neutron wireline tools are used to determine water saturation, neutron porosity and formation mineralogy in both open and cased holes. In addition to the rich variety of measurements, pulsed-neutron technology eliminates the radioactive-chemical sources used in conventional nuclear tools. Source-storage pits, transport shields, loading shields, collar shields and handling tools are therefore not required. Transportation logistics, wellsite safety and wellsite efficiency are all improved.
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