Novel Approach to Measuring Irreducible Water Salinity in Gas Reservoirs

Abstract

A volume fraction of pores in gas reservoir is occupied by water at irreducible saturation. This volume fraction can be challenging to calculate and it impacts on the reserve calculation and the flow rate of production. Water at irreducible saturation does not flow during fluid sampling and therefore cannot be investigated. Interpretation of wireline logs such as electrical resistivity or spontaneous potential measurements are classicly used to derive water saturation but requires knowledge of the salinity of the irreducible water. Salinity of the aquifer below the gas-water contact may be used however, samples are frequently contaminated and errors may be introduced if the salinity evolved since gas emplacement. Drainage capillary pressure measurements on core plugs provide can provide water saturation data, however core samples may be inappropriate or the measurements unreliable. To provide additional constraints on salinity, we propose an alternative new laboratory technique using laser Raman that directly measures the salinity of irreducible water trapped in mineral microcavities known as fluid inclusions. The liquid water spectrum is strongly influenced by dissolved anions (Cl− being the most dominant) and quantification of this overcomes limitations of metastability when using conventional microthermometric techniques. To be robust, the water measured must contain dissolved methane at a concentration in equilibrium with free gas. Through a simple example using a sample from a gas reservoir in Australia, we demonstrate the viability of the technique by presenting a methodology that leads to derivation of a salinity value, methane concentration and a pressure-temperature constraint of entrapment.