1. A method of performing an oilfield operation of an oilfield having at least one wellsite, each wellsite having a wellbore penetrating a subterranean formation for extracting fluid from an underground reservoir therein, the method comprising: obtaining a plurality of real-time parameters from a plurality of sensors disposed about the oilfield, the oilfield receiving command signals from a surface unit to perform the oilfield operation, wherein the plurality of real-time parameters comprise real-time flow rate data and real-time pressure data of the wellbore; identifying a simulation session marked by identified transients in the plurality of real-time parameters, wherein the identified transients are identified using wavelet decomposition; identifying, within the simulation session, a first time period during which the real-time flow rate data is not available and a second time period during which the real-time pressure data is not available; obtaining offline flow rate data for the first time period by performing flow rate re-construction based on at least one selected from a group consisting of tubing head pressure measurement and bottom hole pressure measurement; obtaining offline pressure data for the second time period based on historical data and spot measurement; configuring, in the surface unit, a gridless analytical simulator for the simulation session to simulate the underground reservoir based on the plurality of real-time parameters supplemented by the offline flow rate data during the first time period and the offline pressure data during the second time period; generating real-time simulation results of the underground reservoir and the at least one wellsite in real-time using the gridless analytical simulator; and sending, from the surface unit, the command signals to the oilfield to manage the underground reservoir in real time by performing the oilfield operation, wherein the command signals are based on the real-time simulation results.