Identification of SWF Sands by Elastic Inversion of Conventional 3D Seismic Data
Authors: Lu Shaoming , McMechan George A. , Liaw Alfred 
Published: October 2008

As oil and gas exploration moves into deeper offshore areas, Shallow Water Flow (SWF) from overpressured, unconsolidated/uncemented sands becomes a high profile drilling hazard. Laboratory measurements show that SWF sands have high seismic compressional to shear velocity (Vp/Vs) ratios. Thus, elastic seismic inversion for Vp/Vs provides a quantitative identification of SWF sands, and predrill predictions of SWF.

A field data example uses conventional 3-D seismic data from the Garden Banks area of the Gulf of Mexico. Two wells provide constraints for the seismic data inversion. A known SWF zone in Shellâ??s well GB920 #1 shows high Vp/Vs values. In Kerr-McGee's well GB877 #1, Vp/Vs anomalies match fairly well with Fugro's predrill SWF sand predictions based on geological information and seismic amplitudes. Thus, the high Vp/Vs anomalies in the seismic data, calibrated at the wells, may be used to identify potential SWF sands away from the wells. The Vp/Vs values are quantitative, and are more diagnostic than seismic amplitude alone for SWF sand detection.

Introduction

As oil and gas exploration moves to offshore areas where the water depth is more than 1500 ft, SWF from shallow overpressured, unconsolidated and uncemented sands becomes a challenge to drilling. About 70% of all deep water wells have encountered SWF problem (Huffman and Castagna, 2001). The SWF problems result mainly from the narrow window between pore pressure and fracture gradient in SWF sands (Ostermeier et al., 2002): if the drilling mud-induced pressure is too low when drilling through these sands, water and sand may flow into the borehole and lead to the erosion of the hole sides and create a large cavity. This cannot be overcome simply by increasing the mud weight. The sands may fracture and lead to lost mud returns and loss of well control if the mud weight is too high (Bruce et al., 2002). Both situations are serious drilling hazards.

Several hundred million dollars may have been spent on SWF prevention and remediation (Ostermeier et al., 2000) since SWF problem was first recognized in 1985 in the Gulf of Mexico. In our study area, significant cost was involved in Kerr-McGeeâ??s well GB877 #1 in Pump and Dump Operations to control the potential shallow water flow in the shallow section. An accurate prediction of SWF sands before drilling is critical for the selection of well sites, well design, and operation procedures, and therefore has a major financial impact on the drilling of deep water prospects.

The petroleum industry commonly predicts SWF sands by interpreting the amplitudes of high resolution 2D and conventional 3D seismic data within the stratigraphic framework (McConnell, 2000). However, shallow sands can not be reliably detected directly from seismic amplitude alone, because of the relatively low contrast in acoustic impedance at sand/shale interfaces at shallow burial depth (Ostermeier et al., 2002). More diagnostic seismic attributes for SWF sands are desired to reduce the ambiguity.

Laboratory measurements show that SWF sands have high Vp/Vs values and high attenuation (Huffman and Castagna, 2001; Prasad, 2002). Huffman and Castagna (2001) have used the Vp/Vs anomalies inverted from multi-component seismic data to identify the potential SWF sands.