Downhole Heater Assembly and Power Line Communications System

Abstract

A hydrocarbon heating system comprises a surface control panel and a subsurface heating system, connected by an external power cable which also serves as the data communication link between the two parts. The subsurface heating system includes a heating element and two separate, thermally isolated temperature sensors. One thermal sensor, mounted near the heating element, monitors the temperature of the heating element. Another thermal sensor monitors the ambient temperature of fluid in the borehole. The temperature of the heating element may be varied to maintain an optimal well temperature for the production of hydrocarbons. Downhole electronics convert the thermal sensor outputs to digital data, and modulate the digital data onto a power cable that carries power from the surface to the heating element. A control system at the surface controls the heating element via nested control loops monitoring the temperatures of the borehole fluid and the heating element.

What is claimed is: 
     
         1 . A downhole hydrocarbon heating system operative to heat hydrocarbon fluids in an oil or gas well comprising a borehole, the system comprising:
 a surface control panel;   a subsurface heating system operative to be deployed down the borehole and including
 a heating element; 
 a first thermal sensor operative to sense the temperature of the heating element; and 
 a second thermal sensor operative to sense the ambient temperature of fluid in the borehole; and 
   a power cable connecting the surface control panel and the subsurface heating system, the power cable operative to selectively carry power from the surface control panel to the heating element, and further operative to carry temperature data from the first and second thermal sensors to the surface control panel.   
     
     
         2 . The system of  claim 1  wherein the heating element and the first thermal sensor are disposed in a lower housing operative to be deployed in the borehole at the depth of a hydrocarbon production zone. 
     
     
         3 . The system of  claim 1  wherein the heating element is disposed centrally in the lower housing and secured by a thermally conductive concrete. 
     
     
         4 . The system of  claim 1  wherein the lower housing comprises standoff elements secured to the exterior thereof in radially spaced positions, the standoff elements operative to maintain the lower housing generally centered in the borehole. 
     
     
         5 . The system of  claim 1  wherein the second thermal sensor is disposed in a spaced relationship with the heating element such that the temperature sensed by the second thermal sensor is not directly influenced by the heating element. 
     
     
         6 . The system of  claim 5  wherein the subsurface heating system further comprises an insulating section interposed between the heating element and the second thermal sensor. 
     
     
         7 . The system of  claim 1  wherein the subsurface heating system further comprises a downhole printed circuit board (PCB) containing electronics to convert the first and second thermal sensor outputs to digital data, and to modulate the digital temperature data onto the power cable. 
     
     
         8 . The system of  claim 7  further comprising a transformer operative to tap power from the power cable and to power the downhole PCB. 
     
     
         9 . The system of  claim 7  wherein power cable carries 3-phase power, and wherein the electronics on the downhole PCB are operative to modulate digital data across two phases of the power cable. 
     
     
         10 . The system of  claim 1  wherein the subsurface heating system is deployed below production tubing operative to extract hydrocarbons from the well. 
     
     
         11 . The system of  claim 1  wherein the surface control panel comprises:
 a power line controller operative to receive sensed temperature data from the power cable; and 
 a temperature controller operative to control power to the heating element by selectively connecting the power cable to a power source in response to the sensed temperature data received from the power line controller. 
 
     
     
         12 . The system of  claim 10  wherein the temperature controller is operative to control power to the heating element by:
 in an outer control loop, applying power to the heating element in response to the ambient temperature of fluid in the borehole being below a predetermined lower limit; and 
 in an inner control loop, applying power to the heating element only if the temperature of the heating element is at or below a predetermined setpoint, without regard to the outer control loop. 
 
     
     
         13 . The system of  claim 10  wherein the temperature controller is operative to control power to the heating element by periodically applying power to the heating element for brief durations without regard to the ambient fluid or heating element temperature values. 
     
     
         14 . A method of controlling the temperature of hydrocarbons in a well comprising a borehole, in which is disposed a subsurface heating system including a heating element, a first thermal sensor operative to sense the temperature of the heating element, and a second thermal sensor operative to sense the ambient temperature of fluid in the borehole, the method comprising:
 monitoring the temperature of the heating element and the ambient temperature of fluid in the borehole;   applying power to the heating element in response to the sensed ambient temperature of fluid in the borehole being below a lower limit, and the sensed temperature of the heating element being below a setpoint; and   removing power from the heating element in response to the sensed ambient temperature of fluid in the borehole being above an upper limit or between the lower and upper limit, or the sensed temperature of the heating element being above the setpoint.   
     
     
         15 . The method of  claim 11 , further comprising
 periodically applying power to the heating element for brief durations without regard to the values sensed by the first and second thermal sensors.   
     
     
         16 . The method of  claim 12  wherein periodically applying power to the heating element for brief durations comprises applying power to the heating element for two seconds every thirty seconds.