Measuring and Interpreting High-Temperature Shear Strengths of Drilling Fluids

Deeper drilling for oil is becoming more and more the rule rather than theexception. With deeper drilling come additional problems, perhaps the greatestbeing those brought on by the higher temperatures encountered down the hole, particularly in the Gulf Coast region of Texas and Louisiana. Temperaturegradients of the order of 1.8? to 2.0?F/100 ft are not unusual, and a gradientof 2.3?/100 ft is found in some areas of Texas. With a mean surface temperatureof 74?F, the following temperatures could be expected for a geothermal gradientof 2.0?F/100 ft: at 10,000 ft, 274?F, 12,000 ft, 314?F; 14,000 ft, 354?F; and16,000 ft, 394?F. Severe gelation of lime-base drilling fluid in wells thathave high bottom hole temperatures has become perhaps the most seriousdifficulty encountered in drilling under such conditions. Lime-base drilling fluids have been very successfully and widely used in thedrilling of wells in the Gulf Coast region because of their inherent stabilitytoward contaminants, their ability to suppress the swelling dispersion ofbentonitic shales, and their ease of maintenance. The gradual recognition, during the past few years, that these muds were, in themselves, the cause ofmany difficulties experienced in drilling has led to widespread efforts by thedrilling industry to determine the reasons for the failure of these mud systems and to develop mud systems capable of performingsatisfactorily under high-temperature conditions. Manifestations of High-Temperature Gelation It is generally possible to recognize the symptoms of high-temperaturegelation early enough that advance predictions can be made of seriousdifficulties in mud control, and the useful life of the drilling fluids can beextended by proper treatment. Following is a list of the manifestations ofhigh-temperature gelation:The drill string ‘takes weight’ while going in the hole after a trip. Inearly stages of high-temperature gelation it is possible to notice a slightreduction in drill string weight as the drill pipe is lowered near the bottomof the hole.Excessive pump pressure is required to start the circulationof drilling fluid at or near the bottom of the hole when going back to bottomafter a trip. T.P. 3638