Effect of Temperature on Drilling Mud Resistivities

A cell is described for measuring drilling mud resistivities at temperatures up to 350°F. Rm, Rmf and Rmc were measured at temperatures from 75 to 225°F with 100 psi pressure maintained on the mud system. Hence, Rmc is measured on the undisturbed mud cake under differential pressure. Fifteen synthetic mud samples and 25 field samples were measured. Some of the more important results areThe effect of elevated temperature on Rmf and Rmc is not the same as for NaCl solutions having corresponding resistivities at 75°F.The resistivity ratios, Rmf/Rm and Rmc/ Rm, vary with temperature in a different manner for different muds.Resistivity ratios may be widely different for different muds.Rmc may be different for undisturbed mud cake under differential pressure than when it is disturbed for measurement.Some chemical additives greatly affect the resistivity values as well as resistivity ratios.Mud resistivities may vary widely while drilling. Introduction Quantitative interpretation of electric logs requires reliable values for the mud resistivity, Rm, the mud filtrate resistivity, Rmf, and the mud cake resistivity, Rmc, under borehole conditions. For example, the resistivity of the formation water, Rw, is often calculated from the self-potential curve by the equation: In this equation, K has a value which is relatively constant for certain conditions of lithology and borehole temperature within a specified area. Thus, in the use of this equation to find Rw, it is evident that an error in the determination of Rmf will result in an error of like magnitude in the calculation of Rw. Rm is sometimes measured in the borehole with the collapsed microsonde but it is more difficult to make routine measurements of Rmf and Rmc under borehole conditions. When these resistivities are determined by sample measurements at the surface the correction to borehole conditions is of major importance. Temperature, pressure and differential pressure are some physical fact is that may affect resistivity values but temperature is by far the most important. Loan found no measureable effect of pressure on Rm up to 1,000 psi, but he found that Rmf and Rmc are affected by differential pressure. For differential pressures of 100 psi and 1,000 psi, Rmf was as much as 20 per cent greater for the higher pressure but Rmc was as much as 10 per cent smaller for the higher pressure.