Measuring Well Head Temperature

By Nathan Waldman | Thu, 8 Nov 2007

Petroleum engineers need to know flowing well head temperatures (FWHT) in order to determine choke sizes and line heater capacities.  It is also necessary to know WHT in order to calculate gas flow rates through orifice meters and for converting well-head pressures to bottom-hole pressures.  However, in spite of the importance of obtaining WHT's, it is very difficult to obtain accurate measurements in the real world of oil and gas production.  In the simplest case of an onshore well, the flowing WHT is the temperature in the centerline of the tubing at a depth at least 50 feet below the surface.  Temperatures measured radially from the centerline of the tubing string to the inside wall of the tubing have a distinctive parabolic profile with the centerline being hotter than the wall.   As the flowing well stream approaches the well-head it begins to lose heat at a faster rate than experienced at lower depths.  This cooling effect which is present in onshore wells is much greater in offshore wells and is a function of the water depth.  Heat loss from the well stream may be greater between the mud-line and the surface than between the perforations and the mud-line.

In chemical plants and refineries, flowing pipeline temperatures are important and are usually obtained by the use of  thermowells or from pipeline skin temperature.  A thermowell, as the name implies, is a small diameter tube inserted through the pipe wall perpendicular to the pipe axis.  The tube is sealed at the end projecting into the pipe and a thermocouple is inserted into the open end.  Although this a common industrial practice, it is rarely used in oil and gas production because it prevents pigging or wireline operations and it weakens the pipe wall.  Pipeline skin temperature measurements are very easy to obtain and very useful provided the user understands what is being measured and how the measured value is being used.

If the well head temperature measurement is being used to determine design of downstream equipment, the amount of heat loss occurring in the well stream on its journey to the well head is not important.  It is only important to know the temperature of the well stream as it leaves the tree.  However, the engineer must understand that the external skin temperature of the flow line will be significantly cooler than the average temperature of the flow stream.

When the well head temperature is used as an input for the conversion of WHP to BHP, no surface measurement is accurate, whether the flowing or shut-in.  Data Retrieval Corporation, with the help of many of its customers, designed a test program in which temperature recorders were run on wireline to varying depths above and below the mud line while flow rates were varied.   The results of these studies were used to construct models of the flowing well head temperature as a function of flow rate, liquid loading, gas gravity, tubing I.D., T.V.D. and BHT.  These models are used offshore and onshore in DRC's WHP to BHP conversion algorithms.