Barite Sag Measurements

摘要

Incidences with sag of solid weighting agents in drilling fluids can lead to potential drilling impedimentsincluding; loss of wellbore control, lost circulation, stuck pipe and high torque. The presence of sag hasrelatively often been the cause for gas kicks and oil-based drilling fluids are known to be more vulnerablefor sag than water-based drilling fluids. Separation of weighting materials in a non-moving fluid column isreferred to as static sag while sag in a flowing fluid is normally referred to as dynamic sag. An approach toobtain static and dynamic barite sag measurement protocols is presented to examine the effects of rheologicaland viscoelastic properties of typical field oil-based drilling fluids on barite sag performance. Static sagresults are computed based on modified Stokes settling theory while dynamic sag results are compared forrotational and oscillatory ultra-low to low shear conditions. For static sag measurements, an optical scanninganalyzer, which is based on the principle of multiple light scattering was used to characterize the stability andparticles settling speed of the drilling fluid samples. A cylindrical glass cell containing 20 mL of drilling fluidsample was scanned at the entire height of the sample for 7 days at 1-hour interval to acquire transmissionand backscattering data every 40 um. Under the dynamic sag measurements, a rheometer with a measuringsystem applying a cup and grooved bob was used to conduct rheological and viscoelastic measurementssuch as flow curves, oscillatory amplitude sweep, oscillatory frequency sweep, rotational and oscillatorytime tests on the drilling fluid samples. The drilling fluid properties were characterized under atmosphericconditions before and after dynamic aging. The aging temperature and pressure conditions in the roller ovenwere 120°C and 100 psi respectively for a period of 2-1/2 days. A high precision density meter was used tomeasure the density of the drilling fluid samples before and after each test. Dynamic sag index (DSI) resultswere compared between time-dependent rotational shear test with shear rates from 10.22 to 0.001 s~(-1) andtime-dependent oscillatory shear test with angular frequencies from 10.22 to 0.001 rad/s at constant strainamplitudes of 0.05% within linear viscoelastic (LVE) range and 100%, over a period of 10,800 s to closelycompare to sag-prone conditions during drilling operations. We observed that heat-activated chemicals inthe hot-rolled fluid sample increased the viscosity and elasticity which contributed to lower barite sag andlonger suspension of particles than before hot rolling. Moreover, rotational shear test increased barite sagin 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 thefluid samples. The time-dependent oscillatory shear test can provide new insight on the structural characterof drilling fluids to predict barite sag tendencies during the fluid design phase.