A current industry practice generally referred to as Pumping & Dumping speaks to a method of establishing subsea locations via riserless drilling. Riserless drilling of top holes and often by batch drilling a number of locations within a lease block is an attractive practice because a much less expensive floating rig may be utilized. Without the weight of a marine riser system and subsea BOP stack to buoy, smaller rigs can be efficiently used to establish subsea locations. Later, a larger semi submersible or dynamically positioned drill ship with marine riser and subsea (or surface) BOP drills to the total depth objective.
Under the best of circumstances, seawater is used as the drilling fluid. Mud and cuttings are discharged at the mudline onto the sea floor, thus the term "pumping & dumping". Where there is a risk of drilling riserless into a relatively shallow-to-the-mudline geohazard of abnormal pressure, a heavy, viscous mud may be required, this as a means of overbalancing the zone of abnormal pressure. Another purpose for overbalance may be to enhance wellbore stability. Shallow geohazards may be shallow gas or abnormally pressured aquifers. For example, continuing to drill riserless with seawater in the presence of a shallow water flow hazard may result in loss of the well. A volcanic-like eruption may flow from the subsea wellhead with such volume and velocity that the particulate fall-out could cover this and nearby batch-drilled locations.
A technology that offers the ability to overbalance abnormally pressured zones when drilling riserless with seawater that enables more precise wellbore pressure management with fewer interruptions to drilling ahead is indicated. A technology that recovers perhaps expensive mud and avoids the discharge of annulus returns into the sea further defines a growing industry need for a dual gradient riserless drilling system.
This paper discusses several approaches to accomplishing dual gradient riserless drilling. The drilling medium, whether seawater or a heavy viscous mud, is returned to the rig rather than single passed onto the seafloor. Such a method may also be referred to as "zero discharge riserless drilling".
Deepwater drilling has uncovered a variety of troublesome phenomena specific to those operations. Establishing the top hole (wellhead, surface conductor, conductor casings) by riserless drilling is no exception. In particular, riserless drilling coupled with geological anomalies in the first 3000 ft below mudline has resulted in various difficulties. One of the most important potential problems is shallow water flow (SWF) hazards, labeled as such because the origin of these flow problems is generally within the near-seafloor layers.
Shallow Water Flows have been reported in the Gulf of Mexico, in the southern Caspian Sea, the Norwegian Sea, and the North Sea. These abnormally pressured aquifers have been found in water depths ranging from less than 700 ft to more than 6000 ft. and may lie at seafloor penetrations between 900 ft and 3300 ft. (below the seafloor, e.g., mudline).