Benefits of an integrated petroelastic model

By Graeme Eastwood

The decision to take a spudded well to completion depends on the likely economic payout, estimated through a thorough analysis of all available well data. The result of reservoir characterization, performed at about the midpoint of financial investment, in large part drives whether to proceed with well completion and incur the associated cost. Clearly, quality and accuracy in the analysis are paramount.

The stakes, and the potential error, are even higher when evaluating multiple wells or an entire field. Integrating petrophysical analysis with rock physics provides the necessary information for reservoir engineers, geologists, and geophysicists to optimally judge the risks and opportunities involved. This integrated approach, where data, processes and models are all consistent, dramatically increases quality and makes it easy to iterate as much as needed to achieve the best match to all the data. As a result, integrated Petroelastic model directly reduces risk, improves process efficiency, enhances overall production, and increases the return rate of economic assets.

BACKGROUND

Petrophysics and rock physics, together, identify rock properties necessary to construct a model of the subsurface. Petrophysics typically transforms resistivity, gamma ray and porosity tool measurements into reservoir properties. Rock physics typically transforms petrophysical properties into compressional and shear velocities, and density - elastic rock properties. The process relies on seismic data in combination with sonic and shear logs. Where these are not available, synthetic logs are constructed. The goal is to construct a unified petroelastic model that is consistent with both basic physical principles and the petrophysical model.

The sophistication of the modeling depends on the objectives and the quality of available data. Typical objectives include:

• QC of the measured elastic logs
• QC of petrophysical interpretation
• Synthesis of elastic logs
• Correcting logs for invasion effects and dispersion
• Scenario modeling
• Establishing a framework for interpretation of seismically derived elastic properties

The accuracy and consistency of petrophysical and rock physics models determines the quality and reliability of all subsequent evaluation.

THE INTEGRATED WORKFLOW

The integrated workflow consists of data gathering, editing, conditioning, modeling, analysis, comparisons between petrophysics and rock physics results. When petrophysics and rock physics are performed in a tightly integrated and iterative loop, the consistency, veracity, and utility of the petrophysical and rock physics results are enhanced.

Fig.1 Traditional workflow                                      Fig.2. Truly integrated approach makes iteration practical   

Step 1.      Data gathering ensures that the proper data is available.

Step 2.      Data editing, and conditioning improves well data, correcting for gaps and other inevitable problems, requiring visualization tools and interactive manipulation widgets to make the process smooth, easy, and accurate.

Step 3.      Petrophysical modeling and analysis. Parameters such as water resistivity and lithology are used with resistivity saturation models for the field, region, or rocktype encountered.

Step 4.      Rock physics modeling Elastic properties are constructed and compared with measured logs to correct and calibrate elastic property logs and to demonstrate the correctness of the petrophysical results. Steps 2, 3 and 4 are iterated until the results honor all data in both petrophysical and rock physics analysis.

Typical problems that this integrated approach can discover and correct include borehole and invasion effects, unknown mineral mix and properties, thin and laminated beds, missing logs or gaps, and 4D time lapse effects.

Process iteration is key to building one model,that is truly consistent with all the data and with the lowest uncertainty.

Fig.3. Initial logs showing incpmplete datasets, cycle skipping and spikes, corrected by integrated workflow              

Characteristics of a truly integrated approach are:

• Ability to quickly shift between petrophysics and rock physics
• Parameters, zones, equations, etc. are held constant between models
• Both theoretical and empirical data are supported

Consider a simple case of three wells, all with measured p-sonic and s-sonic. The porosity and volume of clay can be calculated from Neutron-Density. However, deciding what values to choose for the clay points is interpretive and may vary from well to well because of normalization issues. In this case the normalization differences are very subtle when it comes to choosing the clay points but more obvious when the resultant porosity and volume of clay are used in a rock physics model to predict the sonic data. With an integrated approach, the clay points can be quickly adjusted to provide a consistent match to the sonic data. Here, rock physics is used to guide petrophysics and the final model is integrated and consistent.

INTEGRATION BENEFIT 1: REDUCED UNCERTAINTY

Using an integrated approach minimizes uncertainty in the correction process by challenging the initial results with independent data. Brute-force corrections are not necessary. Easy iteration encourages a thorough process. An integrated environment ensures control.

INTEGRATION BENEFIT 2: GREATER ACCURACY

When all the data is in agreement, there is greater confidence in the results. With a realistic underlying model, it is possible to build a better assessment of an asset's economic value. Better calibration of seismic attributes to reservoir properties offers a clearer picture of what is in the field and how much of the resource can be extracted.

INTEGRATION BENEFIT 3: TRUSTWORTHY RESULTS

Integration provides a thorough understanding of the relationship between reservoir properties and seismic data, greatly enhancing the reliability of interpretations. Well-to-seismic ties are improved. Models more reliably map seismic responses that are due to vertical, lateral and temporal changes in the subsurface.

INTEGRATION BENEFIT 4: INCREASED EFFICIENCY

Petrophysics and rock physics are the foundation of reservoir evaluation. Efficiency, gained through integration, and the superior models generated, propagate efficiency all the way through the reservoir development process.

INTEGRATION BENEFIT 5: MORE PRECISE RESERVOIR MANAGEMENT

Field productivity is increased because a better drilling program can be devised. Precision analysis leads directly to precision reservoir management.

Achieving the Benefits with PowerLog^®

Fugro-Jason's PowerLog^® software combines the disciplines of petrophysics and rock physics in one integrated tool. It enables a fundamental modeling approach - a single theoretical petroelastic model can be used to simultaneously derive both petrophysical and elastic properties of reservoir and surrounding rocks.

CHOICE OF MODELING ALGORITHMS

PowerLog offers a choice property modeling algorithms. Solids can be mixed with traditional methods such as Wyllie, Voigt, Reuss, and Hashin-Shtrikman. Fluids properties based on any of a number of models, including the DHI Fluids Consortium relations, can be mixed with methods such as Linear, Homogeneous, and Brie. Fluids and solids can be mixed together with inclusion (porosity) or contact (grain boundary contacts) elastic models such as Xu & White, Berryman's Self-Consistent, and the Stanford contact models. In addition, the suite of rock physics algorithms includes Greenberg-Castagna relations, configurations of Gassmann's equation, Gardner's relation, and many more.

FLUID SUBSTITUTION

In addition to being able to accurately model elastic formation properties, PowerLog incorporates petrophysical formation properties representing formation conditions at the time of data acquisition, and then extrapolates these conditions into the past or future. Formation invasion effects can be modeled and corrected.

Fluid substitutions can be performed to produce logs that reflect an earlier time or project future conditions. Additionally, lithology substitutions can also be easily performed in PowerLog to predict what logs might look like in various undrilled parts of the field. These substitutions are critical for performance of time lapse reservoir monitoring and verification of AVO anomalies and seismic inversion results. PowerLog also supports the estimation of anisotropy parameters from deviated well curves and corrects sonic and resistivity curves for anisotropy influence.

SUMMARY

PowerLog provides significant technical and business benefit by integrating rock physics with petrophysics. This integration ensures the most realistic model of in situ reservoir and surrounding rock properties, forming a solid foundation for all subsequent, interdisciplinary analyses. As a result, interpretations are more accurate and timely, drilling programs are more efficient, and potential economic return is maximized.