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.
Barite–Micromax mixture, an enhanced weighting agent for the elimination of barite sag in invert emulsion drilling fluids
Weighting agents are mixed with the drilling mud to provide the high density required to control high-pressure high-temperature (HPHT) wells throughout the drilling operation. Solids sag occurs when the weighting agent separates from the liquid phase and settles down, causing variations in the drilling fluid density. This study evaluates barite–manganese tetroxide (Micromax) mixture to eliminate solids sag issue encountered with weighted invert emulsion drilling fluids at HPHT conditions. Micromax additive was added to barite-weighted fluids in different concentrations, 0, 15, and 30 wt% of the total weighting agent. Static and dynamic sag tests were used to evaluate the sag tendency of the new formulation under static and dynamic conditions. The performance of the new formulation was evaluated by measuring the electrical stability, density, rheological, viscoelastic, and filtration properties of the drilling fluid. The obtained results showed that Micromax additive improves drilling fluid stability by reducing the sag tendency. Adding only 30 wt% of Micromax additive eliminated barite sag issue in both dynamic and static conditions at 350 °F. 30 wt% Micromax increased the base fluid density by 5.4% and the yield point by 115% and maintained the gel strength value at 12 lb/100 ft 2 , while it reduced the plastic viscosity by 30%. The addition of Micromax additive improved the viscoelastic properties of the drilling fluid by maintaining a higher storage modulus to the loss modulus ratio when compared with the barite sample (in the range 4–4.5). Furthermore, 30 wt% Micromax improved the filtration performance by reducing the filtrate volume, filter cake weight, and filter cake thickness by 50%.
Determining relaxation modes in flowing associative polymers using superposition flows
Oscillations superimposed on steady shear flows have been used repeatedly in the past to determine the relaxation modes in flowing associative polymers. In these experiments, the oscillatory motion has been parallel to the steady-state flow. Here, parallel superposition moduli on associative polymers will be compared with superposition experiments in which the oscillatory motion is perpendicular to the steady-state flow. In the latter experiments, there is less interference between the steady flow and the superimposed oscillations, which has drastic consequences for the results. Data are shown for a HASE polymer (hydrophobic alkali-swellable emulsion). As in other fluids, the limiting viscosities at zero frequency differ drastically, and negative storage moduli can be obtained in parallel superposition. The apparent relaxation frequencies during flow, as derived from parallel superposition measurements, are an order of magnitude smaller than those derived from orthogonal superposition. The effect of shear...