ABSTRACT: An understanding of the mechanical properties of petroleum reservoirs is important for drilling, well stimulation and horizontal well development. These mechanical properties can be measured in the laboratory from triaxial tests ("static" tests) or they can be estimated from wireline measurements of compressional and shear acoustic velocities and rock density ("dynamic" tests).
There can significant differences between static and dynamic mechanical properties in most rocks. Static measurements on cores are much more indicative of the mechanical properties of the reservoir than the dynamic results, however, the information from acoustic well logs covers much more of the reservoir than core measurements and is less expensive. This makes it important to be able to translate the "dynamically" derived mechanical properties to the static values which better represent the reservoir.
A comparison of static and dynamic mechanical properties was carried out on 85 core samples from two wells in the Chase and Council Grove carbonate sequences of the Hugoton and Panama fields, Kansas. The purpose of the study was to characterize the mechanical properties of the different facies and calibrate the dynamic mechanical properties so acoustic well logs from other wells can be utilized more effectively in determlning areal and lithologic variation in mechanical properties of the field.
Results of the tests show that Young's modulus correlates strongly with lithofacies and porosity and that the carbonate sequences can be separated into six "mechanical facies", defined by their modulus-porosity trends and their lithology. Poisson's ratio is less sensitive to lithology but correlates with porosity. The dynamic Young's moduli are 15% to 40% higher than the static values. Dynamic and static Poisson's ratios correlate well with each other for liquid saturated samples.
Dynamic to static transforms based on lithofacies were developed from these data to correct the acoustic log derived mechanical properties to static values appropriate for reservoir deformations. The corrected mechanical properties along with the trends observed between mechanical properties, lithofacies, and porosity allow for the design of more effective hydraulic fracture treatments by utilizing accurate values for the reservoir's mechanical properties and their variability.
