Measurement of the mass flow rate and enthalpy from two-phase geothermal wells is very important for monitoring individual well performance and for optimum reservoir management. Existing techniques are either expensive (separator and tracer dilution), low accuracy (tracer dilution) or require the geothermal well to be taken out of production (calorimeter and lip pressure methods). The two-phase orifice plate is the most widely examined alternative method and has been implemented in several geothermal fields worldwide. In this work, extensive geothermal field testing data is used to evaluate existing correlations using the concentric sharp-edge orifice plate. A new simplified correlation was then developed with high accuracy for the full range of geothermal reservoir enthalpies (600-2800 kJ/kg) and an analytical model was proposed to predict the pressure drop across the two-phase orifice. Computational fluid dynamics (CFD) simulation using ANSYS Fluent was used to investigate the pressure profiles, velocity and discharge coefficients of two-phase flow of geothermal fluids through the sharp-edge orifice plate. The model results showed good agreement between the CFD simulations, field test data and the newly proposed pressure drop model. CFD modelling also show that using an eccentric orifice plate results in lower pressure drop than the concentric orifice. (C) 2018 Elsevier Ltd. All rights reserved.