Schlumberger

Case Study: Identify Water Entry to Control Unwanted Production

RSTPro Measurements Accurately Detect Water Entry and Flow Behind Casing in Centro Lago Field, Venezuela

Challenge: Identify water entry and flow behind casing in a reservoir with fresh formation water, injection water of various salinities, and 13-pu average porosity. The varying mixed salinity prevented using traditional pulsed neutron tools for reservoir evaluation.

Solution: Run the 2 1/8-in RSTPro reservoir saturation tool through 2 7/8-in tubing to log the well through 7-in casing. The tool’s salinity-independent carbon/oxygen ratio determines saturation values through casing at any water salinity.

Result: Identification of water entry and production points on the RSTPro log guided reperforation, resulting in the well producing 100 bbl/d of oil and no water.

High water cut

Initial oil production from a well in the Centro Lago field of Venezuela was 300 bbl/d from Zones A, B, C, and D. Water injection was introduced to the field in 1972. By 1992 fingering had resulted in increasing water production.

In an attempt to eliminate the water production, the operator cemented all perforations and then reperforated Zones A and B. However, in a few months the water cut had increased to almost 90%.

Salinity-independent reservoir evaluation

The RSTPro tool was run with the PS Platform production services string as a last effort to identify the unwanted water entry. Unlike traditional pulsed neutron logging, the RSTPro tool determines saturation values from salinity-independent measurements, so the varying salinity of the mixed waters was not a factor.

The RSTPro log clearly identified the problem: a shale barrier at X,200 was acting as an impermeable barrier, isolating the top sand (X,160 to X,200 ft) from the bottom sand (X,204 to X,254 ft). Water was entering at the bottom of each sand and being produced through the perforations in Zones B and D.

Water-free production restored

Based on the RSTPro evaluation, Zones A and C were reperforated. Following recompletion, the well produced 100 bbl/d with no water.


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