Drilling Fluid Technology Of Suitable For High Pressure Salt-gypsum Layer In Halfaya Oilfield

Many researches have been made to develop inhibitive drilling fluid systems with the features of high temperature resistance; gypsum salt resistance; high pressure resistance in China. However, the results are not satisfactory. The formulas of the drilling fluid systems developed are complex and the amount of the mud chemicals used is large. Although the temperature resistance data of some systems acquired in lab has even reached240℃, high concentration gymsum salt and clay resistance ability of these systems are weak. As a result, these drilling fluid systems developed are not suitable for site application.Researches on geology and relevant drilling engineering document in halfaya oilfield in Iraq show that the incidents such as bit balling, saline water contamination, gypsum salt creepage, well caving, lost circulation and C02^H2S contamination are likely to happen during drilling. Since the lithology in upper formation is mudstone which is likely to disperse and expand, drilling fluid used in this section must be inhibitive enough; since the lithology in the middle formation is high pressure massive pure salt interbedded with gypsum salt and soft mudstone, high density drilling fluid applied in this section must have approved rheology and prevent gypsum salt from creeping as well as resist gypsum pollution; the lower formation is pay zone and the lithology is cracking limestone which is likely to cause the incident of lost circulation. Meanwhile, based on the specific requirement of drilling many extended reach horizontal wells and branch horizontal wells with slim borehole in halfaya oilfield, many lab researches and site applications have been made to customize drilling fluid systems with approved lubricity, cuttings carrying ability, wellbore cleaning ability and reservoir protection ability.To customize drilling fluid systems and mud chemicals, lab experiment must be made for chemicals optimization and evaluation. The properties such as inhibition, lubricity, sealing ability, anti-caving ability as well as effect on mud viscosity must be taken into consideration. High viscosity caused by high solid content, dispersion of solid particles and interaction between solid particles results in the difficulty of conditioning high density drilling fluid rheology. Furthermore, since high density drilling fluid is more sensitive to contamination than common drilling fluid, its rheology conditioning is more difficult.High density complex organic salt drilling fluid has been developed and successfully applied in Halfaya block in Iraq. This drilling fluid system is mainly composed of complex organic salt BZ-WYJ; filtration reducer (gypsum and high temperature resistance) BZ-Redu; salt resistance viscosifier BZ-VIS; inhibitive anti-caving agent BZ-YFT; inhibitive lubricant BZ-YRH. The organic salt base fluid with relatively high density and approved inhibition is the key technology. The temperature resistance of this drilling fluid system can reach more than200℃; gypsum doesn’t dissolve in this drilling fluid; the density can be weighted to more than2.60g/cm3only by barite. This drilling fluid system has the features of low viscosity, low solid content, approved inhibition and approved sealing ability. It can make low ECD available and is helpful for drilling speed increasing. The contradictions between "solid and rheology";"rheology and filtration;"inhibition and dispersion" in traditional high density drilling fluid have been completely solved by using this drilling fluid system. It is an ideal system to be applied to drill high pressure, gypsum and strong water sensitive formation. Lab experiment such as inhibition evaluation of expansibility and double electrode layer; comparison of lubricity; clay, gypsum and salt, cement contamination resistance test; variation of interfacial tension; return permeability test and biotoxicity test are used to evaluate gypsum and salt resistance mud chemicals and optimize complex organic salt drilling fluid formulas.9wells including4horizontal wells were successfully drilled and completed in2011by using complex organic salt drilling fluid. However, the system still had some weak points:Compared with other inorganic salt drilling fluid systems, the ROP was slower, sometimes bit balling was likely to happen; the calcium-ion value tested could reach even more than14000mg/l during drilling even though the gypsum hardly dissolved in organic salt solution; the viscosity was likely to increase in later spud sections and was difficult to condition; high density complex organic salt drilling fluid could give off pungent smell; the cost was higher than that of inorganic salt drilling fluid. To deal with these weak points above, some optimizing measures have been taken as follows:1. Optimizing the formulas of complex organic salt:bit balling preventing agent has been brought in to increase ROP without affecting inhibition ability of complex organic salt.2. Researching and developing compound organic salt and inorganic salt drilling fluid formulas:ROP has been increased and the total cost has been reduced by using new compound formulas. As a result, this kind of drilling fluid can be more competetive.3. Improving and optimizing drilling fluid formulas:PAC-LV has been brought in as the filtration reducer used in newly developed saturated inorganic salt drilling fluid which can be used together with complex organic salt drilling fluid and compound salt drilling fluid based on the company’s requirement.Technical Innovation Points:1. Since the density of complex organic salt base fluid is high, there is no need to add a lot of weighting materials. As a result, the solid content in the drilling fluid prepared by this kind of fluid can be reduced effectively, which makes high density drilling fluid with approved rheology truly feasible.2. The gypsum salt and acid gas contamination resistance ability of mud chemicals have been effectively enhanced with the help of synergism between organic salt and these chemicals3. Incidents prevention measures, problems solution technology as well as drilling fluid formulas have been being optimized based on formation lithology features of halfaya oilfield to customize drilling fluid systems that is easy to condition and helpful for ROP increasing and incidents decreasing. The development of customized drilling fluid systems can shorten drilling period and make safety drilling truly feasible.