Geohazard Detection in Deepwater Clastics Basins Using a Seismic Technique Guided by Geology and Rock Physics Model: Methodology and Examples
Authors: Dutta Nader C. 
Published: October 2008

Detection of hazardous zones, associated with high-pressured fluids in unconsolidated sands and shales, prior to drilling, is essential for environmental as well as health and safety. Drilling for deepwater targets is associated with high cost and risk, while margins of commercial operations are small. Therefore, it is imperative to control cost through accurate well planning and reliable anticipation of geohazards.

This paper deals with a novel seismic approach that uses the full bandwidth and the entire offset range of the conventional 3D seismic data to detect the presence of hazardous zones. Both in shallow and deeper zones, P- and S-velocities are determined using seismic full waveform prestack inversion. Shallow water flow (SWF) layers in the deepwater are identified through the associated high ratios of P- to Svelocities. A new, rock model-based approach especially suited for deepwater pore pressure imaging was applied to predict the presence of both shallow and deeper over-pressured zones.

This paper covers the essential elements of the workflow - a "five-step process" for hazard identification that integrates a special data processing flow with prestack and full waveform inversion of 3D seismic data with geology and rock models appropriate for shallow water flow, gas hydrates and deeper geohazards. Real data examples from deepwater elucidate the approach.

Introduction

During the last two decades, the oil and gas industry has reported major discoveries in deepwater basins worldwide: Gulf of Mexico, Angola, Brazil, Nigeria and Mediterranean, for example. Along with these discoveries, the industry also encountered many risks. These risks are generally of two types: prospect risk associated with 'seal' integrity of potential reservoirs and various drilling hazards, such as those associated with Shallow Water Flow (SWF) sands, Gas Hydrates (GH) and geopressure. The former is due to geopressure build up in the sediments, especially in the shales surrounding the reservoir rocks, causing weakening of seals. The later is also associated with high pressure build up in sediments in the shallow portion of the stratigraphy and in an environment conducive of gas hydrate formation (appropriate thermal and pressure regimes). Pre-drill detection of these hazards and careful well planning are keys to managing these risks. Below we outline briefly the use of seismic data to help in this process.

Shallow Water Flow Sands

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. Shallow water flow sands are known to occur in water depths of 450 m or more, and typically between 300 m and 600 m below the mud line. They are known to be present in almost all deepwater ocean basins where the rate of sedimentation is high. Loose and unconsolidated sediments with high rates of sedimentation characterize the overburden. Compacted shales or mudstones for which the rate of sedimentation is low create a low permeability seal.