The gypsum-salt formations are complex in drilling fluid. The gypsum-salt formations are characterized by salting-in property, plastic creep and anisotropy. Meantime, the rheological properties of drilling fluids become worse when contaminated by high pressured brine in the gypsum-salt formations. In the process of drilling operation in the long-section gypsum-salt formation, reasonable mud weight and good rheological properties of drilling fluids should be used to ensure the safety drilling in this complex formation. The main matters needing attention is as follows:
- The drilling fluid system will be contaminated by complex salts. Corresponding measures should be taken to control the rheological properties of drilling fluids in time.
- An appropriate amount of salts should be added to the drilling fluids, and the salt is not allowed to be added directly to the recycle mud.
- The viscous shearing of drilling fluids should be adjusted and controlled by conventional additives, the use of FCLS is prohibited when necessary.
- The high-pressured brine layer should be guarded against, high-density drilling fluids should be used in the well killing jobs once discovered immediately.
The maintenance for drilling fluids in gypsum-salt formations should be based on the following process:
- The drilling fluid system is transformed into undersaturation salt-water polysulfonate drilling fluids or saturation salt-water polysulfonate drilling fluids before drilling into the top of gypsum-salt formations, which could avoid a series of downhole complexity and accidents, including hole shrinkage, large borehole, borehole collapse, and so on.
- The transforming of drilling fluid system from polymer drilling fluids into saturation salt-water polysulfonate drilling fluids is very complex. The sand settling in tapered shaped tank should be cleaned up. The volume of polymer drilling fluids in the first stage should be calculated on the basis of bentonite content.
- The dissolved liquid glues of 80-A51 and MAN-101, and a half of stoichiometric salts are added to drilling fluids. SMP-II, SMC, FT-1 are added after 1-2 weeks of circulation, and the remaining half of salts are added to drilling fluids. The properties of drilling fluids are tested whether the performance of drilling fluids could meet the design requirement. Then the drilling fluids are weighted to designed density by barite.
- The shale shaker screen should be used well to prevent the slats and additives from shaking out before solution.
- The friction coefficient of drilling fluids should be tested whether the drilling fluids should be added by lubricant or crude oil.
- Various performance indexes of drilling fluids should be tested to meet the requirements of drilling operation in salt-gypsum formations.
- Various performance indexes of saturation salt-water polysulfonate drilling fluids should be controlled well to maintain borehole stability. The field applications indicate that borehole stability could be achieved when the concentration of Cl- 17~19×104 mg/L, and the recrystallization could be avoided effectively.
- The mud density should be controlled on the upper limit of the design. Reasonable mud density should be used to avoid the hole shrinkage and creep deformation of gypsum-salt formations.
- The filter loss of drilling fluids should be controlled within the design range. The mudcake should be thin and tough. Sulfonated filtrate reducer and anti-sloughing agent should be added to drilling fluids to ensure the safety drilling in gypsum-salt formations.
- The desander and desilter should be used as often as possible to maintain good properties of drilling fluids. The use of centrifuge should be based on the properties of drilling fluids and sand content.