Asghar Shams

Adaptive scaling for an enhanced dynamic interpretation of 4D seismic data

In this study, importance is drawn to the role of engineering principles when interpreting dynamic reservoir changes from 4D seismic data. In particular, it is found that in clastic reservoirs the principal parameters controlling mapped 4D signatures are not the pressure and saturation changes per se but these changes scaled by the corresponding thickness (or pore volume) of the reservoir volume that these effects occupy. For this reason, pressure and saturation changes cannot strictly be recovered by themselves, this being true for all data interpretation. This understanding is validated both with numerical modelling and analytic calculation. Interestingly, the study also indicates that the impact of gas saturation on the seismic can be written using a linear term but that inversion for gas saturation can yield at best only the total thickness/pore volume of the distribution. The above provides a basis for a linear equation that can readily and accurately be used to estimate pressure and saturation changes. Quantitative updates of the static and dynamic components of the simulation model can be achieved by comparing thickness or pore volume-scaled changes from the simulator with the corresponding quantities on the inverted observations

The interpretation of amplitude changes in 4D seismic data arising from gas exsolution and dissolution

This study examines the four-dimensional (4D) seismic signatures from multiple seismic surveys shot during gas exsolution and dissolution in a producing hydrocarbon reservoir, and focuses in particular on what reservoir information may be extracted from their analysis. To aid in this process, hydrocarbon gas properties and behaviour are studied, and their relationship to the fluid-flow physics is understood using numerical simulation. This knowledge is then applied to interpret the seismic response of a turbidite field in the UK Continental Shelf (UKCS). It is concluded that for a repeat seismic survey shot 6 months or more after a pressure change above or below bubble point (as in our field case), the gas-saturation distribution during either exsolution or dissolution exists in two fixed saturation conditions defined by the critical and the maximum possible gas saturation. Awareness of this condition facilitates an interpretation of the data from our field example, which has surveys repeated at intervals of 12–24 months, to obtain an estimate of the critical gas saturation of between 0.6 and 4.0%. These low values are consistent with a range of measurements from laboratory and numerical studies in the open literature. Our critical gas-saturation estimate is also in qualitative agreement with the solution gas–oil ratios estimated in a material balance exercise using our data. It is not found possible to quantify the maximum gas saturation using the 4D seismic data alone, despite the advantage of having multiple surveys, owing to the insensitivity of the seismic amplitudes to the magnitude of this gas saturation. Assessment of the residual gas saturation left behind after secondary gas-cap contraction during the dissolution phase suggests that small values of less than a few per cent may be appropriate. The results are masked to some extent by an underlying water flood. It is believed that the methodology and approach used in this study may be readily generalized to other moderate- to high-permeability oil reservoirs, and used as input in simulation model updating.

Calibration of Simulator to Seismic Modeling for Quantitative 4D Seismic Interpretation

Realistic quantitative match between the results of simulator to seismic modelling and observed seismic can be used to update the reservoir model. For this purpose, it is vital to accurately adjust the parameters involved in petro-elastic model (PEM) and seismic modelling as the two controlling components of Sim2seis analysis. The PEM parameters are calibrated from a Sim2seis perspective using well log data. It is important to adapt the PEM parameters according to the lithology definition at the simulation cell scale, which is different from the log scale. The convolutional model (CM) is calibrated by comparison of the results with a full waveform pre-stack finite difference (FD) seismic modelling approach. The results show that if enough care is taken, CM can produce reliable results for 4D analysis. CM amplitudes are consistent with FD, while for time shift, in the case of small time shifts the error between the two methods can be of the same magnitude as the 4D signature.

Updating the Reservoir Model Using Engineering-consistent 4D Seismic Inversion

An updating strategy is designed to iteratively close the loop between the fluid flow simulation predictions and measured production history, predicted and observed 4D seismic data, and finally predicted and inverted impedance/impedance changes. The central ingredient in this scheme is the computation of elastic property changes that are inverted from the seismic in an engineering-consistent manner. The geometry, volumetrics and transmissibility multipliers for the reservoir model are updated in three successive stages by sequential comparison between the seismic and fluid flow domains. The workflow is implemented on a West African field, where reservoir model improvements are obtained in combination with a consistency between model, impedance and seismic domains.

Closing the Loop Using Engineering-consistent 4D Seismic Inversion

An updating strategy is designed to iteratively close the loop (CtL) between the fluid flow simulation predictions and measured production history, predicted and observed 4D seismic data, and finally predicted and inverted impedance/impedance changes. The central ingredient in this scheme is the computation of elastic property changes that are inverted from the seismic in an engineering-consistent (EC) manner. The geometry, volumetrics and transmissibility multipliers for the reservoir model are updated in three successive stages by sequential comparison between the seismic and fluid flow domains. The EC-CtL workflow is implemented on a West African field, where reservoir model improvements are obtained in combination with a consistency between model, impedance and seismic domains.

An Interpretation of the 4D Seismic Response to Gas Exsolution and Dissolution

This study examines the 4D seismic signatures of gas exsolution and dissolution in a producing hydrocarbon reservoir. The physical mechanisms for these reservoir processes are investigated using a series of simulation studies, and the primary controls on the seismic response are thus identified. It is concluded that gas can attain a steady state approximately three to six months after a major reservoir pressure change, and that in this state the seismic responds to the thickness of gas accumulations existing at either the critical and/or maximum gas saturation. Application to multiple repeated surveys over a North Sea turbidite field demonstrates the practical consequences of our findings and how this insight can be used to interpret the seismic amplitudes. Interpretation of the field example confirms a low critical gas saturation of less than 1% for the reservoir rocks. However, it is not found possible to quantify the maximum gas saturation using the 4D seismic amplitude only. Quantitative estimates of the volumes of gas liberated and dissolved in the oil are found possible by integrating material balance principles with the seismic response. It is shown that the insights gained from this application can be generalised to other moderate to high permeability hydrocarbon reservoirs.

Finding a Petro-elastic Model Suitable for Sim2seis Calculation

The petro-elastic model (PEM) is a necessary step in simulator to seismic modelling, intended to close the loop between the seismic and engineering domains. In this work, we discuss some fundamental issues within the conventional PEM algorithm, not commonly covered by published literature. Firstly, we explain the importance of the porosity rock model for the PEM. It is shown that both total/effective porosity models are able to generate satisfactory seismic results, provided that the density and bulk/shear moduli of the solid components are set correctly using an optimisation problem. We find the underlying connections between the simulation model parameterisation and the effective porosity model from the petrophysical domain. Finally, we discuss the effect of vertical upscaling on the seismic domain. We highlight the differences between property upscaling and reflectivity upscaling, and challenge the idea of developing a scale-dependent PEM based on Backus averaging. In addition to a sim2seis analysis, the results of this work have direct impact on seismic inversion via the PEM for pressure and saturation change or impedance change onto the reservoir grid. Keywords: PEM, rock porosity model, simulation model, effective porosity, total porosity, upscaling, Backus averaging, reflectivity upscaling.

An Engineering-consistent Inversion of Time-lapse Seismic Data

Engineering-consistent (EC) concept is built into a “coupled” 4D inversion workflow to represent the reservoir changes. The inversion, with geology and reservoir engineering constraints, is performed on a consistent reservoir grid. The result takes time shift into consideration and shows less non-uniqueness according to the production activities. We applied the method to a West Africa field and updated the reservoir model by improving its dynamic behavior and the consequent match between the synthetic and observed 4D seismic data.

Adaptive Engineering-based Scaling for Enhanced Dynamic Interpretation of 4D Seismic

In this study, importance is drawn to the role of engineering principles when interpreting and estimating dynamic information from 4D seismic data. It is found that in clastic reservoirs the principal parameters controlling mapped 4D signatures are not pressure and saturation changes per se, but the changes scaled by the corresponding thickness (or pore volume) of the reservoir volume that these effects occupy. Indeed, pressure and saturation changes cannot be recovered by themselves, and this is true for all data interpretation and inversion procedures. This understanding is validated both with numerical modelling and analytic calculation. Fluid flow studies also indicate that the impact of gas saturation on the seismic can be written using a linear term, and that inversion to gas saturation can only yield the thickness of the distribution. The above has provided a basis for a linear equation that can be used to easily invert for pressure and saturation changes. Quantitative updates of the simulation model can be achieved by comparing scaled dynamic changes from the simulator with the inverted observations.

Towards Quantitative Evaluation of Gas Injection using Time-lapse Seismic

Of particular concern in the monitoring of gas injection for the purposes of storage, disposal or IOR is the exact spatial distribution of the gas volumes in the subsurface. In principle this requirement is addressed by the use of 4D seismic data, although it is recognised that the seismic response still largely provides a qualitative estimate of moved subsurface fluids. Exact quantitative evaluation of fluid distributions and associated saturations remains a challenge to be solved. Here, an attempt has been made to produce mapped quantitative estimates of gas volume injected into a clastic reservoir. Despite the accuracy of the calibration using three repeated seismic surveys, time-delay and amplitude attributes reveal fine-scale differences though large-scale agreement in the estimated fluid movement. These differences indicate disparities in the nature of the two attributes themselves and highlight the need for a more careful consideration of amplitude processing for quantitative 4D seismic interpretation.