The term Barite sag refers to the slow process of settling of the barite particles and the other solid particles these results in an increased fluctuation of the density of the drilling fluid that exits the drilling bore (Hemphill, T. at al - 2004). Under normal circumstances the density of the drilling fluid that is at the base of the wellbore might be higher than that is exiting the wellbore or the other-way round whereby the density of the fluid at the base is slightly less dense than that exiting the wellbore. These fluctuations are usually of an insignificantly small margin of about 0.012 sg (0.1 lbm/gal) this margin so narrow that it’s usually not associated with barite sag. When barite sag occurs these margin widens up and it may attain an average peak of 0.36 sg (3 lbm/gal).
Barite sag is one of the common challenges that are facing the drilling industry currently due to its inconveniences it causes in time wastage and expense since clearing a barite sag is quite an expensive process which will require a rig circulation to be run which will also consume an lengthy period of time to be complete the drilling fluid should also be conditioned during the treatments if these problems are not sorted out in time they may result in unwanted well control complications. Over years to date the researchers in the drilling industry have a difficulty to accurately predict a barite sag in a deviated well bore though many researchers have come to terms that barite sag a phenomenon that is considered dynamic. Currently the rheological measurements of the drilling fluids or the static measurements are being used in modeling that problem whereby the fluid dynamics are applied to measure or predict the occurrence of a barite sag in a wellbore that is deviated. It is very crucial to understand the behaviour of the drilling fluids in the conduit walls since this is where the occurrence probability of barite sag is high currently there are more sophisticated and complicated hydraulic modeling in the drilling fluids in axial flow.
The correlations of barite sag as a function of the shear stress and velocity of the drilling fluid have been used to help in the predicting the occurrence of barite sag and thence reduce the inconveniences of cost and time wastage that come about as a result of barite sag
The probability of occurrence of barite sag (Hemphill, T. at al - 2004) is high in a number of conditions which include: wellbore that have been deviated with sharp angles averagely between 40-75º, conditions in drilling that are dynamic as compared to static drilling conditions, when using fluid that have a low viscosity as drilling fluids as compared to viscous drilling fluids. Barite sag has high chances of occurring in drilling where they use invert drilling as opposed to drilling whereby water-based drilling fluids are used. The annular velocity for circulation during drilling should be maintained above 0.51m/ (100ft/min) annular velocities below this are prone to lead to barite sag.
Several ideas have been put forward by researchers in the drilling industry and found out to prevent or rather help in prediction of barite sag reduce occurrence. One major thing that the drillers should do is to ensure that the drilling fluid’s Herschel-bulkley yield stress ( ) rheological parameter should always be maintained in the range between 7.2 (7 to 15 ibf/100 ) when drilling and circulating,
The viscosity drilling fluid should always be maintained at shear rates that are as low as 0.001 within a given range a barite sag window can be used where a viscosity shear rate range has been prescribed. During drilling the circulation rates and the rotation rates should always be high in order to avoid the development of the barite sag. Testing equipment should also be put in place to record changes at the core of drilling fluid inside a tube that is suspended and the results should be computed and analyzed as a function of time and the results got should then be correlated with field data.
Barite sag still is a big problem to the drilling industry since none of the above ideologies have fully curbed the problem of barite sag prediction or even prevention. Of the above named ways of preventing and predicting of barite sag the Herschel-bulkley rheological model yield stress ( ) has proved to be the most efficient with a correlation parameter being 0.7 to 0.8 but this results of correlation are not satisfactory enough therefore there is need for development in this sector.
There are certain physical principles that are used in the study of barite sag development they include the occurrence of barite sag which usual occur in low shear rate in the environment. Often the barite sag particles will accumulate at the conduit wall on the lower sides of the angled wellbores. The rheological properties of the drilling fluid also play a role in barite sag prediction. The ‘barite sag window’ can also be used with a set of upper and lower range.
Accumulation and sub-merging of barite beds leads to sliding in the lower part of the hole and it usually begins with a very low angle compared to beds with similar material in the air. The beginning of sliding is determined by various factors: inclination of the borehole such that when the angle is between 40 to 50 degrees then sliding is predominant, the type of mud that eventually influences how much the particles get wet hence the angle of sliding and the type of the weighing material whose shape as well as size is important due to the internal friction it may cause.
Hydraulic Modeling in Eccentric Wellbores
The calculation of the pressure drop required in shearing of a pseudoplastic drilling fluid has some parameters that are used as guidelines in the calculations, (Hemphill, T. at al - 2004) these include: the flow rate, the drilling fluid’s Herschel bulkley rheological parameter (n, K, ), the shear stress of the wall ), the inner tube OD and conduit ID. The formula for calculating the shear stress of the wall of fluid in the lamina flow regime is: =R/2 * dP/dL
Stating the problem
Sag problems are encountered more frequently in oil and water mud as compared to other types of problems such as pipe sticking and stability ones. Oil mud is much more viscous compared to the water based hence much is done to reduce the viscosity by reducing on the additives that suspend barite. In addition the oil ones do not produce strengths of gel like those of water consequently barite sag problems in oil mud is higher than in water based ones which has led to research in oil mud being more common. The trials to get important parameters of rheology especially those of a low rate of shear have been problematic to date (Nguyen, 2006).
Tools for Barite Sag Prediction
One of the tools that has been used successfully is by drilling fluids sag ability and measurements of rheology. The static sag which was plotted which was given as number of sag that was 3 RPM function and ten minutes gel reading (this was for several drilling fluids). The end results finally determined that there was a relationship between the gel that was measured, the rate of shearing and the overall sag that was observed. Dynamic sag, which also was presented by the sag number, was plotted in a similar manner (Nguyen, 2006). This brought a conclusion that the viscosity can determine dynamic sag avoidance. Some others like Dye and others had their focus on the dynamic conditions and came to a conclusion that a shear rate of about four brings about the beginning of dynamic sag of barite. In conclusion it has been said that viscosity with a low rate of shear is a parameter which is rheological that is essential for knowing whether the mud or fluid is capable of reducing or completely preventing the barite sag, more importantly the dynamic one.
Prevention Window is also a tool that determines the sag directly by ultra low rate of shear in viscosity. This is from the fact in equivalence in the shear rates of measurements of viscosity and that of barite sag that is dynamic. Prediction of the sag can be achieved by comparison between the limits it the prevention window and that of viscosity that is rational. When the levels of viscosity are in the lower limit, that is, of the prevention window it implies very bad or rather severe barite sag. Indications of low or bearable amounts of barite sag are shown when the levels of viscosity are past the upper limit of the prevention window. However when prevention of barite sag is considered then these results are excessive. Thus in conclusion, results obtained where the levels of viscosity are in between the limits are the most preffered as this means there is a very low chance for the barite sag.
(Dye., et al 2002)
In approaching the mud rheology, there are steps to be followed which can be of experiment or theoretic terms. The first stage involves laboratory studies where familiarity and study of phenomena about sag of barite in either Newtonian or non Newtonian fluids by a microscope is considered. Simplification in the experiments is usually achieved by methods of statistics which optimize tests’ numbers. The density ρ, index of flow n, the index of consistency K, and also the yield stress in the second stage are used the second stage when they are identified in each of the fluids. Here the angle of hole on the barite sag and the annular velocity which is low are analyzed, but the rotation of the pipe is not taken into account.
The first step in the stage is mixing of mud samples of the above mentioned parameters taken. The section of test is changed to a favorable angle followed by circulation of the mud in a high speed so that it is uniform. The rate of flow is adjusted to get a good annular velocity and mud is circulated at constant velocity. With time (T1) the bottom and top density are measured by different densitometers that are nuclear. A maximum rate of flow is then ensured and uniform density confirmation is done at this step. The flow rate then adjusted to zero and this remains this way for some time (T2) followed by top and bottom density measurement.
The third stage depends on the second stage results where very good ranges in velocity as well as inclination angle are determined. Finally there is analyzation of these data so that the effects of the angle of the hole, velocity, the pipe rotation and rheology are used for prediction.
Hydraulic modeling is crucial in determining the shear rates shear stress and pressure drops at the conduit wall using the individual fluid rheological properties.
(Hemphill, et al., 2004)
The simulated annular velocities are within range of the lamina flow regime thus when the pressure drops are plotted against the AV. the profile is linear for each fluid that is simulated and the conduit shear stress were calculated in the cases where the annulus was sheared fully.
The following comparisons of the speed of rotation, annular velocity and the angle of inclination should be considered: with annular velocity beds are not easily formed with turbulent flow, inclination angle of between 40 and 50 degrees makes slumping very prevalent. Also, the low velocity, a range of 60 to 70 degrees as well as a stationary pipe of drilling exacerbates the barite sag. In addition, barite sag is severe at approximately 30 feet per minute and at a speed of rotation of 3 revolutions per minute. Prediction of sag:
R is the sag register, Δ is a change in weight of mud at a time while M is initial weight of mud. For example = , meaning R is 20. (Nguyen, 2006)
Correlation between the conduit wall shear stress and the dynamic barite sag that is measured is very important in the prediction of barite sag incidence especially in the invert emulsion drilling fluid. The levels of the wall shear stress can be insufficient in some drilling fluids according to the Herschel-Bulkley rheological model if these particular fluids are sheared at annular velocities (AV) that are low and this can result in barite sag