System and method for measuring downhole mud flow density

A system for determining a density of a drilling fluid in a wellbore, comprising:

• a drill string in the wellbore, the drill string comprising a downhole rotor;
• the drilling fluid flowing through the drill string in the wellbore causing the downhole rotor to rotate relative to the drill string;
• a brake configured to apply a brake torque to the downhole rotor;
• a torque measurement system configured to measure the brake torque;
• and
• a processor configured to:
• control the brake to apply a first brake torque to the downhole rotor to place the downhole rotor in a first state having a first rotor speed;
• measure the applied first brake torque;
• control the brake to apply a second brake torque to the downhole rotor to place the downhole rotor in a second state having a second rotor speed;
• measure the second brake torque;
• and
• determine the density of the drilling fluid from a difference between the first brake torque and the second brake torque.

A method of determining a density of a drilling fluid in a wellbore, comprising:

• conveying a drill string into the wellbore, the drill string comprising a downhole rotor;
• flowing the drilling fluid through the drill string in the wellbore causing the downhole rotor to rotate relative to the drill string;
• applying a first brake torque to the downhole rotor to place the downhole rotor in a first state;
• determining the applied first brake torque;
• measuring a first rotor speed of the downhole rotor having the drilling fluid flowing therethrough with the downhole rotor in the first state;
• and
• determining the density of the drilling fluid from the measured first rotor speed and the determined applied first brake torque.