The impact of faults representation on history match and future generated seismic impedance response in reservoir models. Case study for Pierce Field, North Sea

Transmissibility multipliers provide a simple way of accounting for the effects that faults have on fluid flow in production simulation models. Recent studies have shown that transmissibility multipliers can be quantified from parameters such as fault thickness and permeability (Knai and Knipe, 1998; Manzocchi et al., 1999). Generally, these latter parameters must be approximated using empirical relationships with some host rock properties such as V-shale or net-to-gross ratio as well as fault displacement magnitude (Manzocchi et al., 1999). In this study, the effects of two different representations of faults on the history match and the predicted (simulated) seismic impedance are compared for the Pierce Field, North Sea. In the first case, fault transmissibilities were adjusted and the faults were extended in a trial and error manner to improve the match to fluid contacts and production history. In the second case, a step-by-step derivation of the fault transmissibilities in the Pierce Field was adopted based upon the integration of collected and upscaled properties of the deformed and undeformed reservoir as well as empirical relationships between fault offset and thickness. Historical production data were used to compare the effectiveness of the two methods. Furthermore, a synthetic seismic impedance response was generated from both simulation models, using a velocity model based on the Gassmann-Biot theory of elastic wave propagation in fluid saturated porous solid (Gassmann, 1951; Biot, 1956). This allows the assessment of different fault representations when trying to predict optimal times for further time-lapse seismic surveys. The results demonstrate the effectiveness of the derived transmissibilities model in generating a satisfactory history match in a relatively short time period. The generated seismic models show that, by acting as baffles/barriers to fluid flow, faults can play a major role in creating residual seismic features that can be observed on 4D seismic surverys. Incorporating realistic fault rock properties into the simulation model, which is used to create synthetic seismic, can therefore allow geophysicists and engineers to better plan the timing of 4D seismic surveys so that they have the greatest influence on field development decisions. Introduction After a production simulation model has been constructed, it must be tested to determine whether it can reproduce the previous production behavior (i.e. fluid flow rates, pressures etc.) of the reservoir. Modelling past performance will identify weakness in the input parameters to the model, suggest possible modifications and demonstrate the quality of the reservoir description that is eventually accepted before future reservoir performance is modelled. In structurally complex reservoirs (e.g. densely faulted), history matching can be a time-consuming and frustrating process due to the large number of parameters that affect performance. Indeed, history matches are inevitably non-unique and therefore the best efforts should be made to ensure that all input parameter are physically/geologically realistic. New technologies are providing increased amounts of data that allow this nonuniqueness in the history match to be reduced. For example, time-lapse (4D) seismic surveys can be used to place more constraints on the fluid flow behavior of reservoirs (Greaves et al., 1987). To use time lapse seismic effectively, acquisition and processing must be as similar as possible for each survey in the series to ensure that differences can be related to production effects and not merely survey artifacts. The power of this technique lies in its ability to map fluid movements in the lateral and vertical space of the reservoir thus enabling reservoir engineers to locate unswept zones. Of particular interest for the present study is possibility that time-lapse seismic can sometimes be used to identify the presence of sealing faults (Lumley and Behrens, 1998; Koster and Gabriels, 2000). Most benefit from a 4D is gained if the timing between the two seismic acquisitions coincides with a period in field development that the results are likely to have a significant Copyright 2005, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Europec/EAGE Annual Conference held in Madrid, Spain, 13-16 June 2005. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s).