Multivariate optical computing (MOC) is a compressed sensing technique with the ability to provide accurate spectroscopic compositional analysis in a variety of different applications to multiple industries. Indeed, recent developments have demonstrated the successful deployment of MOC sensors in downhole/well-logging environments to interrogate the composition of hydrocarbon and other chemical constituents in oil and gas reservoirs. However, new challenges have necessitated sensors that operate at high temperatures and pressures (up to 230°C and 138 MPa) as well as even smaller areas that require the miniaturization of their physical footprint. To this end, this paper details the design, fabrication, and testing of a novel miniature-sized MOC sensor suited for harsh environments. A micrometer-sized optical element provides the active spectroscopic analysis. The resulting MOC sensor is no larger than two standard AAA batteries yet is capable of operating in high temperature and pressure conditions while providing accurate spectroscopic compositional analysis comparable to a laboratory Fourier transform infrared spectrometer.

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This paper presents a new optical sensor configuration using a multivariate optical computation (MOC) platform in order to enhance chemical analysis during formation tester logging operations. The platform allows access up to the mid-infrared (λ ~ 3.5 microns) optical region, which has previously not been accessible for in-situ real-time chemical measurements in a petroleum well environment. The new technique has been used in the field for the analysis of carbon dioxide and synthetic drilling fluid components such as olefins. MOC is a technique that uses an integrated computational sensor to perform an analog dot product regression calculation on spectroscopic data, optically, rather than by electronic digital means. Historically, a dot product regression applied to spectroscopic data requires both a spectrometer and a digital computer in order to perform a chemical analysis. MOC sensors require neither and because the key sensor component, the multivariate optical element (MOE), is a stable temperature robust solid-state element, the technique is well suited for downhole petroleum environments. A new dual-core configuration using two MOEs designed to work in parallel enhances the sensitivity of the measurement enabling a mid-infrared analysis. Spectroscopic measurements were performed on 32 base oils that were reconstituted to reservoir compositions over a wide temperature and pressure range up to 350°F and 20,000 psi for a total of 12 combinations for each base oil. Live gas compositions (i.e. reservoir conditions) were achieved by adding multiple methane, ethane, propane, and carbon dioxide charges to each base fluid. The reconstituted petroleum fluids were further mixed with olefin-based synthetic drilling fluid (SDF). This rigorous experimental design data therefore allowed for solid state MOEs to be designed to operate under the same reservoir conditions. Laboratory validation showed measurement accuracy of +/-0.43 wt% for a range of 0 to 16 wt% CO2 and +/-0.4% from 0 to 10 wt% SDF. A wireline formation tester optical section was modified with the MOC dual-core configuration to enable the mid infrared detection of both carbon dioxide and olefins. This formation tester was then deployed in five wells collecting seven samples from various locations. The downhole SDF and carbon dioxide measurements were subsequently found to be in good agreement with laboratory analysis with field results for valid pumpouts showing an accuracy of 0.5 wt% CO2 and 1.0 wt% olefins cross a range of 1.2 to 22 wt% CO2 and 1.4 to 9.7 wt% SDF. Carbon dioxide is an important component of petroleum whose presence and concentration affects completion options, surface facilities, and flow assurance, which thereby affects operational costs of petroleum production. Olefins are a primary component of synthetic drilling fluid (SDF), although other mid-infrared active components such as esters, ketones, alcohols, and amines also can be present. High concentrations of SDF in openhole formation tester samples lower the quality of laboratory phase behavior analysis and thereby force greater monetary risk in development of assets, especially when conducting reservoir production simulations. Therefore, it is important to monitor both components during formation tester sampling operations.

Downhole fluid analysis has the potential to resolve ambiguity in very complex reservoirs. Downhole fluid spectra contain a wealth of information to fingerprint a fluid and help to assess continuity. Commonly, a narrowband spectrometer with limited number of channels is used to acquire optical spectra of downhole fluid. The spectral resolution of this type of spectrometer is low due to limited number of narrowband channels. In this paper, we demonstrate a new type, compressive sensing (CS) based broadband spectrometer that provides accurate and high-resolution spectral measurement. Several specially designed broadband filters are used to simplify the mechanical, electrical, optical, and computational construction of a spectrometer, therefore provides measurement of fluid spectrum with high signal-to-noise ratio, robustness, and a broader spectral range. The compressive sensing spectrometer relies on reconstruction technique to compute the optical spectrum. Based on a large spectral database, containing more than 10000 spectra of various fluids at different temperature and pressure conditions, which were collected using conventional high resolution spectrometer in a lab, the basis functions of the optical spectra of three types of fluids (water, oil and gas/condensate) can be extracted. The reconstruction algorithm first classifies the fluid into one of three fluid types based on multichannel CS spectrometer measurements, the optical spectrum is reconstructed by using linear combination of the basis functions of corresponding fluid type, with weighting coefficients determined by minimizing the difference between calculated detector responses and measured detector responses across multiple optical channels. 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 spectrum covers wavelengths from 500 nm to 3300 nm, which is a wider spectral region than has historically been accessible to formation testers. The expanded wavelength range allows access of the mid-infrared spectral region for which synthetic drilling-fluid components typically have higher optical absorbance. This reconstruction spectra may allow contamination to be directly determined. This paper will discuss the CS optical spectrometer design, fluid classification and spectral reconstruction algorithm. In addition, the applicability of the technique to fluid continuity assessment, sample contamination assessment and digital sampling will also be discussed.

Accurate reservoir fluid identification and sampling of hydrogen sulfide (H2S) contaminated fluids is difficult to achieve due its consumption by the interior of downhole tool surfaces prior to sampling or measurement. For low PPM level concentrations, this fact does not change, despite recent tool advances utilizing NACE compliant materials. Consequently, H2S concentrations are typically under-reported which adversely affects production and presents significant health safety and environment concerns. Historically, only sampling bottles have been coated to preserve H2S concentrations during transit to laboratories with a material that is resistant to H2S reactivity to enable more representative measurements. However, only very recent efforts have transitioned the focus toward successfully coating the interior of the tools. This paper details a state-of-the-art technology, initially developed and heavily leveraged from the semiconductor industry. The technology is adapted to coat the interior surfaces of downhole tools with a chemically resistant dielectric thin film. New developments now provide the benefit of the process being safe, able to be performed at atmospheric pressure and temperature conditions, and portable; thus, allowing the coating process to be deployed to field locations. The method involves atomic layer deposition (ALD) technology to be plumbed in directly to a downhole tool and conformally deposit a thin layer (e.g. < 1 micron) of highly durable H2S-resistant sapphire to the entire interior tool surface. An automated procedure has been developed allowing the versatility to accommodate a number of unique geometries inherent of different formation tester configurations. New advances in Quartz Crystal Microbalance sensors are also realized in-situ to optimize (in real-time) the efficiency of the process and ensure uniform and conformal coverage is obtained in the fastest and safest manner. Laboratory testing on a prototype system demonstrated uniform and conformal coverage of a ~ 500 nm thick sapphire film resistant to flaking and scratching. Accelerated lifetime stress testing demonstrated high durability relative to expected tool life. Testing of coated and uncoated tools show the coating is successful at the 50ppm level H2S for up to 4 days. These results are contrasted with similar tool body samples not coated with the H2S-resistant ALD sapphire and subject to the same H2S conditions. To show the coating’s durability, subsequent experiments flowed mud-based drilling fluid through both the tool body and sample chambers, followed by thorough cleaning and successful repeating the same 50ppm H2S test. Exposure of the sapphire coated tool body and sample chambers to various concentrations of H2S demonstrated zero loss. Ultimately this technique represents a new opportunity to gather representative formation samples containing low concentrations of H2S.