The objective of this study was to characterize formation water resistivity (Rw) by validating water sample analyses, calibrating log derived Rw and mapping variations areal and vertically in order to reduce uncertainties regarding this property when determining water saturation (Sw) in the Tambaredjo oil field. Current calculations issued high Sw which originated low STOIIP and high actual oil recovery did not synchronize with the production of the field.

A database with analyzed water sample and log-derived (Dielectric and Conventional logs) Rw data was created. Validation of the sample data was conducted by a Water Sample Analysis (WSA) tool, where the ionic balanced results were used. The log-derived Rw were normalized to the surface temperature and calibrated with the water sample analyzed Rw. To identify the initial Rw, groups of several wells (selected from the current produced water salinity maps) from total Tambaredjo Central Area (TCA) were made. With these initial Rw values the areal and reservoir distribution maps were determined and used for calculating the water saturation in the wells.

Creating the WSA tool was very important to determine which samples were ionic balanced and were useful for further steps. These samples delivered different salinities which could clarify the water sources for the produced sand intervals. Three main groups of water sources were established based on the salinity; S-sands, T-sands and Cretaceous. Based on this classification, the log derived Rw were grouped and the salinities derived from the logs were compared with the produced water salinities retrieved from the Production Data Base. The log derived salinity that did not match the produced water salinity, the intervals from these logs were checked, re-selected and the salinity calculations were updated. Based on produced water salinity maps, well groups from the total TCA were created, comparing latest produced water salinity with initial ones. Assumptions were made of possible communications of water sources due to salinity values, where initial Rw did not match with the latest produced Rw. An initial Rw map was created and variable Rw proposed, instead of using a constant Rw for reservoir intervals of Tambaredjo oil field.

Sw improves in all T-unit reservoirs of TCA from 41.9 to 40.4%, mainly in T1 and T2 sands (36.7% to 33.4%). In the T3 interval, Sw increases from 52.3 to 54.5% as formation water was found to be slightly fresher than originally estimated.

Regarding stock tank oil initially in place (STOIIP) in TCA, with the variable Rw, it was estimated 265 MMSTB considering only T1 and T2 intervals. This is 17 MMSTB more than using the average value (Rw = 1.18ohm.m @ 67°F). The actual recovery is 14.2% and 14.9%, respectively, indicating that with the variable Rw, the actual recovery is closer to the expected primary recovery for this of field (10 to 13%) according to Ambastha (2008).

Formation water salinity is a very important input in Sw equations. When geological features like top erosions, channel stacking or leaking seals are not tracked, and it is assumed that formation water resistivity is constant, water saturation calculations might not match production performances. This research proposes a methodology to identify sources of formation water mixture determining if a well was at initial reservoir conditions when it was logged, establishing Rw by areas.

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