A major problem encountered in the drilling of high-temperature wells is the control of the rheological properties of high-density deflocculated water-base muds. This paper discusses the mud flow properties and describes the use of a low-molecular-weight copolymer, the sodium salt of sulfonated styrene maleic anhydride, to stabilize the mud system under high-temperature conditions.
The effect of high bottomhole temperatures on water-base drilling muds may be evidenced first by high-viscosity mud "off bottom" after tripping the pipe or by abnormal thinner treatments to maintain pipe or by abnormal thinner treatments to maintain desirable flow properties during drilling. In more severe cases, it may prove difficult to "break" circulation after trips or result in failure of the logging tool to go to bottom.High temperature is one of the seven mud contaminants listed by Rogers. Though one may not think of high temperature as a contaminant in the same sense as gypsum or salt, it nevertheless can be highly detrimental to the flow properties and filtration control of the mud. High temperature results in the flocculation of the mud solids, just as contaminating salts do, but it causes this indirectly through degradation or desorption of the rheological stabilizer. Unlike flocculating salts, however, high temperature cannot be treated out; therefore, the mud system must be capable of performing in its presence. presence. The effects of elevated bottomhole temperature became quite evident in the late 1930's and early 1940's when the molecular dehydrated phosphate and polyphosphate-type deflocculants were standard mud thinners. These phosphate thinners undergo thermal degradation or reversion to the ortho form in the 150 to 260 degrees F (65.5 to 93.3 degrees C) ranger and, thus, become ineffective in maintaining satisfactory flow properties as the borehole temperature exceeds these limits. Tannin compounds, primarily quebracho extract, were popular thinners for the higher-temperature wells in the 1940's. These "red muds," so named because of the color imparted by quebracho, were more temperature-stable and more resistant to contamination than the polyphosphates, but their range of effectiveness also was surpassed as well depths increased.Lime-treated muds utilizing either quebracho or calcium lignosulfonate became popular during the early to mid 1950's, but this mud system exhibits a type of high-temperature instability that results in severe gelation. Grays described this effect as the high-temperature solidification of lime muds. McCaleb's work showed that this solidification process was the result of a lime/clay reaction process was the result of a lime/clay reaction enhanced by high temperature to produce tobermorite, a mineral commonly found in portland cement. Unfortunately, this gelation problem still exists since lime hydrate may be incorporated to some degree through the drilling of cement.In an attempt to eliminate the lime gelation problem, muds treated with gypsum and a problem, muds treated with gypsum and a ferrochrome lignosulfonate were used extensively in the late 1950's.