Guy Duncan

Seismically constrained petrophysics in the Scarborough area

Despite continued improvements in acoustic logging technology, logs often remain influenced by formation damage and mud-filtrate invasion. Seismic data, another type of measurement that is not affected by drilling, can be integrated in the understanding of incorrect log readings resulting from damage in the immediate vicinity of the wellbore. An iterative workflow of log conditioning, petrophysical interpretation, rock physics modelling, and synthetic-to-seismic matching is applied to ensure the P-sonic and density logs represent the true in situ properties of the rocks. Key to this integration is the development of consistent petrophysical interpretations and rock physics models. In this paper, we discuss how we arrived at a seismically constrained petrophysical interpretation in the Scarborough gas field in the Exmouth Sub Basin, offshore North West Australia. The logs of these relatively old wells were badly affected by deep invasion of the drilling fluids into the gas sand reservoirs. Conditioning of the well logs was essential to replace the poor quality data. The conditioned logs were used to create an initial rock physics model. Synthetic seismograms were then created using the conditioned logs from the rock physics modelling. Through iterating between petrophysics, rock physics, and synthetic-to-seismic matching we arrived at the final interpretation that is consistent with all available subsurface data. The final synthetic-to-seismic matches for the wells were significantly improved. The integration of different subsurface data types through rock physics modelling significantly reduced the uncertainty in the reservoir properties of the Scarborough gas field.

Estimation of Azimuthal Anisotropy from VSP Data Using Multicomponent Velocity Analysis

Observation of azimuthal shear wave anisotropy can be useful for characterisation of fractures or stress field. Shear wave anisotropy is often estimated by measuring splitting of individual shear-wave events on VSP data; however this method may become un

Estimating azimuthal stress-induced P-wave anisotropy from S-wave anisotropy using sonic log or vertical seismic profile data

Most sedimentary rocks are anisotropic, yet it is often difficult to accurately incorporate anisotropy into seismic workflows because analysis of anisotropy requires knowledge of a number of parameters that are difficult to estimate from standard seismic data. In this study, we provide a methodology to infer azimuthal P-wave anisotropy from S-wave anisotropy calculated from log or vertical seismic profile data. This methodology involves a number of steps. First, we compute the azimuthal P-wave anisotropy in the dry medium as a function of the azimuthal S-wave anisotropy using a rock physics model, which accounts for the stress dependency of seismic wave velocities in dry isotropic elastic media subjected to triaxial compression. Once the P-wave anisotropy in the dry medium is known, we use the anisotropic Gassmann equations to estimate the anisotropy of the saturated medium. We test this workflow on the log data acquired in the North West Shelf of Australia, where azimuthal anisotropy is likely caused by large differences between minimum and maximum horizontal stresses. The obtained results are compared to azimuthal P-wave anisotropy obtained via orthorhombic tomography in the same area. In the clean sandstone layers, anisotropy parameters obtained by both methods are fairly consistent. In the shale and shaly sandstone layers, however, there is a significant discrepancy between results since the stress-induced anisotropy model we use is not applicable to rocks exhibiting intrinsic anisotropy. This methodology could be useful for building the initial anisotropic velocity model for imaging, which is to be refined through migration velocity analysis.

The value of using relative amplitude changes

There is a rich history of using differential measurements to improve the signal-to-noise (S/N) ratio when there is correlated signal to improve the signal-to-noise ratio. With respect to seismic amplitude measurements, this is done in a practical sense by comparing the amplitude of reflections on structure to those off structure, or by comparing anomalous amplitudes to an average background. In fact, anomalous is defined in reference to the background. While this is intuitively the thing to do, the value of this methodology needs to be calculated by the quantitative effect on risk and uncertainty and the interaction of this effect with business decisions. It is only by influencing the business decisions with the information that value is realized.

4D Seismic over the Pyrenees Fields

In this paper we present a case study of 4D seismic acquired over the Pyrenees Fields, offshore Western Australia. The Pyrenees trend was discovered with the drilling of the West Muiron-5 discovery well in 1993 which found oil and gas within the Pyrenees member sandstones. Production at Pyrenees started in 2010. Before the start of production, a dedicated 4D baseline survey was recorded over the fields in 2006. A detailed modelling study concluded that a 4D monitor survey would provide useful information for reservoir surveillance and infill drilling decisions. The monitor survey was acquired in 2013, and the overall quality of the 4D was excellent with high 4D signal strength and low 4D noise. The 4D response at Pyrenees is broadly consistent with the modelling. The main response is softening of the reservoir caused by gas coming out of solution produced by a pressure drop within the reservoir. The 4D response to changes in oil saturation is small. Incorporating the 4D interpretation into field development is ongoing, and so far it has been useful for refining the stratigraphic model, determining fault seal integrity, and determining the sealing capacity of intra-field faults.

Estimation of Stress Induced Azimuthal Anisotropy – AVO Modeling

The analysis of rock anisotropy in terms of seismic velocities and within the context of rock-physics (Biot-Gassmann theory of poroelasticity) provides important information for the evaluation of the stress state (tensors) of rocks, detection of the directions of formation weaknesses, helps in the estimation of overall permeability and failure prediction. Understanding the influence of stress and pore pressure on seismic velocities is important for 4-D reflection seismic interpretation, AVO analysis and reservoir modeling. Laboratory measurements were carried out on spherical shale samples from the overburden under confining stress up to 400 MPa, by means of ultrasonic soundings in 132 independent directions. Such an approach enables the estimation of 3-D elastic anisotropy. Assuming VTI symmetry approximation, from the measured velocities the stiffness tensor was inverted. Since the sandstones were partly unconsolidated, it was not possible to take ultrasonic measurements . To overcome this, we developed a method for stress induced azimuthal anisotropy estimation using only cross-dipole logging data. These results give the possibility for anisotropic correction in AVO analysis.

Bayesian inversion whispers

Many times we are faced with the business decision of whether or not to develop a sand that is at the limit of seismic resolution and near the noise level of the data. The critical issue is developing a reasonable certainty that there is enough volume of hydrocarbons to develop. A popular approach is to use Bayesian methods to determine the probability of an economic volume of hydrocarbons being present. A problem with this approach when it is applied to these marginal cases is a bias to the answer. Often, this comes from a relatively strong sophomoric prior constraint on the gross thickness and net-to-gross (N/G) of the sands, imposed to keep the inversion focused on the correct seismic reflector. The data are whispering what the answer should be through the Bayesian apparatus, but this whisper is overwhelmed by the sophomoric prior constraints. We found a simple solution to this problem—run the seismic inversion several times using the output mean of the previous inversion as the input mean of the next ...

A new paradigm for rapid technology onboarding

The emergence of Java as a serious numerical and 2D graphics language and the general acceptance of open source maintenance agreements have laid the foundation for a new paradigm that allows rapid technology onboarding. This paradigm has a backplane of Seismic Unix as the data standard enhanced to allow fast random access and to store horizons as well as layer based models. Attached to this backplane are proprietary Seismic Unix modules (e.g., wavelet based seismic lithofacies identification, and stochastic model based inversion) as well as commercial and consortia modules (e.g., Delphi, Colorado School of Mines, advanced full wavefield migration, and Monte Carlo driven map migration). This data format allows for multidimensional indexing critical for uncertainty, 4D, wavelet and AVO analysis. A final ingredient is an open sourced Java based viewer of the data (synchronized map and cross section views). This unifies the user interface and frees the internal, consortia, and commercial development to improve the core technology, not develop yet another user interface. The final result is rapid delivery of technology to the user that is determined by what is of value to him.