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Results
Sourceless Neutron-Gamma Density SNGD: A Radioisotope-Free Bulk Density Measurement: Physics Principles, Environmental Effects, and Applications
Evans, Mike (Schlumberger) | Allioli, Frangoise (Schlumberger) | Cretoiu, Valentin (Schlumberger) | Haranger, Fabien (Schlumberger) | Laporte, Nicolas (Schlumberger) | Mauborgne, Marie-Laure (Schlumberger) | Nicoletti, Luisa (Schlumberger) | Reichel, Nicole (Schlumberger) | Stoller, Christian (Schlumberger) | Tarrius, Mathieu (Schlumberger) | Griffiths, Roger (Schlumberger)
Abstract For many years there has been a need to find an alternative to the radioisotope-based gamma-gamma density (GGD) measurement. The traditional GGD measurement uses the scattering of 662-keV gamma rays from a Cs radioisotopic source to determine formation density. A statistically precise measurement requires a 40-GBq or higher source strength and such a logging source, with a 30.17-year half-life, may pose health, security, and environmental risks. Pulsed-neutron generators have been used in the industry for several decades in wireline tools and more recently in logging-while-drilling tools. These generators produce 14-MeV neutrons, many of which interact with the nuclei in the formation through inelastic collisions. These inelastic interactions are typically followed by the emission of a variety of high- energy gamma rays. Similar to the case of the GGD measurement, the transport and attenuation of these gamma rays is a strong function of the formation density. However, the gamma-ray source is now distributed over a volume within the formation, where gamma rays have been induced by neutron interactions and the source can no longer be considered to be a point as in the case of a radioisotopic source. In addition, the extent of the induced source region depends on the transport of the fast neutrons from the source to the point of gamma-ray production. Even though the physics is more complex, it is possible to measure the formation density if the fast neutron transport is taken into account when deriving the density answer. This paper reviews the physics underlying the sourceless neutron-gamma density (SNGD) measurement, explains the various facets of the algorithm used for its computation and details the different environmental effects that may influence the measurement. The successful application of the method is shown in several log examples.
- Europe (0.68)
- North America > United States > Texas (0.28)
Inversion-Based Workflows For Interpretation Of Nuclear Density Images In High-Angle And Horizontal Wells
Shetty, Sushil (Schlumberger) | Omeragic, Dzevat (Schlumberger) | Habashy, Tarek (Schlumberger) | Miles, Jeffrey (Schlumberger) | Rasmus, John (Schlumberger) | Griffiths, Roger (Schlumberger) | Morriss, Chris (Schlumberger)
ABSTRACT: We have developed multi-step inversion-based workflows for the interpretation of nuclear density images in high-angle and horizontal wells. The key component of the workflow is the model-based parametric inversion using a newly developed fastforward model based on second-order 3D sensitivity functions. For the first time, a layered formation model and borehole are included simultaneously in the analysis resulting in accurate layer thicknesses, shoulder-bed corrected layer densities, and borehole geometry consistent with all the data. The parametric model used for interpretation includes a multi-layer dipping formation, mud properties, borehole geometry, and 3D well trajectory. Measurement sensitivities are used in the design of a flexible and robust four-step iterative procedure for determining optimum parameter values. In the first step of the procedure, an initial guess for the formation layering and dip is derived from the compensated density measurement by extracting sinusoidal features of the image and squaring the bottom quadrant profile. In the second and third steps, the optimum mud properties and borehole geometry are derived from the shallow sensing measurements. In the final step, the optimum formation layering and dip are derived from the deeper sensing measurements. The workflow utilizes an adaptive sliding window whose length is determined after segmentation of the images along the trajectory based on the relative dip. The workflow is especially tuned for interpretation in horizontal wells, when potential ambiguity in interpretation is increased because of the difficulty in determining the dip, lateral changes in layer properties and the influence of stand-off and nearby non-crossed boundaries. In scenarios where the wellbore trajectory is nearly parallel to the boundary, the parameterization includes the non-crossed boundaries. The inversion window size is small and the processing enforces the lateral continuity of the layer thickness or formation densities locally.
- Europe (0.98)
- North America > United States > Texas (0.28)
- Europe > Norway > North Sea > Northern North Sea > North Viking Graben > Block 30/6 > Veslefrikk Field > Statfjord Group Formation (0.99)
- Europe > Norway > North Sea > Northern North Sea > North Viking Graben > Block 30/6 > Veslefrikk Field > Dunlin Group Formation (0.99)
- Europe > Norway > North Sea > Northern North Sea > North Viking Graben > Block 30/6 > Veslefrikk Field > Brent Group Formation (0.99)
- (3 more...)
ABSTRACT: Logging measurements are axially focused and generally deep reading. In near vertical boreholes, measurement volumes are approximately parallel to formation layering (Fig. 1, upper panel). In this environment, logging measurements provide optimal vertical resolution and information about formation properties beyond the mudinvaded zone. Logging-while-drilling (LWD) measurements are predominantly acquired in high angle and horizontal (HaHz) wells. In this environment, measurement volumes are approximately perpendicular to formation layering and deepreading measurements may respond to multiple layers, creating complications for subsequent interpretation (Fig. 1, lower panel). For many years LWD measurements were considered unsuitable for quantitative petrophysical evaluation, however the problem was not so much with the measurements but with the interpreters' assumption that measurements respond to a single layer, as had been their experience with vertical wells. With an increasing proportion of HaHz wells drilled for field development, a workflow that compensates for geometrical effects is required to determine true formation properties from the logs. A new workflow has been developed to address these issues and is now available in commercial software. Starting with the acquired logs, a layered earth model of the structural geometry proximal to the wellbore is created. Log responses are used to identify boundary intersections with the well trajectory. In the case of multiple crossings of a single boundary, the user simply "joins the dots" to define the formation geometry. Formation dips extracted from LWD images are plotted on the layered earth model to define the relative dip between the wellbore and layering. Formation geometry from other sources can also be imported to guide the structural interpretation. Once the approximate geometry is defined, initial estimates of the formation properties (such as gamma ray, horizontal resistivity, vertical resistivity, bulk density and neutron porosity) for each layer are obtained from the measured logs.
- Europe (0.71)
- North America > United States (0.68)
- Geology > Geological Subdiscipline > Stratigraphy (0.66)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.48)