Abstract:

We investigated the potential use of a spectroscopy-based chemical sensor where a fiber optic waveguide would be installed within the lower marine riser package/blow-out preventer (LMRP/BOP) to provide early warning of an impending blow out. The light source and the spectrometer would be located at the platform and only the fiber optic cable and only the sensing tip or ‘probe’ would be located within the LMRP/BOP would be within a harsh environment (elevated pressure and temperature) and need to be ‘ruggedized’. A chemical sensor located in the LMRP/BOP would give adequate time for a response by the drilling crew should an influx of formation hydrocarbons be detected. Our planned tasks (a) through (d) involved testing with water-based mud (WBM) and oil-based mud (OBM). Many potential adaptations were conceived and we investigated those that could be accomplished within the project duration and budget constraints. We completed work on the two tasks (a) and (b) related to WBM with an approach that yielded promising results for WBM and transitioned to tasks (c) and (d) using that same approach: a) confirm that we can discern between low-molecular weight hydrocarbons (C5 through C9) and water-based mud (WBM) on their signal, or characteristic spectra; b) discern these hydrocarbons within WBM; c) discern between low-molecular weight hydrocarbons and oil-based mud (OBM) based on the spectra, and; d) discern between these hydrocarbons within OBM. Our results of tasks (a) through (d) incorporated adaptations of spectroscopic methods that include, broadly, transmission spectroscopy, absorption spectroscopy, reflectance, and Raman spectroscopy. We confirmed the ability to discern between water and low molecular weight hydrocarbons using NIR spectroscopy (task (a)). We confirmed difficulty using transmission spectroscopy for WBM due to the expected high degree of light scattering leading to the use of hydrophobic/lipophilic porous coatings that would allow detection using the Evanescent wave. A cyclic olephin copolymer (Topas® brand name) coating onto bare fiber (dip-coated) yielded promising results although response times were too long (over 30 min.) for this application. (Figure ___). We initially investigated a porous metallic coating (either aluminum or silver) but were unable to develop a technique to apply the coating uniformly. We were successful in sputter-coating bare fiber with carbon to develop a 10, 14, and 16 nanometer-thick coating. This approach yielded a fast response time (within 1 second) and spectra that could be used to differentiate between an alkane within the WBM. These results are best summarized in Figure ___ below that presents the spectrum. This result shown for the 10nm coating was conducted by first acquiring the spectrum for clean WBM as background (blue), consisting of 5 g of barite and 0.2 g xanthan gum in 200ml de-ionized Water. Xanthan gum is a viscosifier that we find helps to keep the hydrocarbon droplets suspended in the WBM during measurements. The subsequent spectrum (orange) for the WBM containing 16.7% by vol of heptane that was agitated to form suspended droplets prior to measurement. The spectrum represents a change to the background upon introducing the heptane that diffused through the coated sensing region and absorbs energy from the transmitted NIR light (Evanescent wave). We also investigated a backscatter approach (FIGURES___) and use of Raman shift (Figures___) that yielded promising results. FUTURE WORK: a)investigate methods of implementing this and the Evanescent wave approach into a small-scale laboratory model; b)adapt a sapphire crystal cell in future tests that will allow us to pressurize and heat the mud while interrogating it with a laser through the sapphire wall.