As E&P companies moved into deeper water, they encountered geologic and environmental obstacles never seen before. Other operating issues they ran into were not new but were more complicated to resolve because of the water depths and subsea conditions. Some still vex them.
The Loop Current and eddies
A clockwise current of water from the Caribbean Sea into the Gulf of Mexico between Cuba and the Yucatan creates a flow known to ocean scientists and meteorologists as the Loop Current. Panel 1 of figure 1-1 shows three stages of the Loop Current as it extends further into the Gulf. The movement of this current is more or less random both in timing and the extent to which it penetrates the Gulf. On its exit from the Gulf, it changes name to the Florida Current, a tributary of the Gulf Stream.
Fig. 1-1. The Loop Current and eddy currents in the Gulf of Mexico
As in panel 2, eddy currents spin off the Loop Current and work their way westward as a huge column of warm water, spinning in a clockwise fashion. In panel 3, the larger eddy current creates smaller, cold-water eddies, rotating in a counterclockwise direction. Eventually, both the cold and warm water eddies move westward into shallow water (panel 4) and dissipate.
The velocity of the current as the eddy spins can be 2 to 4 knots. As the eddies pass by an oil and gas drilling or producing operation, they subject the facilities to unusual stress and vibration. In addition, depending on the position of the platform and the eddy, the current may increase in one direction and then perhaps change to the opposite some time later.
The swirling eddy currents drift slowly westward in the Gulf at about one knot. Their diameters vary. As a result, a platform or drilling operation may see the effects of one eddy current for a day and the next one forperhaps a week or a month.
During an eddy current episode, the drilling riser from a semi-submersible or a drillship may bend or bow from the current to such an extent that the vessel has to change positions to stay connected. In some cases, the distortion can be so exaggerated that the drill pipe rubs against the drilling riser, a situation that warrants shutdown of operations before failure. At a producing operation an eddy can cause a riser to vibrate, inducing worries about metal fatigue and ultimate failure. Some mechanical devices can deflect the vibration effect of the currents on risers, but add to the drag.
The random nature of the eddy currents vexes the industry and begs for mechanical solutions to mitigate its effects. No known method to prevent the eddy currents themselves is on the horizon or even likely. E&P people just learn to live with them.
Occasionally drillers run into drill sites that have special geologic problems at depths of less than 1,000 or 2,000 feet below the sea floor— excessive faulting makes drilling and well control more difficult; thin layers of gas can disrupt what should be easy drilling.
Special processing of seismic data helps identify the presence of these hazards, and when they are found, the solution is often to spud the well in an offset location and directionally drill the well around them to the target bottom hole location. Eliminating the delays that these shallow hazards sometimes cause usually offsets the extra cost of the directionally drilled well.
Shallow water flows
Another geologic quirk prevalent in the Gulf of Mexico happens when sand layers in the first 2,500 feet became slightly over-pressured during their original deposition. As the drill bit penetrates these layers, the contained water wants to flow into the well bore. Increasing the weight of the drilling mud is the normal antidote to prevent inflows, but often the rock and sands at these depths are very young geologically and therefore have little strength. The extra mud weight can fracture other layers in the vicinity of the shallow water flows, causing loss of drilling mud and other well control problems. The situation calls for carefully setting casing at selected depths to isolate the troublesome sands, a time consuming and costly solution, but a necessary procedure if the well is to reach the target depth.
Many deepwater reservoirs have much more compartmentalization within themselves—more faulting, less homogeneous sediment and less continuity. That results in a larger number of smaller reservoirs, making the field development more difficult and costly. Using multiple completion or multilateral wells and smart well technology in these cases can reduce the number of wells and total cost.
Many deepwater reservoirs are also very young in geological age. As they are produced, the reservoir pressure drops and the sands can compact due to the weight of the overlying rock. The result can be a discouraging reduction of the porosity and permeability and the productivity of the well, as well as the ultimate recovery. This phenomenon can even cause subsidence on the ocean floor, threatening the integrity and safety of a platform. Engineers pay careful attention to possible subsidence in the design phase of both fixed and floating platforms above or near these reservoirs.
In some reservoirs, even as deep as they are, the pressure is so low that artificial lift is required from the beginning of production, as in Shell’s Perdido Field in about 8,500 feet of water.
In other installations, gas is pumped down the annulus between the production tubing and the casing and injected into the production tubing at predetermined ports. The gas reduces the density of the fluid moving upward. That lowers the pressure needed to push the oil to the surface and facilitates flow from the reservoir and allows longer life and higher ultimate recovery.
Gas pressure issues
Water temperatures at the deepwater sea floor are typically less than 40°F, sometimes as low as 32°F. For wells with high proportions of water and natural gas, the formation of gas hydrates, a crystalline form of methane with the behavior of icy slush, can block wellheads and flowlines. If the equipment is designed for it, injection of chemicals such as methanol can revert these hydrates back to gaseous form.
Drilling and completing wells in the deepwater continually encounters new problems and challenges. All have to be dealt with in real time, and all lead to incremental learning in preparation for the next well.