Investigation of reservoir temperature in a gas reservoir in Middle East: case study

Abstract

Having a good estimation of geothermal gradient and the reservoir temperature has a great impact on the methodological reservoir management in the entire reservoir life, from natural depletion phase up to draining the last portion of hydrocarbon by applying an appropriate EOR method. This issue could become crucial in a gas condensate reservoir management as the reservoir temperature has great influence on the time and amount of precipitated condensate in the reservoir and on the surface. It is usual to consider a constant reservoir temperature throughout the field. Also, encountering a constant geothermal gradient, with a similar fluid in a reservoir, is expected. During drilling campaign of two appraisal wells (one in the crest and other in the flank) of one of Middle East gas reservoir, it is aproximately found one geothermal gas gradient in both well; however, with some displacement in the wells. To find out a scientific justification for this phenomenon, a study was set out and results were presented in this paper. By using the analytical solution to unsteady-state conduction heat transfer for a cylinder with a radius of rm and infinite length and the available temperature data from FBDSTs, the exact reservoir temperature at different depths is calculated. It is also shown that by using the analytical solution ‘affected thermal well bore radius’ can be estimated. The affected thermal well bore radius is defined as the radius that beyond it, formation rock is not affected by any down-hole temperature variation. Based on the elaborated work, in addition to confirmation of having one geothermal gradient in the reservoir, it is found out that the ‘affected thermal well bore radius’ in wells is about 20–60 cm. Also by using the basic steady-state conduction heat transfer equation and assuming that the amount of transferred heat via layers of earth is constant, it is qualitatively shown that having different reservoir temperatures at constant depth throughout a giant deep-reservoir is normal and it should be accounted for the reservoir simulation.