Monitoring Oil/Water Fronts by Direct Measurement (includes associated papers 23943 and 24103 )
Authors: Neil K. , Dunlop B. , King Geoffrey A. , Breitenbach E. Allen 
Published: December 2007

Summary.
A monitoring techniquethat locates moving reservoir fluidsby taking the difference betweensuccessive seismic surveys isdescribed. Field measurements areprovided by repeatedsurface-seismic-reflection surveys of limited extent. The seismic measurementsare processed to minimize noise and toderive the difference betweensuccessive surveys. All acoustic, production, and reservoir data are thenintegrated by reservoir simulation todefine fluid interfaces and volumes. The simulation model provides improved performance predictions. Theseismic process has been validated in a field under waterflood. Field-trialresults are discussed.

Introduction
Direct measurement of the position of reservoir fluids during production has greatpotential for improving recovery because itprovides advance warning of changes inproduction behavior that may be used to prolong well life, or to place new wells moreeffectively. Knowledge of fluid movement in thereservoir is a key to enhanced productionmanagement and determines the quality of performance predictions. Currently, wellsprovide most of the data available duringproduction. Fluid breakthrough at a well, however, usually signals a decline in thewell's production rate. Reliable advanceknowledge of the locations of fluid interfacesaway from wells would allow remedialactions to be taken to extend the economiclives of these wells. This paper describes amonitoring technique that uses repeated seismic surveys to detect saturation changes atlocations distant from wells. The monitoringtechnique integrates the data currently usedfor prediction of reservoir behavior withmeasurements made by differencing repeated surface-seismic-reflection surveys. Thecontinuity of seismic information along crosssections is integrated with the 3D coverageprovided by simulation. This creates a fieldmodel in which the fluid locations can beverified by direct measurement. Repeatedsurveys and simulations provide feedbackthat refines the accuracy of the field model. Simulation then provides more reliablepredictions of future field behavior.
The technique is novel because itsystematically integrates production datagathering-seismic and all available engineeringdata-to produce a verified field model. Theuse of software to integrate the engineering and seismic measurements encourages jointcontributions from many disciplines.
Surface seismic monitoring of water/oildisplacements has not been reported in theliterature. A number of authors havereported the use of seismic for monitoring EORprocesses, in some cases with multiplesurveys. Greaves and Fulp reported use of multiple 3D surveys to track the effects of a combustion pilot in Texas. Alderman etal., Pullin et al., and Singhal and Cardreported on combustion and steamfloodmonitoring in the Athabasca tar sands of Alberta, Canada. Lawyer and Henleyreported the successful monitoring of gasmovements in the case of a gas cap and of injected CO2, respectively.
Theoretical and experimental studies onthe petrophysical parameters suggest thatseismic monitoring can identify water/oildisplacements. Nur and Wang and Tosayaet al. provided experimental resultsdescribing the acoustic velocities of compressional (P) waves in both pure hydrocarbons and natural crude oils over a range of pressures and GOR's. Surprisingly largechanges in acoustic velocity were exhibitedwith variations of temperature, pressure, anddissolved gas content. Rafipour examinedthe variation of P-wave velocity at differentfluid saturations in clastic core samples and noted the significant change in seismicresponse that may be expected.
Seismic monitoring requires that a number of variations be made to the conventionalseismic data-gathering scheme. Thesevariations result from the need for goodamplitude resolution and repeatability between measurements made at different times. Processing is also altered because improvement of the signal-to-noise ratio from the initialseismic survey, which is used as a baseline, is achieved by summation over multiplesurveys in areas of the field that are unaffectedby saturation changes.
In this paper, seismic monitoring isdescribed and validated through application ina waterflood in a field producing onshorefrom a thin shaly sand reservoir buried at1,850 ft [564 m] subsurface. Results areextended in a theoretical discussion to showthat the technique may be applied to reservoirs of varying thicknesses and fluidcontents and at a wide range of depths in mostlithological environments.

Monitoring Technique
Reservoir monitoring, as presented here, isa complete technique for gathering andanalyzing data during production. Its aim isto provide the information necessary tomake informed production-managementdecision.
JPT
May 1991
P. 596^