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.
Prevention of Barite Sag in Oil-Based Drilling Fluids Using a Mixture of Barite and Ilmenite as Weighting Material
Drilling high-pressure high-temperature (HPHT) wells requires a special fluid formulation that is capable of controlling the high pressure and is stable under the high downhole temperature. Barite-weighted fluids are common for such purpose because of the good properties of barite, its low cost, and its availability. However, solids settlement is a major problem encountered with this type of fluids, especially at elevated downhole temperatures. This phenomenon is known as barite sag, and it is encountered in vertical and directional wells under static or dynamic conditions leading to serious well control issues. This study aims to evaluate the use of barite-ilmenite mixture as a weighting agent to prevent solids sag in oil-based muds at elevated temperatures. Sag test was conducted under static conditions (vertical and inclined) at 350 °F and under dynamic conditions at 120 °F to determine the optimum ilmenite concentration. Afterward, a complete evaluation of the drilling fluid was performed by monitoring density, electrical stability, rheological and viscoelastic properties, and filtration performance to study the impact of adding ilmenite on drilling fluid performance. The results of this study showed that adding ilmenite reduces sag tendency, and only 40 wt.% ilmenite (from the total weighting material) was adequate to eliminate barite sag under both static and dynamic conditions with a sag factor of around 0.51. Adding ilmenite enhanced the rheological and viscoelastic properties and the suspension of solid particles in the drilling fluid, which confirmed sag test results. Adding ilmenite slightly increased the density of the drilling fluid, with a slight decrease in the electrical stability within the acceptable range of field applications. Moreover, a minor improvement in the filtration performance of the drilling fluid and filter cake sealing properties was observed with the combined weighting agent. The findings of this study provide a practical solution to the barite sag issue in oil-based fluids using a combination of barite and ilmenite powder as a weighting agent to drill HPHT oil and gas wells safely and efficiently with such type of fluids.
The costs of identifying undiagnosed prostate cancer in asymptomatic men in New Zealand general practice
Background. Screening for prostate cancer (PCa) using the prostate-specific antigen (PSA) test is widespread in New Zealand. 3 informed testing history in 2007–09 were NZ$8887 compared to NZ$13 870 if the men had PSA tests in 2007–09. If we assumed a PSA test involved a full 15-minute general practice consultation, the estimated costs increased to NZ$26 877 per PCa identified. Conclusions. Screening of asymptomatic men for PCa is widely practiced. Most of the costs of screening were incurred in general practice. Calls for men to receive increased information on the harms and benefits of screening will substantially increase the costs. The current costs could be reduced by better targeting of screening.