Complementary Core Information

Salinity of Pore Water

It is important to know the salinity, and thus the resistivity, of the formation water present within a reservoir rock, especially to enable accurate water saturation calculations using LWD and wireline well logs. With knowledge about the salinity/resistivity of both the formation water and the coring fluid, measurement of the salinity of the water recovered in the core will indicate the extent of filtrate flushing.

Analysis of the water salinity can be applied only to cores cut with oil-base mud. In core samples acquired with oil-base mud, where usually no extraneous water has been added to the core, the salinity of water in the sample should represent that in the reservoir. Bloys (2011) described low invasion coring techniques.

In situations where the formation has not been previously penetrated by wells and no water production has occurred, the salinity of the formation water may be initially unknown. This salinity can sometimes be estimated from regional studies or calculated from the spontaneous potential (SP) log if a sufficient salinity differential exists between the coring fluid and the in-place formation water. Even in this case, further verification of the calculated salinity is desirable.

A relatively rapid laboratory technique for estimating the water salinity uses a sample of core taken adjacent to those samples used for the Dean-Stark or summation-of-fluids analysis (University of Liverpool, 2014). If it is assumed that the porosity and water saturation present in the adjacent sample are the same as that in the summation or Dean-Stark (Royal Society of Chemistry, 2010) sample previously analyzed, the quantity of water in the salinity sample can be estimated. The sample can then be crushed and washed with a known volume of distilled water, leaching the salt from the sample. This solution can then be titrated to generate a measure of water salinity. This technique is documented in API RP 40 (1998).

Salinity measurements on cores have also been used to estimate the vertical sweep efficiency, an important input into the hydrocarbon recovery efficiency calculation. In those reservoirs where the salinity of the injection water varies sufficiently from that of the formation water, determination of the core salinity on a foot-by-foot basis will establish that portion of the formation that has been contacted by the injected water and, thus, has been swept.

There are practical limitations to making these calculations on low porosity formations that contain moderate water saturations. In such cases, the quantity of water present in the test plug may be quite small and a small error in estimating its volume will generate a large error in the calculated water salinity. Water derived from a drill stem test, or sometimes from a formation tester if conditions are ideal, will generally be more definitive for determining the resistivity of the formation water, .