Review of Deliverability Models Used in Geothermal Reservoir Simulations

Production wells on deliverability are often used in modelling geothermal fields, especially for simulations of future scenarios. The mass flow rate is calculated by multiplying the productivity index by mobility and the difference between flowing bottom-hole pressure and the reservoir block pressure. Here three options are considered for calculating the bottom-hole pressure: (i) constant, (ii) dependent on enthalpy, and (iii) dependent on enthalpy and flow rate. To implement Models (ii) and (iii) a wellbore simulator is used to generate the functional dependence of the bottom-hole pressure on enthalpy and enthalpy and flow rate, respectively. The standard method for implementing Model (iii) is to use a table look-up for the bottom-hole pressure but we have also used a 2D rational polynomial representation. All three methods are tested on future scenario simulations of the Wairakei-Tauhara geothermal system. It is found that the most complex method (iii) gives the least conservative results i.e. the most optimistic predictions of future production.