Saturation Evaluation with RSTPro Tool Finds Bypassed Oil and Gas Zones | SLB
Case Study

Challenge: Identify bypassed hydrocarbons in a North Sea well shut in 6 years earlier because of high water cut and low pressure

Solution: Run the RSTPro reservoir saturation tool to evaluate saturations and determine depths for shutting off existing perforations and reperforating

Results: Identified three intervals for reperforating that are producing 27.5-MMcf/d gas, 997-bbl/d oil, and only 1.7% water

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Saturation Evaluation with RSTPro Tool Finds Bypassed Oil and Gas Zones

Sigma and C/O logging finds 27.5 MMcf/d of gas and 997 bbl/d of oil in a North Sea well previously shut in for high water cut

Looking for bypassed hydrocarbons

An operator wanted to identify any bypassed hydrocarbons in a North Sea well that had been shut in because of high water cut and low pressure. The plan was to shut off the water-flushed producing zones and recomplete the new zones.

Measuring water and hydrocarbon saturations

The innovative dual-detector spectrometry system of the through-tubing RSTPro reservoir saturation tool records carbon and oxygen and Dual-Burst thermal decay time measurements in the same pass. The C/O ratio is used to determine the formation oil saturation independent of the formation water salinity. The sigma log of the thermal neutron capture cross section is used to determine water saturation behind casing. Porosity is also measured for formation evaluation.

Identifying new gas and oil zones

The well was logged with the RSTPro tool under shut-in conditions. Schlumberger petrotechnical experts conducted an integrated evaluation of the data. Formation sigma and related gas indicators were used to identify gas-bearing zones, and the C/O data were used to characterize the oil- and water-bearing zones. The source of the water was confirmed, and new potential oil and gas reservoirs were identified.

On the basis of the petrotechnical evaluation, the operator set a bridge plug to isolate the water-bearing zone and perforated three intervals using a 2 7/8-in PURE perforating system for clean perforations with deep penetrating PowerJet Omega shaped charges to perforate through any formation damage and create clean perforation tunnels.

Post intervention, the three new intervals were producing 27.5-MMcf/d gas, 997-bbl/d oil, and only 1.7% water.

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In sigma mode, the RST tool runs an optimized Dual-Burst timing sequence and measures the time decay of the population of capture gamma rays after each burst. The time decay of late-capture (long-burst) gamma rays is dominated by the macroscopic sigma of the formation and is used to deduce water saturation if the water salinity is known and sufficiently high. The time decay of early-capture (short-burst) gamma rays is primarily sensitive to borehole salinity. The ratio of near- to far-detector capture count rates (TRAT) is used to compute the thermal neutron formation porosity. The ratio of far- to near-detector burst-on count rates (IRAT) provides a gas indicator that is independent of thermal neutron absorbers.
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Three intervals were perforated where expert petrotechnical evaluation of the RSTPro tool’s measurements indicated oil and gas. Post intervention, water production had significantly declined to 1.7%.
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