Case Study: Downhole Fluid Analysis and Sampling in <1 h from Low-Resistivity, Tight Acacus Sand

Saturn 3D radial probe quickly extracts high-purity reservoir fluid from thin layers with mobility as low as 0.04 mD/cP

Challenge: Efficiently extract fluid and conduct downhole fluid analysis (DFA) to provide critical information for determining saturations in laminations that exhibit abnormally low resistivity responses because they have been extensively diagenetically altered.

Solution: Use the Saturn 3D radial probe with its self-sealing drain assembly to quickly and reliably access individual low-mobility thin layers and induce and sustain flow for pressure testing, DFA, and fluid sampling.

Result: Efficiently and unambiguously differentiated oil-bearing zones from water-wet intervals in a rugose borehole through quantified DFA in less than 60 min for each station.

Low resistivity prevents saturation determination

OMV needed more information to correctly model saturations in the Acacus Formation in Tunisia. This Upper Silurian formation is the primary producing oil and gas reservoir in the Ghadames basin, but the interbedded sand and shale layers have been extensively subjected to diagenesis, including chloritization, siderite cementation, and quartz overgrowths, which have altered the routine porosity-permeability relationship for the clastic deposits. Previous attempts to use a wireline formation tester to identify thin layers with good permeability for fluid sampling have required long operating times and have not been successful. A conventional large-diameter single probe does not seal well in the typically rugose boreholes and generally is unable to sample fluids from the low-mobility layers. OMV wanted a more efficient, effective way to extract reservoir fluid and conduct DFA for the accurate determination of fluid saturations in the Acacus Formation.

3D radial probe easily extracts reservoir fluid for DFA

The Saturn 3D radial probe features four self-sealing elliptical ports with the industry’s largest surface flow area, totaling more than 79 in2. The drain assembly reliably seals to the circumference of the wellbore to quickly create true 3D flow around the borehole—even in very low-permeability, low-mobility formations—for pressure measurements and fluid sampling. The resulting quick cleanup to representative reservoir fluid is verified by DFA using the LFA live fluid analyzer to identify and differentiate hydrocarbon types and confirm single-phase validity for sample collection. The probe design also minimizes storage volume effects to not bias measurements.

DFA and fluid sampling completed in 0.04-mD/cP mobility in less than 1 h

The Saturn 3D radial probe performed flawlessly for OMV in the low-permeability laminated sands. The average time to establish a seal for the drain assembly was less than 11 min, DFA was completed in 15–45 min, and samples collected in only 60 min, including DFA, in mobilities as low as 0.04 mD/cP. With the oil- and water-bearing intervals now identified, completion designs can be precisely targeted.

Download: Downhole Fluid Analysis and Sampling in <1 h from Low-Resistivity, Tight Acacus Sand (0.26 MB PDF)

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Fluid Flow and Pressure Data Where Not Previously Possible

Saturn 3D Radial Probe
The self-sealing Saturn 3D radial probe flows fluid circumferentially from the reservoir to extend formation testing to low permeabilities, heavy oil, unconsolidated reservoirs, near-critical fluids, and rugose boreholes.
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