A High Density Undersaturated Saltwater Drilling Fluid for Salt and Gypsum Drilling in Well ZQ2

Well ZQ2 is an exploratory well located at the Qiu-2 substructure of the Qiulitage Structure in the Kuche sag. This well penetrated the mudstone Jidike Formation-Kumugeliemu Group (N1j² -E_1-2km¹) which are buried at depths from 4545 m to 5827 m. This section is characterized by long salt/gypsum formations, high content of gypsum, high formation pressure, soft mudstones embedded with salt and unconsolidated soft high argillaceous content high water content rocks. The diameter of the hole penetrating this section is φ333.375 mm. This section was previously designed to drill with oil base mud, and was then changed to water base mud. High-and low-pressure formations exist in the same interval. All these challenges demand more of the water base drilling fluid to be used. An undersaturated polymer sulfonate drilling fluid was prepared using a sodium polyallyl sulfonate filter loss reducer MYK, a modified plant gum encapsulating inhibitor NXX and an organic salt Weigh2 which was first used to improve the undersaturated sulfonate drilling fluid to high density undersaturated polymer sulfonate drilling fluid. This drilling fluid have shown itself strong inhibitive capacity, high resistance to salt and gypsum contamination, stable properties and simple maintenance of rheology. Drilled cuttings acquired by the mud loggers were accurately relevant to the formation being drilled. The rheology of this drilling fluid during drilling was also much easier to control than the rheology of the drilling fluids used nearby. Application of this drilling fluid in drilling the well ZQ2 was a success, without encountering problems encountered when using other undersaturated drilling fluids, such as the use of thinners which leads to "adding mud-dumping mud-adding mud again, viscosifying-thinning-viscosifying". Using this drilling fluid, the salt/gypsum formations were successfully and safely drilled through, borehole wall was stable during drilling, wireline logging and casing running were successful at the first try. This drilling fluid technology has provided the base for casing program optimization in the Kuche sag.