Industry experience has emphasized the necessity for quick detection and quick control of shallow water flows. Pressure While Drilling (PWD) instruments showing an increase in annular pressure when drilling riserless with seawater will indicate produced sand. Shallow, well-developed, clean, geopressured formations will produce very large quantities of brine during an uncontrolled drilling operation. Because of high fluid velocities and the unconsolidated nature of shallow formations, large volumes of produced sand—and subsequent borehole enlargement—should be expected.
There are options for successfully drilling shallow water flow zones. A common scenario is to drill riserless with seawater using high viscosity sweeps while also having kill fluid ready. If a saltwater flow occurs, evaluate the risk and cost scenarios of continuing to drill under-balanced versus containing the flow with large volumes of weighted fluids. This paper reviews a number of approaches using specialty fluids for drilling, along with their case histories.
Hundreds of case histories have proven that foam cement is the preferred approach to cementing casing across shallow water flow zones. Benefits include but are not limited tovastly-improved displacement efficiency;flow-control owing to the fact that an expansive fluid is in place during the phase transition from a liquid slurry to a gel with strength sufficient to suppress water flows;ease of on-location job modifications.
However, specialized blends are often required so that the cement (foamed or non-foamed) will function as intended at the low temperatures encountered across these zones. Use of these blends can complicate material handling logistics, especially on rigs with limited bulk volume capacity. The paper discusses the benefits of an all-liquid additive approach that allows the use of any Class A or Class H neat cement that may be on a rig. The chemistry of the additive package produces a slurry with the same capacity to control shallow water flows as the specialty blends. Case histories of these systems will be discussed.
The paper also discusses a decision flow chart designed to help the engineer better plan for controlling water flows related to drilling through shallow water flow environments.
Introduction
Shallow water flows (SWF's), which are formed in a variety of rapid-depositional environments including channel and turbidite sands and rotated slump blocks, present drilling challenges in many deepwater locations around the world.1 There is also evidence that some flow zones are the result of water communicating upwards via faults from thousands of feet lower being trapped under a shallow impermeable cap sediment. The problem has been established as costly to prevent, but even more so to remediate once a flow reaches the mud line. One study reported that out of 123 wells drilled in deepwater areas known for shallow water flow problems, 71% were successfully contained once behind pipe, 24% could not be repaired, and only 4 to 5% were found to not have encountered water flows.2 In this study, it was also found that $30 million had been spent in prevention, while some $137 million had been spent on remediation.
When drilling wells in deepwater, the first one or two strings of pipe are usually drilled riserless. This is not only due to the lack of large-diameter risers, but it also allows deeper setting depths for surface casing, as the weak, shallow formations may not tolerate the hydrostatic head exerted by a fluid-filled riser. The absence of the riser means that all drilling fluid is diverted to the seafloor and cannot be re-circulated. Therefore, high volumes of weighted muds must be built on-the-fly or held on standby vessels. If the later approach is used, inclement weather can interrupt operations.
