Innovative Non-Aqueous Fluids Technology Improves Drilling Efficiency Significantly in Dealing with Divalent Complex Salt Formation Under HTHP Conditions
Authors: Yin Da , Peng Shuangjiu , Liu Guoqing 
Published: March 2015

Abstract
This paper presents a case study on the successful application of Non-Aqueous Fluids (NAF) in the Tian Mountain Front Block in North Border of Tarim Basin. Traditionally the wells in this area are drilled with water based mud. Due to the unpredictable divalent complex salt formations, the high temperature (up to 180 °C) and high pressure (mud weight up to 2.6 s.g.) conditions, significant non-productive time (NPT) and low rate of penetration (ROP) were encountered in dealing with wellbore instability, kicks, stuck pipe and downhole losses in the salt formations and fractured pay zone. The water base fluids would become unstable when they are heavily contaminated by divalent salts and/or brine influx.

Introduction
Tarim Basin is the biggest oil and gas bearing basin in China. Total area is approximately 560,000 km2, including "dead sea" desert of around 337,000 km2. One of the most challenging field in terms of drilling difficulties lies in the north border of Tarim Basin: the Tian Mountain Fronts. The reservoir depths are over 7000 meters TVD and bottom hole temperatures are up to 180 °C. Analysis of a large quantity of offset well data1,2, operator internal reports3,4,5 and presentations6 was comprehensively carried out and it was observed that frequent downhole complications were encountered in three major formation sequences / intervals:Shallow formations above the salt formations:Heavy losses and extremely low ROP are common in drilling the hard conglomerates above the salt formations. In the event that the salts are inadvertently penetrated, the high pressure brine influx would require higher mud weight to kill the well. This is normally followed by downhole losses and stuck pipe events.Inside the salt formation sequences:When the water base mud is used, downhole losses and stuck pipe events would normally follow a high pressure brine influx. Stuck pipe is also often encountered when the highly mobile divalent salt formation is penetrated, depending on the mud weight and time exposed. It is believed that the salts are dominantly divalent and their creep rate is much higher than the monovalent salts (believe to be 20 times faster). The risk of stuck pipe is significant higher when divalent mobile salts are drilled.
Another hazard causing the heavy downhole losses and pipe stuck in drilling the salts formation is when the pay zone is inadvertently penetrated, since the mud weight required in drilling the salts is much higher than the mud weight required in drilling the pay zone. It has been a long standing problem to confirm the bottom of the salt sequences. It is common that the number of salts sequences is unpredictable and hence require technology for confirming the end of salt sequences is highly challengeable. Often salts are encountered again in the next interval after the casing has been set to isolate all salt sequences, because the determination of the end of the salt sequences has been made mistakenly.