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.
Rheological Analysis of Static and Dynamic Sag in Drilling Fluids
Methods exist for evaluation and mitigation of barite sag in drilling fluids; however, the increased use of nontraditional weighting agents and systems presents increased challenges for fluid design. An indirect evaluation of sag is presented for various types of fluids , using viscometry and oscillatory testing, with and without superimposed shear.
Thixotropy and Yield Stress Behavior in Drilling Fluids
Drilling fluids are commonly recognized as complex fluids which exhibit both yield stress behavior and varying degrees of thixotropy. Traditional models for yield stress behavior have been used extensively, with the Bingham plastic model remaining prevalent for description in the field and the Herschel-Bulkley model becoming the standard for computer simulations of fluid behavior. Neither of these models represents the full behavior of drilling fluids, either missing aspects of the shear-thinning behavior or inaccurately predicting the yield stress of the fluids. These errors are exacerbated through variations in measurement techniques by technicians who do not fully appreciate the thixotropic nature of these fluids. An improved understanding of the rheological behavior, in particular the yield and thixotropic nature, of drilling fluids would result in improved models and enhanced drilling performance, thereby reducing drilling costs. An examination of some typical drilling fluids will be presented. The intertwined effects of thixotropy and yield stress on rheological measurements will be highlighted. These fluids will also be evaluated with traditional yield stress models as well as with several recently proposed models.