In-Situ Bubblepoint Measurement by Optical Spectroscopy
Authors: Gisolf A. , Dubost F. X. , Dumont H. , Achourov V. , Daniele N. , Anselmino A. , Crottini A. , Aarseth N. A. , Fjeld P. H. , Molla S. 
Published: October 2023

Representative fluid properties are required for a wide range of field life aspects such as initial sizing of reservoir hydrocarbon reserves and production planning. Fluid properties are routinely obtained from laboratory sample analysis, but some fluid properties can also be measured in situ with formation testers. A new downhole bubblepoint technique has been developed to supplement traditional downhole fluid analysis (DFA) measurements. Bubble-initiation pressure is measured on reservoir fluids enabling early estimations and sample representativity.

The method outlined consists of two parts—bubble generation and bubblepoint-pressure detection. After the isolation of a volume of contamination-free fluid in the fluid analyzer module of a formation tester, a downhole pump is used to reduce flowline pressure at a low and precise flow rate. Bubble initiation is detected using optical spectroscopy measurements made at a 128-ms data sampling rate. Even very small bubbles scatter visible and near-infrared light directed through the flowline, ensuring that the initiation of bubbles is detected. Flowline decompression experiments are performed in minutes, at any time, and on a range of downhole fluids.

Downhole bubblepoint pressure measurements were made on four different fluids. The gas/oil ratio (GOR) of the tested fluids ranged from 90 m3/m3 to 250 m3/m3. In each case, the downhole bubblepoint obtained from the flowline decompression experiment matched the saturation determined by constant composition expansion (CCE) in the laboratory to within 350 kPa. We observed that bubble initiation is first detected using near-infrared spectroscopy. As the pressure drops, gas bubbles coming out of the solution increase in size, and the bubble presence becomes identifiable on other downhole sensors such as the live fluid density and fluorescence, where it manifests as signal scattering. For each of the investigated fluids, pressure and density measurements acquired while the flowline pressure is above saturation pressure are also used to compute compressibility as a function of pressure.

This downhole bubblepoint pressure measurement allows optimization of real-time sampling operations, enables fluid grading and compartmentalization studies, and can be used for an early elaboration of a fluid equation-of-state (EOS) model. The technique is suitable for black oils and volatile oils. For heavy oil with very low gas content, the accuracy of this technique may be reduced because of the energy required to overcome the nucleation barrier.

Prior documented techniques often inferred downhole bubblepoints from the analysis of the rate of change of flowline pressure. Direct detection of the onset of gas bubble appearance without requiring additional dedicated downhole equipment and validated against laboratory measurements is shown for the first time. The measurement accuracy is enabled by the combination of 128-ms optical spectroscopy with low and accurate decompression rates.