Lee Hunt

Quantitative estimate of fracture density variations in the Nordegg with azimuthal AVO and curvature: A case study

We investigated the accuracy of surface seismic attributes in predicting fracture density variations within the Nordegg Formation in west central Alberta. We know from core, drill samples, well-log, and drilling data that the Nordegg zone is fractured to some degree. These fractures are of interest because the reservoir has very low permeability, and therefore natural fractures may materially affect well performance. 3D surface seismic techniques such as amplitude variation with azimuth or azimuthal AVO (AVAz), variation of velocity with azimuth (VVAz), curvature, and coherence techniques are all tools that have been used to predict fractures in a qualitative fashion. In this study, we wanted to understand how well these attributes predicted the reservoir quality in a quantitative fashion. Previous quantitative studies have used image log orientation data or estimated ultimate recoveries (EUR) in vertical wells as validation data. The conclusiveness of these studies has been subject to several problems: f...

The effect of interpolation on imaging and AVO: A Viking case study

The use of prestack interpolation prior to prestack migration to improve AVO analysis on image gathers is demonstrated on an exploration play. The interpolation achieves this improvement by reducing migration artifacts. AVO analysis attempts to estimate fundamental information from surface seismic data and likely will be used more frequently if the estimates can be more accurately produced. Land 3D seismic typically has poor and irregular sampling. This poor sampling creates migration noise, which is a material cause of inaccurate AVO estimates. Prestack 5D interpolation is applied prior to prestack migration and AVO analysis on the imaged gathers to address this noise problem. The interpolation algorithm includes offset and azimuth dimensions that preserve AVO information. This method is evaluated bycomparing the results to those of alternate approaches, such as superbinning, that suppress this kind of noise in AVO analysis. The evaluation is determined by comparing our ability to predict the reservoir q...

Causal fracture prediction: Curvature, stress, and geomechanics

We propose scaling volume curvature measurements with material property estimates to produce a superior prediction of natural fractures. Curvature is one of many, indirect, fracture-inferring attributes. It does not detect fractures, but is causally related to them through the assumption that increasing curvature relates to increasing strain. There are many other variables that are causally related to fractures. We propose that it would be advantageous to create combinations of these causal variables with curvature. Some of the most well known and important causes of variations in natural fracture density are material properties relating to brittleness. Material properties are critical geologically at all scales, from large-scale regional studies to prospect-level inquiries because the properties may vary significantly within individual formations and between formations. These vertical and lateral changes in material properties may be important and should be considered in fracture estimation, along with c...

The Effect of Interpolation On Imaging And Azimuthal AVO: A Nordegg Case Study

To address the issue of inadequate sampling, typical of land seismic data, an azimuthal AVO (AVAZ) processing flow should include interpolation and prestack migration prior to the AVAZ inversion. It is well established that seismic data should be prestack migrated before AVO, but the irregular sampling inherent in land data can introduce migration artifacts which distort the estimates of the AVO inversion. With the introduction of the extra dimension of azimuth this is even more a concern for AVAZ analysis. By performing 5D minimum weighted norm interpolation before PSTM, the wavefield is better sampled leading to better migration and AVAZ results. This has been demonstrated on synthetic examples. This paper demonstrates the benefits of this on a real dataset by comparing AVAZ processing sequences with and without interpolation and correlating the predictions quantitatively to well control. The interpolation/PSTM flow proceeding AVAZ inversion produces better correlations to the well control than only PSTM.

Resolution on multiples: Interpreters' perceptions, decision making, and multiple attenuation

We investigated our ability to remove a specific short-period multiple from the Nisku and Blueridge formations in West Central Alberta, Canada. This problem is commercial in nature, and has persisted because it was believed that the multiple had too little moveout to be removed, rendering interpretation of the thin Blueridge zone impossible. Associated with this issue was the belief that the modern high-resolution Radon transforms do not materially affect the stack response of real data in this area despite their excellent performance on synthetics and on other data in the literature. Serious technical work seldom affords a discussion of “beliefs”, but this work is concerned with the decision-making of the interpreter. We show that in order to address a specific, real, short-period multiple problem, the interpreter was required to challenge previously held technical assumptions. This required the interpreter to consider the nature of the multiple itself, the nature and limitations of the multiple suppression technology used, and to objectively measure the level of success in suppressing the multiple.

The effect of stretch free stacking on a clastic exploration play in Alberta, Canada

A 3-D seismic survey was shot over a set of spatially complex channel leads that were thought to be charged with both gas and water. The prospects being investigated suffered from spatial and temporal reservoir resolution problems as well as fluid risks. In the attempt to address these challenges, the seismic data was processed using a new algorithm that promised to handle the long offset information advantageously. This algorithm is a new process that is meant to eliminate NMO stretch effects in the stack. Since fluid estimation was of such concern in this dataset, the algorithm was also used to create NMO stretch free gathers for AVO analysis. The results of this work seemed to be excellent. Numerous new prospects were identified on the 3-D survey, one of which could not have been identified without the stretch free stack process, or SFS (for brevity), and AVO analysis. As is sometimes the case, the most difficult and interesting task became that of understanding what the SFS process actually did to the data and why.

5D Interpolation, PSTM And AVO Inversion

To address the issue of inadequate sampling, typical of land seismic data, an AVO processing flow should include interpolation and prestack migration prior to the AVO inversion. It is well established that seismic data should be prestack migrated before AVO, yet the irregular sampling inherent in land data can introduce migration artifacts which distort the estimates of the AVO inversion. By performing 5D minimum weighted norm interpolation prior to the PSTM, the wavefield is better sampled leading to better migration and AVO results. By working in five dimensions, the algorithm can interpolate through gaps that are problematic for lower dimensional interpolators. The 5D interpolation is amplitude preserving and appears to improve the signal-to-noise ratio with minimal evidence of smearing. In order to support these assertions, a series of parallel processing test flows were performed and compared on a 3D seismic survey from Alberta, Canada with extensive well control. For each of these flows, Ostrander gathers at key wells, AVO attributes, and their ties to 29 wells were examined. The interpolation PSTM flow prior to AVO inversion produced the best correlation to the well control.

Sensitivity of rock properties in AVO analysis and prospect evaluation

Sensitivity of rocks properties responding to pore fluid and sensitivity of seismic attributes to a given lithology contrast are two important issues for defining a reservoir. This study attempts to quantitatively determine these sensitivities. The evaluation of the sensitivities was used to predict seismic response and amplitude variation with offset. It is demonstrated that the analysis of these sensitivities is useful for seismic interpretation and prospect evaluation. A methodology is suggested to evaluate these sensitivities by using absolute and relative variations of rock properties with respect to porosity, pore fluid and rock composition. It indicates that the orthogonality of rock properties with given a pair of lithologic units is a measure of lithologic contrast. It could be enhanced through mathematical manipulations.

Precise 3D seismic steering and production rates in the Wilrich tight gas sands of West Central Alberta

We evaluate the controls on production performance of the Wilrich tight gas sand play in West Central Alberta, and show that careful steering using 3D seismic to place the wellbore within the upper reservoir is the most important geophysical contribution to production outcomes. Geologic, geophysical, drilling, and production data from more than 20 wells are used in the analysis. The completion and production parameters within the study area are relatively invariant, creating a control experiment relative to other productivity factors. We thus isolate the effects of varying bottom hole pressures, porosity, wellbore length, number of stimulations, mud gas response, gamma ray measurements while drilling, mud weight, curvature, amplitude versus offset (AVO), amplitude versus azimuth (AVAz), velocity versus azimuth (VVAz), and position of the horizontal wellbore within the reservoir. These variables are treated separately and in a multivariate fashion to determine their relative and combined effect on the productivity of the wells. Several methods of statistical evaluation are used to test confidence in the results. The Wilrich sand is approximately 20-m thick, and it was expected that the multistage fracture stimulation would have minimized the importance of vertical permeability variations by adequately accessing the entire vertical reservoir section. Such is not the case; precise placement of the wellbore in the most permeable stratigraphy of the thin reservoir is of material importance. The pressure and porosity strongly affect the production performance, but to a lesser degree than vertical position within the reservoir. This suggests that stratigraphic concerns as they relate to permeability variation can be critical, even in thin fracture-stimulated reservoirs. Interesting relationships were observed between the AVAz and curvature measures, but neither they nor the AVO or VVAz attributes yielded statistically significant correlations to the production data.

Short -period land 3D multiple attenuation: a case study with interp olati on and sparsity

Summary Exploration and development of the Nisku and Blueridge in West Central Alberta is challenging because these zones are deep, underlie thick coal sequences, and are contaminated with short period multiples. The advent of high resolution sparse Radon Transform multiple attenuation techniques allows us to examine the multiple issue. The Blueridge zone was found to be contaminated by multiples with a variety of small move-outs. The Radon Transform may benefit from well sampled, regular gathers, as input. This can typically only be achieved in land data by borrowing traces from neighboring CMP locations (superbinning) over a significant area. We became concerned that this superbinning might limit the resolution of the transform through a structural smearing effect. We performed 5D interpolation prior to multiple attenuation to eliminate the need for superbinning and reduce the potential effect of smearing on the Radon Transform. Our processing flow resulted in an improved interpretation of the Nisku and Blueridge reservoirs relative to legacy processing flows. A significant improvement was gained from the aggressive AVO compliant noise attenuation and resolution enhancement we applied to all our new products. The interpolation-sparse Radon Transform approach produced superior Tau-p spaces, but improvements appear to be caused by an improvement in signal to noise ratio in the interpolated gathers as well as a reduction in structural smearing.