Marwan Wirianto

Time-domain modeling of electromagnetic diffusion with a frequency-domain code

We modeled time-domain EM measurements of induction currents for marine and land applications with a frequency-domain code. An analysis of the computational complexity of a number of numerical methods shows that frequency-domain modeling followed by a Fourier transform is an attractive choice if a sufficiently powerful solver is available. A recently developed, robust multigrid solver meets this requirement. An interpolation criterion determined the automatic selection of frequencies. The skin depth controlled the construction of the computational grid at each frequency. Tests of the method against exact solutions for some simple problems and a realistic marine example demonstrate that a limited number of frequencies suffice to provide time-domain solutions after piecewise-cubic Hermite interpolation and a fast Fourier transform.

Exploiting the airwave for time-lapse reservoir monitoring with CSEM on land

In the application of controlled source electromagnetics for reservoir monitoring on land, repeatability errors in the source will mask the time-lapse signal due to hydrocarbon production when recording surface data close to the source. We demonstrate that at larger distances, the airwave will still provide sufficient illumination of the target. The primary airwave diffuses downward into the earth and then is scattered back to the surface. The time-lapse difference of its recorded signal reveals the outline on the surface of the resistivity changes in a hydrocarbon reservoir under production. However, repeatability errors in the primary airwave can destroy the signal-to-noise ratio of the time-lapse data. We present a simple and effective method to remove the primary airwave from the data, which we call partial airwave removal. For a homogeneous half space and a delta-function type of source, the surface expression of the airwave does not depend on frequency. For this reason, the primary airwave can be subtracted from the data using recordings at two frequencies, one low enough with a skin depth of the order of the reservoir depth that is sensitive to the reservoir, the other high enough to only sense the near surface. The method does not affect secondary airwave components created by signals that have propagated through the earth and returned to the surface. We show that the method provides a direct indicator of production-related time-lapse changes in the reservoir. We illustrate this for several models, including a general 3D heterogeneous model and one with strong surface topography, for situations where survey repeatability errors are large.

Inversion of 3D time‐domain electromagnetic data: The effect of time‐weighting

In order to mitigate the airwave problem, caused by the interaction between source-excited electromagnetic fields and the air, controlled-source electromagnetic surveys on land are almost exclusively implemented as a transient electromagnetics system (TEM), typically measuring step-off or step-on responses. In this way, the earth response can be well separated from the air response, as the latter primarily arrives at very early times and is then followed by the earth response. Because the air response carries no information about deeper targets, the early-time data are often considered useless and are removed in processing and inversion. In this paper, we show that simply muting the earlytime TEM data may lead to unsatisfactory inversion results. Without the early-time response, inversion cannot retrieve the resistivity of the near surface. Due to the diffusive nature of the electromagnetic fields, this also affects the recontruction of the resistivity at larger depths. We illustrate this with a synthetic example.

A Study of Land CSEM Reservoir Monitoring in a Complex 3D Model

We studied the effect on land CSEM measurements of resistivity changes after oil production by running numerical simulations for several configurations in a complex 3D model. We include estimates of noise levels that can be expected in a field experiment,

A Feasibility Study of Land CSEM Reservoir Monitoring: The Effect of the Airwave

The displacement of oil with saline water creates a resistivity change that might be detectable by time-lapse CSEM measurements. Because the difierence in measured EM signals before and after production is small, acquisition design plays an important role. We carried out numerical experiments to understand how to optimize the acquisition to best capture the time-lapse signal. Our study shows that exciting a VED source at some distance away from the target would be an attractive choice, as a HED source induces strong airwave energy masking the anomalous signal.

Incorporating EM Inversion into Reservoir Monitoring

In the application of controlled source electromagnetics for reservoir monitoring on land, the timelapse signal measured with a surface-to-surface acquisition can reveal the lateral extent on the surface of resistivity changes at depth in a hydrocarbon reservoir under production. However, a direct interpretation of the time-lapse signal may generally be difficult and biased. We investigated if non-linear inversion can use time-lapse responses to characterize the subsurface resistivity changes. We examined two different strategies, using a full non-linear inversion algorithm as the interpretation tool: inverting the reference and monitor data independently or in sequence. In the second case, the inversion result of the reference data set serves as an initial guess for the inversion of the monitor data set. Numerical examples show that independent inversion of the data sets can provide an estimate of the depth and lateral extent of the resistivity changes. The second strategy of sequential inversion produces less satisfactory results. We illustrate the independent inversion approach for an example with large survey repeatability errors are large and another one with a complex overburden.

Strategies for Reservoir Characterization and Production Monitoring using Controlled Source Electromagnetic Data

Exploiting the fact that in a marine environment the source is continuously in action while towed behind a boat has improved CSEM capability as a direct resistor indicator. We have taken this synthetic source aperture concept one step further to show on numerically modeled data that uncertainties in source location and orientation are eliminated using a processing procedure called interferometry by multi-dimensional deconvolution. This procedure also eliminates the effects of the sea-surface. This procedure could work well for acquisition according to the present industry practice, under realistic uncertainties in receiver location and orientation, and realistic levels of noise. This is a data-driven procedure that requires properly recorded data. In case some data are not properly recorded due to receiver clipping, a hybrid model-driven data-driven approach must be used. We show some simple 2D examples to illustrate the concept of this procedure, including its drawbacks and advantages for characterization and monitoring purposes.

Exploiting the Airwave for Land CSEM Reservoir Monitoring

Displacement of oil by saline water leads to time-lapse differences in CSEM measurements. Because the time-lapse effect may be weak and difficult to observe, acquisition optimization plays an important role. Applying a horizontal dipole source on the surface creates a source-induced airwave component. We studied the effect of the airwave on time-lapse land CSEM measurements of resistivity changes after oil production by running numerical simulations for several configurations. Our study shows that the source-induced airwave is the predominant component in increasing the amplitude of the time-lapse fields, the difference between the recorded EM data before and after oil production. Moreover, the lateral extent of the depleted part of the oil reservoir is much better defined when the airwave is present.

Applying Essentially Non-oscillatory Interpolation to CSEM Modelling

Modelling and inversion of Controlled-Source Electro-Magnetics requires accurate interpolation near the resistivity contrasts, where the field component perpendicular to the interface that separates the two resistivities is discontinuous. Simple linear interpolation may produce large errors in that case. Here, we propose to use the essentially non-oscillatory piecewise polynomial interpolation that was designed for piecewise smooth functions that contain discontinuities or have discontinuous first or higher derivatives. The scheme uses a non-linear adaptive algorithm to choose the smoothest stencil. Some examples are presented to illustrate the behaviour of this scheme.

Time‐domain CSEM modeling with a frequency‐domain code

We model time-domain electromagnetic measurements of induction currents for marine and land applications with a frequency-domain code. A robust multigrid solver makes frequency-domain modeling followed by a Fourier transform an attractive choice. An interpolation criterion determines the choice of frequencies. The skin depth for each frequency defines the computational grid at each frequency. Examples illustrate the behavior of the method.