Critical path reviews and optimization efforts are a continuous process that must be directed at solids control operates. The handling of fluids in heavy oil operations remains a significant challenge in terms of solids control and drilling waste management.
The heavy oil characteristics have created challenges in fluid handling and drill solids removal. With a specific gravity equivalent to water, the heavy oil does not float on the surface of the tanks for easy removal but instead is incorporated into the drilling fluid. The heavy oil also contains dissolved gas which evolves slowly once the oil is at surface conditions. This creates a foaming effect, making fluid transfer difficult.
The initial solids control system utilized three cascading shale shakers at a scalping station near the rig floor. A mud cleaner and high speed centrifuge were mounted on the central mud system. The cascading shakers have been run with screens as fine as 84 mesh on the lower shakers when drilling the oil sand and pumping fluid at 500 GPM. These parameters and screen size require that the shakers be monitored continuously. The mud cleaner was generally ineffective due to what appeared to be screen blinding but could possibly have been compaction and adhesion of the sand to itself and to the screens caused by the asphalt content of the heavy oil.
The mud cleaner was replaced by two low speed decanter centrifuges. This system worked well but the abrasive nature of the sand and the high sand content associated with extremely high drilling rates quickly eroded the decanter centrifuges and further use of this system was deemed to be uneconomic. A desilter was then added to the system ahead of the decanter centrifuges in an effort to remove some of the sand. A high level of wear on the decanter centrifuges was still experienced.
The present system replaces the two low speed decanter centrifuges and the desilter with a single two-cone enhanced hydrocyclone that is fed by two 100 HP pumps. Since the discharge from this unit is sufficiently dry, no further
processing is required. It makes a fine cut, possibly as low as 20 microns, greatly reducing wear on the remaining high speed decanter centrifuge which is performing well. Experience indicates that this hydrocyclone works best when very coarse, 38 mesh screens are used on the primary shakers. This also eliminates constant cleaning of the shakers.
Transferring the drilling fluid from the well to the central mud system poses a challenge once the oil content goes above the range of 4 to 5%, which it often does when drilling long sections of productive sand. Several changes to the
centrifugal transfer pump suctions have been made to date, without significant improvements.