High-Quality Pressure Data, Sample Collection, and Formation Testing in Wide Permeability Range

Published: 07/17/2014

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Proving a low-permeability potential reservoir

Although oil columns had been found in the overlying Middle Jurassic and underlying Triassic beds in a deepwater field offshore Norway, the potential reservoir in the Lower Jurassic section consisted of unexpectedly poorer quality, low-permeability rock. Statoil needed to know the zonal contributions, estimated horizontal and vertical permeabilities, and reservoir pressure along with the collection of fluid samples for laboratory analysis to fully evaluate the Lower Jurassic formations.

Circumferentially flowing fluid in low-mobility rock

Even in very low-permeability formations, the Saturn 3D radial probe readily initiates and sustains true 3D circumferential flow around the borehole. Unlike conventional single probes that funnel fluid from the reservoir to one extraction point, the Saturn probe features four self-sealing elliptical ports with the industry's largest surface flow area of 79.44 in2. The result is quicker cleanup times for fluid sampling and the efficient performance of pressure measurements, especially in low-mobility formations where conventional probes cannot function. The design of the Saturn probe also minimizes storage volume effects on testing.

Graph: High-Quality Pressure Data, Sample Collection, and Formation Testing in Wide Permeability Range
The Saturn 3D radial probe flowed fluid for collecting three samples from a zone with a mobility of only 0.6 mD/cP, reducing the contamination to a low 5 wt% after only 6 h of pumping. Drawdown was 35 bar during cleanup at 12 cm3/s and 9 bar during sampling at 3 cm3/s. A conventional probe used in a zone 5 m higher in the same formation with 4 times the mobility required almost twice the drawdown and still had 17-wt% oil-base mud (OBM) filtrate contamination.

Acquiring low-contamination samples and valid testing data

Following an initial run of the MDT modular formation dynamics tester employing conventional probes, the lower mobility reservoir sections were investigated using the MDT tester in combination with the Saturn 3D radial probe. Sampling efficiency was greatly improved by using the Saturn probe to collect three oil samples and six water samples at mobilities as low as 0.3 mD/cP. For example, the conventional probe collected a hydrocarbon sample in a zone with 2.4-mD/cP mobility after 5 h of pumping that contained 17-wt% OBM filtrate. When the Saturn probe extracted fluid from 5 m lower in the same formation at 0.6 mD/cP, the drawdown was about half of that from the single- probe sampling and after only 6 h of pumping the contamination had been reduced to 5 wt%.

The toolstring also incorporated an observation probe at a 1.23-m interval from the Saturn probe for conducting vertical interference tests to evaluate permeability and permeability anisotropy and to estimate the flow potential. All four vertical interference tests returned valid, interpretable reservoir responses at the probes, with particularly good data acquired in a zone with 120-mD/cP mobility. High-quality pretest pressure measurements were made at all Saturn probe stations, in mobility as low as 0.3 mD/cP.

Graph:High-Quality Pressure Data, Sample Collection, and Formation Testing in Wide Permeability Range
A pressure transient test with vertical interference monitoring shows well-developed pressure transient responses at both the Saturn probe and the observation probe. The buildup mobility interpretation yields a mobility of 120 mD/cP, for which a dual-packer configuration might be challenged to create sufficient drawdown for conducting a valid test.
North Sea, Norway, Europe, Offshore

Challenge: Measure pressure data, collect fluid samples, and conduct pressure transient and vertical interference tests to evaluate a low-permeability potential reservoir in the Norwegian Sea.

Solution: Run the Saturn 3D radial probe with its large, circumferential surface flow area for inducing and sustaining flow from both low- and high-mobility formations to enable sampling and formation testing.

Results: Assessed potential productivity by collecting low-contamination hydrocarbon samples and conducting valid tests in a mobility range from 0.3 to 120 mD/cP to evaluate the fluid content and permeability and permeability anisotropy.

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