Historically, the surface holes of the wells in the West Azeri field of the Caspian Sea were drilled conventionally with seawater and gel sweeps. However, seawater destabilized the highly reactive soil formations in the surface interval, resulting in unacceptable movement of the 20-in. casing. Further examination identified mechanical and chemical destabilization of the reactive shallow soils as the root causes of this instability.
This paper describes the application of a silicate-based drilling fluid system1,2,3,4,5,7,8 in tandem with a unique Mud Recovery System (MRS) that combined to stabilize the problematic formation and allowed for the successful setting of the 20-in. casing without the lateral movement experienced in offset wells.
The operator conducted laboratory tests to identify possible fluid alternatives that would provide the necessary chemical stability to provide support for the template and casing. The laboratory investigation considered conventional SBM, glycol/polymer/KCl and silicate/polymer/KCl inhibitive water-based fluids, and a KCl/NaCl/high-performance waterbased mud (HPWBM) as possible alternatives. The oil-based, HPWBM and silicate systems demonstrated excellent chemical stabilization. Offshore discharge restrictions negated the use of oil-based drilling fluids if the MRS was used. The HPWBM provided an alternative but logistics eliminated it from the initial consideration, thus opening the door for the first-time use of the silicate-based system in the Caspian Sea.
The authors also review the application of the MRS system that utilized the rig pumps to deliver the fluid down the drill pipe and up the annulus to specially designed equipment on the sea floor. The latter used a sub-sea disc pump to transfer the fluid to the surface for conventional solids processing and maintenance. This technique provided a dual gradient to control the equivalent circulating density.
Three field tests were conducted to determine the best fluid and mud weight for use in drilling the surface holes for the operator's template in the West Azeri field. The first test with the MRS unit used a conventional silicate/polymer/KCl system; the second test employed a synthetic-based drilling fluid with a pin-connector and a riser. A third trial utilized the silicate/polymer/KCl system with increased concentration levels of silicate and KCl and a higher mud weight. The authors will review the fluid planning, the performance of the three field trials, and the results which have been implemented into an ongoing field development program.
Introduction
The shallow soils in the West Azeri field are highly reactive and deformable; the pore fluid is nearly salt-saturated whereas the Caspian seawater is merely brackish. Initial drilling experience using conventional practice of seawater and high viscous sweeps resulted in severe washouts and pack-offs with eventual lost circulation. The initial template was abandoned due the unacceptable movement of the surface casings.
In order to solve the instability problem, laboratory inhibition tests using Azeri soil samples were run to identify possible drilling fluid solutions. Inhibition tests in the laboratory showed the soil dispersed rapidly in seawater and various brines, thereby, identifying that salinity alone was not the solution.10
Geomechanical studies suggested that an increased mud weight was required in addition to chemical stabilization.9 The application of the mud recovery system (MRS) provided a solution to allow using a higher mud weight, minimizing the effect on the equivalent circulating density (ECD) at the shoe. The unit provides transfer of the fluid and solids from the seafloor to the surface via a sub sea disc pump. This technique provides a dual gradient condition resulting in ECD pressures lower than those observed with a conventional riser.
Laboratory tests evaluated several inhibitive water-based alternatives including glycol, silicate and high-performance water-based muds (HPWBM).
Caspian seawater and kerosene (simulating OBM performance) were used as reference benchmarks. Based on this data, as well as logistics contingency, a silicate system was selected for a trial evaluation in the Azeri field. The objective of the field trial was to set 30-in. conductor and drill a 24-in. hole without disturbing the surrounding formation. Additional criteria for success were satisfactory drilling performance, wellbore strength, and an in-gauge wellbore.
