Passive production logging instrument using heat and distributed acoustic sensing

A system to measure fluid flow in a wellbore, the system comprising:

• a probe including at least a heater;
• a fiber optic cable connected to the probe;
• and
• electronic computer hardware with software programmed to perform operations when the probe is lowered into a wellbore, the operations comprising:
• changing an output of the heater to thereby change a temperature of drilling fluid moving over the fiber optic cable;
• measuring a strain on the fiber optic cable caused by changing the temperature of the drilling fluid to yield a measured strain;
• preliminarily determining a velocity of the drilling fluid from the measured strain to yield a preliminarily determined velocity;
• measuring at least a second parameter of the drilling fluid to yield at least a measured second parameter;
• adjusting the preliminary determined velocity based on the measured at least a second parameter to yield an adjusted velocity;
• and
• determining a flow rate of the drilling fluid based on the adjusted velocity.