Development of New Kill Fluids with Minimum Sagging Problems for High Pressure Jilh Formation in Saudi Arabia
INTRODUCTION Barite sag is an unwanted variation in drilling fluid density after an extended period of static conditions. This problem is experienced frequently in Khuff and pre-Khuff gas wells in the Base Jilh Dolomite (BJD) formation. The Jilh formation is a dolomitic Middle Triassic tight zone that ranges from 8,000 ft to 10,000 ft true vertical depth (TVD) with abnormal saltwater pressure. Several factors add further complication to the problem of well control, such as uncertainty in predicting pore pressure, high temperature (300 °F), well deviation (leading to the Boycott settling phenomenon), drillpipe eccentricity and salt contamination of the drilling fluid. This high-pressure BJD formation requires a drilling fluid with a density up to 150 pcf. For high density fluids, achieving the right fluid formula with the right fluid rheology is difficult since excessive heat, dehydration and solids loading can deteriorate mud properties. The addition of weighting materials, such as barite (BaSO4) or hematite (Fe2O3), is needed to achieve the desired density to overbalance the well and avoid any possible flow from the formation; however, maintaining a high volume of solids content — of BaSO4 particles or any other material — in suspension is challenging, especially for extended periods of time. The separation of weighting material particles — usually BaSO4 — from the mud leads to undesirable density fluctuation as the drilling fluid is being circulated throughout the hole. This phenomenon, often referred as barite sag, may lead to serious well control incidents, stuck pipe, lost circulation and nonproductive time. Previous experience has shown that the barite sag phenomenon is attributed not only to mud-related problems but also to operation related practices1. Drillpipe eccentricity, stationary drillpipes, hole inclination and low annular velocities can dramatically increase barite sag2. Barite sag increases even faster as the hole angle increases to between 45° to 60°. This process was reported by Boycott in 1920, when he noticed that blood cells settled faster in inclined test tubes than in vertical tubes3.
A Combined Barite–Ilmenite Weighting Material to Prevent Barite Sag in Water-Based Drilling Fluid
Barite sag is a serious problem encountered while drilling high-pressure/high-temperature (HPHT) wells. It occurs when barite particles separate from the base fluid leading to variations in drilling fluid density that may cause a serious well control issue. However, it occurs in vertical and inclined wells under both static and dynamic conditions. This study introduces a combined barite–ilmenite weighting material to prevent the barite sag problem in water-based drilling fluid. Different drilling fluid samples were prepared by adding different percentages of ilmenite (25, 50, and 75 wt.% from the total weight of the weighting agent) to the base drilling fluid (barite-weighted). Sag tendency of the drilling fluid samples was evaluated under static and dynamic conditions to determine the optimum concentration of ilmenite which was required to prevent the sag issue. A static sag test was conducted under both vertical and inclined conditions. The effect of adding ilmenite to the drilling fluid was evaluated by measuring fluid density and pH at room temperature, and rheological properties at 120 °F and 250 °F. Moreover, a filtration test was performed at 250 °F to study the impact of adding ilmenite on the drilling fluid filtration performance and sealing properties of the formed filter cake. The results of this study showed that adding ilmenite to barite-weighted drilling fluid increased fluid density and slightly reduced the pH within the acceptable pH range (9–11). Ilmenite maintained the rheology of the drilling fluid with a minimal drop in rheological properties due to the HPHT conditions, while a significant drop was observed for the base fluid (without ilmenite). Adding ilmenite to the base drilling fluid significantly reduced sag factor and 50 wt.% ilmenite was adequate to prevent solids sag in both dynamic and static conditions with sag factors of 0.33 and 0.51, respectively. Moreover, HPHT filtration results showed that adding ilmenite had no impact on filtration performance of the drilling fluid. The findings of this study show that the combined barite–ilmenite weighting material can be a good solution to prevent solids sag issues in water-based fluids; thus, drilling HPHT wells with such fluids would be safe and effective.
Turbulent boundary layer wall‐pressure fluctuations downstream from an abrupt change in surface roughness
Spectra of wall‐pressure fluctuations beneath a turbulent boundary layer have been investigated immediately downstream from an abrupt change in surface roughness, from slightly rough to very rough. Over the measurable frequency band (400 Hz to 7 kHz), the spectra in the first 150 mm scale on wall variables. This indicates that the motions responsible for the pressure fluctuations measured in this region are very close to the wall. Farther downstream this scaling procedure breaks down. It is suggested that the initially thin internal layer behaves independently of the flow external to it until it has grown appreciably and only then interacts with the outer boundary layer. It was found that, for all frequencies measured, the dimensional spectral level decreases initially then rises to a maximum at 250 mm from the roughness change before decreasing once more. The position of the maximum is close to where the internal layer ceases to have a distinct edge.