Abstract

Integration of time-lapse seismic and production data provides an effective tool for reservoir management. However, extensive work is required to build a "common" earth model1,2 honoring both types of data. This typically leads to longer turnaround times for time-lapse seismic projects. This paper demonstrates a method to shorten this time by reconciling production data with time-lapse seismic data in the data domain rather than in the earth model domain. The results from this method demonstrate that this quick evaluation can reveal whether or not the observed seismic differences represent reservoir changes associated with production, and can provide a basis for a more extensive model based evaluation. This method was applied to a gas reservoir in the Gulf of Mexico where preliminary rock physics modeling showed that water replacement would lead to more than 10% impedance change for approximately 30% of gas saturation change. The saturation-amplitude relation used to threshold the seismic difference volume was calibrated at the well or predefined locations. The threshold was optimized through solving the material balance relation obtained from production data with the time-lapse seismic data, thus making it more likely that the seismic differences are representative of saturation changes in the reservoir. This work:

  1. provides a new procedure for gas reservoir time-lapse analysis under certain geological and production conditions; and

  2. demonstrates a new first order method to reconcile time-lapse seismic data and production data without employing time consuming reservoir characterization and flow simulations. This approach greatly reduces the turnaround time involved in the use of time-lapse seismic information for reservoir management. Further, the result could lead to better subsequent model based analyses1,2.

Introduction

A pilot study was initiated to evaluate the feasibility of using legacy 3D seismic time-lapse data as a potential reservoir monitoring tool to assist in planning future development efforts in a shallow oil and gas field in the Gulf of Mexico. The reservoir selected for initial evaluation was the shallowest of a series of stacked oil and gas pays. This choice was based both on seismic data quality and the desire to eliminate any possible effects of production in overlaying reservoirs. The target reservoir produces gas from a faulted anticlinal trap at a depth of about 3000 feet, and is normally pressured by a strong water drive. The reservoir has been producing from three wells, two of which are no longer producing due to high water production. Two 3D seismic surveys have been acquired since the time of initial production; one in 1987 and another one in 1995. During this interim period approximately 26.6 BCFG has been produced. Maps based on the 3D data show good structural conformance of seismic amplitude with known hydrocarbon/water contacts, and indicate potential drilling locations in undrilled fault blocks up-dip from the depleted wells. The technique developed in this study involves use of the two existing (legacy) 3D seismic datasets, in conjunction with well logs and production engineering data, to provide spatial constraints on estimates of produced and remaining reserves.

Methodology

Material balance is a popular and effective tool for reservoir engineering. The use of material balance can help to estimate reserve in the reservoir at a specified time, and analyze the production mechanism. However, it is difficult to provide spatial information for the reserve calculated from production data alone. On the other hand, time-lapse seismic does contain information of the spatial change of fluid, but unfortunately suffers from ambiguity with respect to fluid saturation. Integration of the two types of information will help to resolve much of this ambiguity, providing spatial information for the change in reserve with time. This in turn provides a way to monitor the reserve spatially in an efficient way.

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