Shallow Water Flow Assessment from an Inverted Seismic Cube
Authors: Trabant Peter K. , Lepley Taylor 
Published: October 2008

A proprietary seismic process can identify potential drilling problems before costly after-the-fact drilling disasters are encountered. Considering the financial impact of a single unanticipated drilling problem, such as a shallow gas kick or an uncontrolled shallow water flow (SWF), a smart well plan should include a petrophysical evaluation of the 3D seismic data. It is an inexpensive insurance against those unwanted drilling trouble costs.

Transformed 3D seismic cubes can be used to assess the risk of encountering zones that may be capable of SWF. A recent geohazards study in the Gulf of Mexico (GOM) used exploration 3D seismic as its prime interpretive tool. High amplitudes indicative of a potential SWF zone were identified at a proposed location. Upon further investigation using these 3D cubes, the high amplitudes were found to lack appropriate reservoir indicators. Therefore, the SWF risk was downgraded, and the casing program proceeded as originally planned.

While there are many methods to transform seismic data, this process used proprietary algorithms based on prestack velocities, Vp/Vs ratios and long offsets on a 3D seismic cube. The transformed cubes displayed lithology, porosity and reservoir quality. Values associated with SWF zones were highlighted to emphasize position and magnitude prior to selecting a drill site location.

Though an interpretation can be constrained by seismic data quality, accuracy of velocity picks and assumptions used in converting seismic reflection characteristics to lithology, this technique adds value by offering an additional tool to assess shallow drilling risks. This can be particularly important since the shallow section of a well may be drilled riserless, in the case of deepwater wells, or drilled with only a diverter when using platform or jackup rigs.

Shallow Water Flows

Shallow water flows are a serious drilling problem. They occur when the drill-bit penetrates an overpressured SWF zone at a shallow depth prior to cementing the initial casing string and installing the blowout preventer (BOP). Sand and water flow uncontrollably out of the borehole from the overpressured formation. Generally, SWF formations lie at relatively shallow depths below the seafloor within the tophole range (0-3,000 feet below mud line) in water depths over 1,200 feet. They have been reported in 50 percent of deepwater wells in the Gulf of Mexico (Alberty, 2000) costing the industry millions of dollars due primarily to lost rig time. To reduce loss, it is essential that a detailed geohazards assessment be made prior to drilling and include SWF zone detection.

SWF zones occur primarily within overpressured, underconsolidated sands, deposited less than half a million years ago. Sand deposits produced during sea level lowstands are later covered by fine-grained deposits during sea level highstands. The combined rapid accumulation of thick sandrich deposits topped by fine-grained seals creates an environment in which pressures from the overburden coupled with a thick, impermeable seal prevents the formation from expelling water and generates overpressured conditions. Where sands have high porosity and permeability, SWF zones are produced. Examples of this type of formation are basin floor turbidites and channels.