The Huabei Oilfield Company operates several oilfields in northeastern China including the Cha He Ji field which is made up of low porosity, low permeability, stacked shaly sands. All wells in the area are produced using rod pumps, and many produce with a high water cut. The usual saturation monitoring techniques such as carbon-oxygen and sigma logging have been unsuccessful due to the low porosity, high clay content of the sands and very fresh formation water. The field has been under waterflood for years with variable injection-water salinity further complicating the reservoir monitoring efforts. It is known that large volumes of bypassed oil remain in the field, but a reliable and cost effective reservoir monitoring technique to identify remaining oil has proven illusive until now.

There is a limited amount of open hole log data in most wells, and the data which exists is not of sufficient quality to derive accurate porosity and clay volume in the lower porosity sands, resulting in bypassed pay zones. A fit for purpose suite of cased hole logs consisting of cased hole formation resistivity and epithermal neutron porosity, combined with a fast wellsite interpretation product has proven to be very effective. The resistivity-based saturation calculation is complicated by the variable water salinity caused by the waterflood. Sensitivity analysis has been performed to estimate the accuracy of the calculated fluid saturation, and a method for selecting a suitable formation water salinity value has been proposed.

Over 40 wells have been logged in this and other fields operated by the Huabei Oilfield Company using this technique, with a lot of success at identifying and producing bypassed oil zones. Accessing bypassed, lower quality, hydrocarbon zones is a cost effective method of increasing the productivity of mature fields.

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