A simplified surge and swab pressure model for yield power law fluids

Surge and swab pressures can be generated in different stages of well construction including tripping operations and reciprocation of drillstring in the wellbore. Significant surge and swab pressures can lead to a number of costly drilling problems such as lost circulation, formation fracture, fluid influx, kicks, and blowouts. This phenomenon is of economic importance for the oil industry.Theoretical and field studies indicate that pressure surges strongly depend on drillpipe tripping speeds, wellbore geometry, flow regime, fluid rheology, and whether the pipe is open or closed. Although a large number of studies were conducted in the past to investigate surge and swab pressures, experiments under controlled laboratory conditions have never been reported. This paper presents results of an experimental study aimed at investigating the effects of pipe speed (i.e., tripping speed), fluid properties and borehole geometry on surge and swab pressures under laboratory conditions. Other phenomena such as fluid gelling and pipe eccentricity effects were also examined. Experiments were performed in a test setup that has the capability of varying the tripping speed and accurately measure the surge or swab pressures. The setup consists of fully transparent polycarbonate tubing and inner steel pipe, which moves axially using a speed-controlled hoisting system. Experiments were conducted using mineral oil and polymeric fluids.A new regression model has been developed to calculate surge and swab pressures under steady-state flow conditions. The model is based on the results of approximate numerical solutions obtained by considering the annulus as a narrow slot. Model predictions were compared with experimental measurements and predictions of existing models. A satisfactory agreement has been obtained. Experimental results and model predictions confirm that the trip speed, fluid rheology, annular clearance and pipe eccentricity significantly affect the surge and swab pressures.

► The new correlation provides accurate surge pressure predictions for YPL fluids.
► Surge pressure is substantially affected by the diameter ratio, tripping speed and fluid rheological properties.
► Pipe eccentricity can reduce significantly the value of surge and swab pressures.
► The effect of gelling on surge pressure is negligible with XG based fluids.