Seismic modeling of gas chimneys Børge Arntsen 1 , Lars Wensaas 1 , Helge Løseth 1 , and Christian Hermanrud 1 ABSTRACT We propose a simple acoustic model explaining the main features of gas chimneys. The main elements of the model consist of gas diffusing from a connected fracture network and into the surrounding shale creating an inhomogeneous gas saturation. The gas saturation results in an inhomoge- neous fluctuating compressional velocity field that distorts seismic waves. We model the fracture network by a random- walk process constrained by maximum fracture length and angle of the fracture with respect to the vertical. The gas satu- ration is computed from a simple analytical solution of the diffusion equation, and pressure-wave velocities are locally obtained assuming that mixing of shale and gas occurs on a scale much smaller than seismic wavelengths. Synthetic seis- mic sections are then computed using the resulting inhomo- geneous velocity model and shown to give rise to similar de- terioration in data quality as that found in data from real gas chimneys. Also, synthetic common-midpoint CMP gathers show the same distorted and attenuated traveltime curves as those obtained from a real data set. The model shows clearly that the features of gas chimneys change with geological time a model parameter in our approach, the deterioration of seismic waves being smallest just after the creation of the gas chimney. It seems likely that at least some of the features of gas chimneys can be explained by a simple elastic model in combination with gas diffusion from a fracture network. INTRODUCTION Vertical zones of deteriorated seismic data quality are found on data from several chalk fields of the Central North Sea, such as Al- buskjell D’Heur, 1987, Ekofisk Pekot and Gregory, 1987, Eldfisk Michaud, 1987, Hod Norbury, 1987, Tommeliten D’Heur and Pekot, 1987; see also Figure 1 and Valhall Muns, 1985. These zones of deteriorated seismic data quality are often referred to as gas chimneys. The presence of gas chimneys have been interpreted as hydrocarbon leakage pathways, and mapping of such chimneys by neural network techniques has been established as an exploration tool Meldahl et al., 2002; Ligtenberg, 2003; Heggland, 2004. The disturbed seismic data quality is associated with reduced acoustic velocities Dangerfield, 1992 and gas presence in overbur- den rocks Muns, 1985. The influence of gas on the seismic image was further confirmed by Granli et al. 1999, who described imag- ing through the gas chimney above the TommelitenAlpha field using marine shear wave data. Shear waves show little interaction with fluids and were expected to penetrate gas-filled sediments without being significantly dis- turbed. The imaging of the overburden rocks improved significantly along the edges of the chimney where the raypaths of the downward moving compressional waves did not pass through the chimney, but the raypaths of the upward-moving shear waves did. Gas presence in sediments alone does not lead to deteriorated seis- mic signals, and the seismic imaging within and below gas-filled res- ervoirs are not in general poorer than outside gas-filled reservoirs. Deterioration of seismic signals because of gas presence is thus de- pendent on a specific distribution of gas within the sediments. A theoretical model for acoustic wave propagation through par- tially gas-saturated sediments was proposed by White 1975. This model predicts strong attenuation of seismic waves caused by inter- action of fluid flow with the elastic waves, and requires inhomoge- neous gas saturation on a scale of a few centimeters to yield signifi- cant attenuation. An alternative model by Lerche 1986 uses an acoustic medium with randomly fluctuating gas saturation on a scale of the seismic wavelength to predict properties of seismic waves propagating through gas chimneys. He relates the fluctuating gas saturation to fluctuations in the seismic velocity and derives analytical solutions of the resulting wave equation. The key observation here is that in- creasing gas saturation reduces compressional wave velocity, and even small amounts of gas imply a significant reduction in compres- sional-wave velocity. O’Brien et al. 1999 uses a detailed velocity model to simulate the seismic effect of the gas chimney at the Valhall field. This model is based on the perceived existence of layers with low and laterally variant velocity. Synthetic seismic data reproducing the observed Manuscript received by the Editor November 29, 2006; revised manuscript received March 8, 2007; published online August 23, 2007. 1 Statoil, Trondheim, Norway. E-mail: barn@statoil.no; lawe@statoil.no; heloe@statoil.no; che@statoil.no © 2007 Society of Exploration Geophysicists. All rights reserved. GEOPHYSICS, VOL. 72, NO. 5 SEPTEMBER-OCTOBER 2007; P. SM251–SM259, 19 FIGS. 10.1190/1.2749570 SM251