Integrating well log, seismic, and CSEM data for reservoir characterization

Lucy MacGregor Lucy Macgregor
SEG 2011 Honorary Lecturer
Europe
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 Sponsored by Shell


Improved reservoir management and production optimization requires accurate characterization of reservoir properties and their changes through time. There are many different methods, measuring a plethora of different physical properties that can be used to interrogate the Earth and address these demands. Indeed, in recent years, advances in geophysical data acquisition and interpretation have led to significant improvements in the remote imaging of Earth structure and properties.

However, when only a single data type is considered, ambiguities in the interpretation can remain. Integration of different geophysical data types allows the strengths of each to be exploited to provide a better constrained estimate of Earth properties than can be achieved when only a single data type is considered. In this lecture, we will concentrate on three contrasting methods: surface seismic, marine controlled source electromagnetic (CSEM), and well-log data.

Well logs provide a high-resolution measurement of the properties of a reservoir and the surrounding strata; however, properties can only be determined in a small area local to the well. Often measurements of reservoir properties across the extent of a field are desirable for reservoir management or production optimization. Remote geophysical measurements are therefore required.

Seismic data are most commonly used for this purpose, providing in the first instance high-resolution images of subsurface structure and stratigraphy. Amplitude variation with offset (AVO) and inversion for acoustic and elastic impedance may also be used to constrain properties such as elastic moduli and density. However, seismic data alone in many situations cannot give a complete picture of the reservoir. For example, AVO anomalies may be caused either by fluid or lithological variations, which cannot be separated on the basis of the seismic data alone. In some instances, although the presence of gas in a reservoir can be determined, it is difficult or impossible to determine whether the saturation is sufficient for the accumulation to be commercial (Figure 1). In addition, calibration between impedence and the underlying rock and fluid properties is also a complex process, requiring careful application of rock physics models developed for a given geologic scenario.

Gas saturation

CSEM uses a high-powered source to transmit low-frequency signals through the Earth to an array of receivers. By interpreting the received signals using forward modeling and inversion approaches, the resistivity structure of the sea floor can be determined. Resistivity well logs often show that commercial hydrocarbon deposits may be many times more resistive than surrounding lithologies. In principle, such variations should be readily detected using CSEM tools. In contrast, seismic data are sensitive to boundaries between lithologic units but are often less sensitive to fluid changes within these units. Given high-quality seismic and well data and sophisticated seismic inversion and rock physics tools, we can sometimes relate seismic changes to saturation effects. Nevertheless, the change in resistivity caused by variations in saturation should be much easier to detect.

However, despite the sensitivity of resistivity data for determination of saturation, there are two inherent challenges to interpreting CSEM data. First, the structural resolution of CSEM data is poor. Second, the cause of resistivity anomalies (particularly high-resistivity features) cannot be uniquely linked to the presence of hydrocarbons in the subsurface when taken in isolation. In many situations, these are equally likely to be caused by other high-resistivity material (for example, tight carbonates, salt, or volcanics).

The limitations in each method outlined above can be mitigated using an integrated approach to geophysical interpretation. Seismic information can determine the reservoir structure (but potentially not its content or extent). Well logs provide independent constraints on the reservoir and the overburden structure but these are local to the well. Within this constrained interpretation framework, CSEM can provide further complementary information on the content of a reservoir. This is illustrated in Figure 2 which shows the results of an integrated interpretation workflow in which seismic and CSEM inversion results are combined to provide an estimate of gas volume across a reservoir.

Geophysicists have a range of tools at their disposal to study the Earth, and no single one provides all the answers. Integration of these methods is in itself challenging: different physical processes occurring at different scales must be reconciled into a consistent interpretation, and these challenges have only begun to be addressed. However, early studies have shown the strength of an integrated approach to data analysis in providing quantitative information on rock fluid properties.

Lucy MacGregor
September 2010