Seismic low-frequency effects in gas reservoir monitoring VSP data

Summary Conventional processing of VSP monitoring data at a fractured gas site did not reveal detectable changes, when gas was substituted by water. The application of low frequency analysis allowed us to find such changes which include the effects of stronger reflections and travel time delays at low frequencies. These effects match previously found effects in laboratory data and theory. The existing explanation suggests very low Q values for porous fluid saturated layers primarily as a result of internal friction between grains. This explanation needs further theoretical investigation. The amplitude and phase reflection properties can be used for detecting and monitoring fluid saturated porous layers. Introduction and Background The Northern Indiana Public Service Company (NIPSCO) operates naturally fractured reservoirs for seasonal storage of natural gas. The gas is injected during summer and withdrawn during winter. As part of DOE sponsored research in fractured gas production, Lawrence Berkeley National Laboratory (LBNL) conducted a VSP (Vertical Seismic Profile) experiment to aid delineation of NIPSCO's Trenton Formation reservoir, and to study the seismic effects of variable gas pressures (1).The reservoir is in NIPSCO's Royal Center field in Northern Indiana. The annual displacement of water by gas within the natural fractures of the reservoir make this field a good candidate for time-lapse monitoring. The Trenton formation is a Paleozoic Ordovician dolomite, which is part of the mostly shale and carbonate stratigraphy of the Royal Center field. It is believed by the field operators that the top section of the Trenton dolomite is unfractured and forms a cap for the reservoir. Thickness of the fractured reservoir within dolomite is 30 ft approximately. The depths of the reservoir are 980 - 1010 ft. There were two phases of time-lapse VSP acquisition at the field site with essentially identical acquisition geometry (source/receiver locations) under distinctly different reservoir conditions. During the initial survey in December 1996 the reservoir gas pressure was near its maximum of about 400 psi; during the second survey in May 1997 the reservoir gas pressure was reduced to about 250 psi. Since the natural water pressure within the Trenton formation is about 310 psi, the reservoir was mostly gas saturated in the 1996 survey and mostly water saturated in the 1997 survey. Acquisition There were four 9-component VSP data sets acquired and a single walkaway VSP. The sensors were 14Hz 3- component wall-locking geophones GS-20D in a 5 level string with 8 foot spacing between recording depths. The vibroseis sources (both P- and S-wave) used a 12 to 99 Hz sweep, 12 s long with a 3 second listen time. The S-wave trucks were positioned at each source site such that they generated an in-line and a cross-line polarized shear-wave (relative to the line connecting the source location and the well). The analysis in this paper will focus on the 100 ft. source-offset VSP, which had sensor depths from 296 to 1088 ft.