ABSTRACT

Injectite sands form reservoirs of highly complex nature when it comes to sand distribution and connectivity. An injectite sand reservoir is formed when sand become highly mobile like a suspension of liquid sand and move from areas of higher pressure at deep depths to shallower depths of lower pressure. The fluid sands fracture rocks and are injected into zones of lower permeability. The sands often leave an interconnected trail with intrusions at various depths. Therefore, lower zones flow, and higher zones may have a highly localized interconnected vertical conduit. The resulting semi-interconnected reservoir (think stack structure) is very difficult to assess by conventional means since the interlacing reservoir sands may be either continuous or discontinuous at and across multiple levels. Fluid contacts might vary throughout the system, and fluid type can also vary throughout the system. Traditionally, contrasts in fluid type and varying fluid contacts have been interpreted as a sign of compartmentalization, but this might be an erroneous conclusion in an injectite system. For this reason, it is important to understand fluid trends in situ in these reservoirs. The motivation for reconstruction of a wide spectrum started first with the need for an actual measurement of contamination, but when evaluating new injectites play in the Norwegian North Sea, the reconstructed spectrums were also important in order to better indicate in-situ fluid properties.

Downhole fluid analysis has the potential to resolve ambiguity in very complex reservoirs as observed in injectite systems found in the North Sea. Downhole fluid spectra contain a wealth of information to fingerprint a fluid and help to assess continuity. A new spectral inversion technique has been developed to overcome these limitations. The new spectral reconstruction technology has been developed, providing very accurate spectral profiles with respect to laboratory measurements. The spectral reconstruction technique is accomplished with a novel single-beam optical configuration as opposed to the more common dual-beam reference path optical configuration more commonly used. To enable a single-beam configuration for spectral measurements, a digital twin model of the spectrometer is used to compute a reference profile in real time. The technique uses specially designed channels, tailored to enable the calculation of a digital twin reference. As such, the reconstruction is more independent of drift with respect to devices that use a separate reference path. By also borrowing from compressed sensing techniques, the reconstruction technique also allows for higher-fidelity measurements across a larger wavelength range.

The reconstructed data may then be used for purposes such as contamination measurement, fluid property trends for reservoir continuity assessment, and digital sampling. Digital sampling is the process of extrapolating clean fluid properties from formation fluids not physically sampled. The reconstruction occurs at wavelengths from 450 nm to 3300 nm, which is a wider optical region than has historically been accessible to formation testers. The expanded wavelength range allows access of the mid-infrared region for which synthetic drilling-fluid components have much optical activity. This reconstruction may allow contamination to be directly determined. However, because the spectrum is calculated and not measured directly, care must be taken with the interpretation as some uncharacterized signatures may provide a non-unique inversion problem. This paper will discuss fluid continuity assessment from six depths of an injectite reservoir structure based on the reconstruction of spectral data from fluids at these six depths. The applicability of the technique to contamination assessment and digital sampling will also be discussed.

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