Shallow water flow prediction using prestack waveform inversion of conventional 3D seismic data and rock modeling

Abstract

Shallow water flow (SWF) layers are frequently encountered in deepwater areas when drilling into poorly consolidated geopressured sands (Figure 1). These sands, when flowing, can cause extensive damage to a borehole. More than $200 million has been lost to date for remediation and prevention of SWF problems in the Gulf of Mexico. Lately, this problem has also been a concern in many other deepwater clastic basins in the world. Figure 1. Shallow water flow problem encountered in deepwater drilling. SWF sands are known to occur in water depths of 450 m or more and typically 300–600 m below the mudline. They are known to be present in almost all deepwater ocean basins where the rate of sedimentation is high. Figure 2 shows the formation of SWF layers in a deepwater environment. Loose and unconsolidated sediments with a high rate of sedimentation characterize the overburden and low permeability seal is created by compacted shales or mudstones for which the rate of sedimentation is low. If isolated sand bodies are in this shale or mudstone, water from such bodies will not escape easily due to the presence of low-permeability sediments around them. In addition, the high rate of sedimentation from the overburden exerts an enormous pressure on these sediments, causing these isolated bodies containing large amounts of water to be overpressured. These overpressured SWF layers pose a threat to drilling, and their identification prior to drilling is therefore important in reducing risk. In this paper, we study the feasibility of detecting SWF layers using prestack waveform inversion of seismic data in conjunction with geologic analysis of stratigraphic sequences related to SWF zones. Figure 2. Formation mechanism of SWF sands. In-situ measurements of elastic and other rock properties of SWF sediments are very limited because SWF layers are associated with very low sonic velocities. Measurement of such …