Rheological Characterization of Oil-Based Drilling Fluids – Effect on Barite Sag

Rheology is one of the key drilling fluid properties for controlling sag. Yet, the current understanding of the sag-rheology relationship is limited to a few general guidelines for reducing sag in the field. This paper reviews current understanding of sag and describes the initial stage of work aimed at finding a correlation between fluid rheology and sag performance under laboratory conditions. INTRODUCTION In oilfield terminology, “sag” describes the settling of weight material, which results in significant drilling fluid density variations seen at the flow line. Barite sag is usually observed when circulating the fluid out of the hole after the fluid column has been static for some time. Barite sag can occur over a relatively wide fluid density range, 1400-2400 kg/m, and can lead to density variations as high as 480 kg/m. Sag can occur in both oil-based and water-based drilling fluids, but it is experienced more often in oil-based fluids. Occurrence of sag can lead to potential drilling complications such as well-control problems, lost circulation, induced wellbore instability, and stuck pipe. Sag occurs through dynamic and/or static settling. In an inclined hole, sag may result in the slumping of barite bed. The fact that the density variations are most commonly seen after static periods had previously led to the belief that static settling was the main mechanism for barite sag. However, flow loop tests and field experience have shown that while some static settling may occur, it is less likely to produce the large scale density variations seen frequently at the flow line. Hanson et al. and Jefferson emphasised the potential for dynamic sag occurring while circulating the drilling fluid, and observed that prevention of dynamic sag is more difficult than static sag. The overall potential for barite sag is highest when the drilling fluid experiences low shear rates. Flow loop data and field observations suggest that severe sag (>120 kg/m) occurs under the combined influence of low viscosity and low annular velocity. Well-control and hydraulics considerations often require the drilling fluid pressure to exceed formation pore pressure (to prevent the influx of formation fluids) and to be less than the fracture gradient (to avoid fracturing of formation and drilling fluid losses). These requirements place operating windows on flow rate and/or fluid rheology that may actually create conditions that promote barite sag. Sag may be particularly problematic in extended reach wells where the margin between pore pressure and fracture gradient is small. In flow loop tests, Bern et al. and Dye and Greg identified a critical mean annular velocity of 100 ft/min above which barite bed formation was minimised. Accelerated settling can occur in an inclined wellbore through the well-known Rheological Characterization of Oil-Based Drilling Fluids – Effect on Barite Sag Ahmadi Tehrani, John Chapman and Alison Fraser M-I Fluids UK, Research & Technology Centre, Ashleigh House, 1 Abbotswell Road, Aberdeen AB12 3AD, U.K. Boycott effect. This can lead to “slumping” which is the sliding downward of a bed of solids deposited on the lower side of an inclined wellbore. For well angles 50-80, which are subjected to most of the sag problems, low flow rates will make low density fluid accelerate upwards while the high density fluid is forced downward along the low side of the well creating a slumping barite bed. The barite bed may be disturbed by high annular velocities and with drillpipe rotation. Flow loop tests and field experience shows that sag is worst when drillpipe is stationary. Barite sag is related both to the drilling fluid properties and the drilling conditions and practices indicated above. The fluid properties affecting sag include rheology, solids content, particle-size distribution (PSD), and the chemistry of the fluid system. It is a commonly held view that the lowshear-rate (LSR) rheology of the fluid and its linear viscoelastic properties affect the sag performance of the fluid. An increase in the LSR rheology is thought to be beneficial for mitigating dynamic sag. The LSR rheology has been variously defined by the low-shearrate viscosity (LSRV), yield stress or gel strength. The shear rate to which LSRV should correspond can typically be thought of as the shear rate created by the particle as it settles under gravity in an otherwise quiescent fluid. This may be estimated from Stokes’ Law: