The Skin Factor as a Rock Mechanics Diagnostic Tool

Abstract

Abstract The paper presents a series of well tests performed on various North Sea weak reservoirs and will show how the skin factors measured can be interpreted in terms of sand production. In particular, the results presented show that sand production results in abnormally low skins independently of what the perforation conditions were in terms of under or over balance. First, a series of tests were performed on injectors after a very limited amount of back production for clean up. Such tests were injection tests followed by clean up and a very good correlation existed between skin factor and under balance conditions. However, after important sand production, the wells were tested again under injection and all showed very low skin factors. Second a series of four production tests were performed on a well shot over balance and the evolution is clearly one of decreasing skin factor indicating sand production. Third, for a given field, all tests performed with a total draw down - i.e. draw down plus depletion - superior to 70 bars show an extremely low skin as the other show high skins. The balance conditions for such wells were very diverse. Fourth, it will be shown how on wells producing sand, the stimulation of the levels that produce sand leads to dramatic flow concentrations from very limited intervals and various PLT results will be presented. The practical consequences of such results will then be presented and three essential aspects will be dealt with. The perforation balance policy will be discussed in the light of the results presented above. The use of well test as a systematic mean to predict sand production will be outlined. Finally, it will be shown that test interpretation techniques ought to be refined to achieve a bigger understanding of the results in terms of description of the near wellbore area. P. 375