2.6.1 Introduction

The temperature in the earth's undisturbed crust continuously increases with depth and is characterized by the geothermal gradient. This also holds for a well shut-in for a sufficiently long period. However, as soon as the well starts to produce fluids from an underground reservoir, well temperature continuously changes until a steady-state condition is reached. This is because hot fluids flowing to the well bottom and rising in the well progressively heat up the well's surroundings. Temperature distribution in producing wells, therefore, will always be affected by the factors governing the heat transfer process.

Temperature distribution in and around a producing well is schematically depicted in Figure 2-59, which shows flowing temperatures in the function of the radial distance from the well's centerline at an arbitrary depth. During shut-in, fluid in the tubing string cools down to the geothermal temperature T _r and a uniform temperature profile is observed. When the well starts to produce, the rock at the wall of the wellbore is still at the geothermal temperature and well fluids lose a considerable amount of their heat content. Therefore, at startup conditions, fluid temperature is much less than the inflow temperature of the given pipe section. As seen from the figure, most of the temperature drop occurs across the annulus (usually filled up with water) and the cement sheath. Due to the steel's high thermal conductivity, temperature losses across the tubing and the casing are usually negligible.

Figure 2-59: Radial temperature distribution in...