Barite Sag Measurements
Authors: Ofei Titus Ntow , Lund Bjørnar , Saasen Arild , Sangesland Sigbjørn , Linga Harald , Gyland Knud Richard , Kawaji Masahiro 
Published: February 2020

Incidences with sag of solid weighting agents in drilling fluids can lead to potential drilling impediments including; loss of wellbore control, lost circulation, stuck pipe and high torque. The presence of sag has relatively often been the cause for gas kicks and oil-based drilling fluids are known to be more vulnerable for sag than water-based drilling fluids. Separation of weighting materials in a non-moving fluid column is referred to as static sag while sag in a flowing fluid is normally referred to as dynamic sag. An approach to obtain static and dynamic barite sag measurement protocols is presented to examine the effects of rheological and viscoelastic properties of typical field oil-based drilling fluids on barite sag performance. Static sag results are computed based on modified Stokes settling theory while dynamic sag results are compared for rotational and oscillatory ultra-low to low shear conditions. For static sag measurements, an optical scanning analyzer, which is based on the principle of multiple light scattering was used to characterize the stability and particles settling speed of the drilling fluid samples. A cylindrical glass cell containing 20 mL of drilling fluid sample was scanned at the entire height of the sample for 7 days at 1-hour interval to acquire transmission and backscattering data every 40 μm. Under the dynamic sag measurements, a rheometer with a measuring system applying a cup and grooved bob was used to conduct rheological and viscoelastic measurements such as flow curves, oscillatory amplitude sweep, oscillatory frequency sweep, rotational and oscillatory time tests on the drilling fluid samples. The drilling fluid properties were characterized under atmospheric conditions before and after dynamic aging. The aging temperature and pressure conditions in the roller oven were 120°C and 100 psi respectively for a period of 2-1/2 days. A high precision density meter was used to measure the density of the drilling fluid samples before and after each test. Dynamic sag index (DSI) results were compared between time-dependent rotational shear test with shear rates from 10.22 to 0.001 s−1 and time-dependent oscillatory shear test with angular frequencies from 10.22 to 0.001 rad/s at constant strain amplitudes of 0.05% within linear viscoelastic (LVE) range and 100%, over a period of 10,800 s to closely compare to sag-prone conditions during drilling operations. We observed that heat-activated chemicals in the hot-rolled fluid sample increased the viscosity and elasticity which contributed to lower barite sag and longer suspension of particles than before hot rolling. Moreover, rotational shear test increased barite sag in the fluid sample more than oscillatory shear test, particularly for shear rates exceeding 1.0 s−1. Within the LVE range, oscillatory shear test did not disturb the position of the particles enough to promote sag in the fluid samples. The time-dependent oscillatory shear test can provide new insight on the structural character of drilling fluids to predict barite sag tendencies during the fluid design phase.