| Abstract |
The use of aerated fluids as the drilling medium for geothermal wells is one of the more successful techniques for overcoming drilling problems and improving production. The principal objective in utilising these fluids is to lower the density of the drilling fluid in order to reduce pressures in the annulus during drilling so that they "balance" formation pressures at potential loss zones. Although straight air drilling has been employed to drill dry steam reservoirs for some time, the use of denser fluids, such as aerated muds, to drill in higher pressure reservoirs, has recently received more attention. Unlike straight air drilling which is normally carried out in an underbalanced mode, accurate prediction of the annular flowing pressure profile is of considerable importance when using lightly aerated fluids, to ensure that downhole differential pressures are minimised and substantial losses or gainsdo not occur. Determinationof the annular flowing pressure profile (and therefore any potential pressure differential) is extremely difficult due to the compressibilityof air and the complex two phase flow which occurs in various sections of a well. Computer modelling of these profiles using various scenarios of liquid and air flowrates and annular backpressures, both prior to and during drilling operations, can assist operators in preventing any major over- or under-pressures, thereby ensuring adequate return circulation, minimising well contamination and reducing the likelihood of the drillstring becoming stuck. This paper reviews the principles associated with using aerated fluids in geothermal reservoirs, it discusses the fluid dynamics associated with predicting the pressure profiles and it examines a number of scenariosevaluated by a field proven computer model. |