Oil Base Mud in High Pressure, High Temperature Wells

Abstract

ADCO's experience while drilling the sixth deep Khuff well with water base mud to total depth of 21682 ft and a bottom hole static temperature of 465 F has highlighted mud related problems. The mud has shown gelation, barite sagging, sensitivity to formation water influx and possible instability at densities higher than 122 pcf. These problems were considered in the selection criteria of the drilling fluid for the seventh deep Khuff well which required a drilling fluid density of 110–140 pcf with stable HTHP fluid loss at temperature up to 465 F, capable to tolerate influx of CO2, H2S, and high salinity formation water. Studies on various oil & water base muds were performed in various laboratories including ADCO's, the Shareholders' and the mud companies'. The development of stable water base mud was not possible. Oil base mud formulation was developed and has been used. The well has reached its total depth at 20085 ft & a bottom hole temperature of 440 F after facing operational challenges requiring mud weight of 155 pcf to control the well and during which, the OBM choice proved to be a success. Introduction ADCO's experience in drilling five deep Khuff wells, using water base mud encouraged the drilling of the sixth well utilizing water base mud. The well was drilled to total depth at 21682 ft. Loss of circulation occurred into Jilh formation at 14570 ft and was controlled by decreasing mud weight from 100 pcf to 97 pcf. Intermittent formation water influx from the Lower Khuff and Pre-Khuff formations from 19146 ft to 21682 ft occurred although 7" liner was set between the two zones at 20220 ft, The bottom hole temperature of 465 F caused mud degradation in the from of gelation, barite sagging and failure to tolerate higher densities. Differential sticking occurred across Upper Khuff at 17940 ft after increasing the mud weight from 96 pcf to 110 pcf in an attempt to stop formation water influx from Lower Khuff. Failure to shape the mud density and rheology, as required, probably was the cause of the problem which occurred while cementing the 4 1/2" liner and ended up in cementing the setting tool and drill string in hole and abandoning the well primary objective after excessive fishing operations. P. 257