A New Optical Sensor Configuration Enables First Time Use of the Mid-Infrared Optical Wavelength Region for Chemical Analysis During Formation Tester Logging Operations
Authors: Piazza Ralph , Vieira Alexandre , Sacorague Luiz Alexandre , Jones Christopher , Dai Bin , Price Jimmy , Pearl Megan , Aguiar Helen 
Published: September 2019

Abstract
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.