Abstract
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 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 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 64-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 wide range of downhole fluids.
Downhole bubblepoint pressure measurements were made on four different fluids, all from different reservoirs and regions. The gas-oil ratio of the tested fluids ranged from 500 to 1,500 scf/bbl. In each case, the downhole bubblepoint obtained from the flowline decompression experiment matched the saturation determined by constant composition expansion in the laboratory to within 50 psi. We observed that bubble initiation is first detected using near-infrared spectroscopy. As pressure drops, gas bubbles coming out of solution will 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 optimizing real-time sampling operations, enables fluid grading and compartmentalization studies, and can be used for an early elaboration of a fluid equation of state model. The technique is well-suited for black oils and volatile oils. For heavy oil with very low gas content, the accuracy of this technique may be reduced due to the energy required to overcome the nucleation barrier.
Prior documented techniques often inferred downhole bubblepoints from analysis of the rate of change of flowline pressure. Direct precise detection of the onset of gas bubble appearance without the need to divert fluid flow is shown for the first time on a wide range of fluids. The measurement accuracy is enabled by the combination of 64-ms optical spectroscopy with low and accurate decompression rates.
