Combining burial history and rock-physics modeling to constrain AVO analysis during exploration

A B S T R A C TRelationship between different geomechanical and acoustic properties measured from seven laboratory-tested unconsolidated natural sands with different mineralogical compositions and textures were presented. The samples were compacted in the uniaxial strain configuration from 0.5 to 30 MPa effective stress. Each sand sample was subjected to three loading-unloading cycles to study the influence of stress reduction. Geomechanical, elastic and acoustic parameters are different between normal compaction and overconsolidation (unloaded and reloaded). Stress path (K 0 ) data differ between normal consolidated and overconsolidated sediments. The K 0 value of approximately 0.5 is founded for most of the normal consolidated sands, but varies during unloading depending on mineral compositions and textural differences. The K 0 and overconsolidation ratio relation can be further simplified and can be influenced by the material compositions. K 0 can be used to estimate horizontal stress for drilling applications. The relationship between acoustic velocity and geomechanical is also found to be different between loading and unloading conditions. The static moduli of the overconsolidated sands are much higher than normal consolidated sands as the deformation is small (small strain). The correlation between dynamic and static elastic moduli is stronger for an overconsolidation stage than for a normal consolidation stage. The results of this study can contribute to geomechanical and acoustic dataset which can be applied for many seismic-geomechanics applications in shallow sands where mechanical compaction is the dominant mechanism.

Section: Introductionmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

Effects of stress reduction on geomechanical and acoustic relationship of overconsolidated sands

Narongsirikul

Mondol

Jahren

2019

“…One method to characterize the burial history is to relate the measured velocity to velocity development through time. A framework for such a modelling sequence is described in Avseth and Lehocki (2016), where three phases are described: mechanical compaction, cementation, and uplift. In the workflow by Avseth and Lehocki (2016), neither stress dependence during unloading nor anisotropy was incorporated.…”

Section: Introductionmentioning

confidence: 99%

Integrating rock physics laboratory data and modelling to improve uplift characterization methodology

Torset

Holt

Duffaut

2020

Modelling changes in elastic wave velocities in sediments and rocks as they undergo burial and uplift can reveal aspects of their burial history. This has implications in the petroleum risking procedures, as aspects such as hydrocarbon maturation and reservoir quality are key ingredients. Previous modelling for such a purpose assumes constant velocity and porosity during uplift after the cessation of cementation. Laboratory data of a synthetic sandstone formed under stress, and subjected to loading and unloading cycles are used to test this assumption. The experimental P‐wave velocities show a clear increase in stress dependence during the uplift simulation. Besides, the P‐wave anisotropy transforms from negative to positive. These observations are combined to make an updated modelling workflow to mimic the experimental results better. The modelling is conducted in different phases. Loading before cementation is modelled using a triaxial granular medium theory. For the laboratory data, a novel anisotropic formulation of the patchy cement model captures the observed trends. A modified crack model incorporates the stress sensitivity seen during the unloading that simulates uplift. The results of the integrated rock physics workflow mimic the laboratory data very well. A conceptual field example using the rock physics workflow illustrates how the exclusion of the stress dependence during uplift can lead to an underestimation of the exhumation magnitude.

“…Grude Landrø and Osdal ; Narongsirikul Mondol and Jahren –c; Draege et al . ; Avseth and Lehocki ) and experimental compaction studies (Nygård et al . ; Grande et al .…”

Section: Introductionmentioning

confidence: 99%

“…In addition, in the past few years, several experimental approaches have been developed with the aim of understanding the effects of complex burial histories or stress paths on rock properties, for example using rock physics modelling (e.g. Grude Landrø and Osdal 2013; Narongsirikul Mondol and Jahren 2013a-c;Draege et al 2014;Avseth and Lehocki 2016) and experimental compaction studies (Nygård et al 2004;Grande et al 2011). However, the understanding of the impact of stress unloading on rocks is still immature.…”

mentioning

confidence: 99%

Acoustic and petrophysical properties of mechanically compacted overconsolidated sands: part 1 – experimental results

Narongsirikul

Mondol

Jahren

2019

This paper part one is set out to lay primary observations of experimental compaction measurements to form the basis for rock physics modelling in paper part two. P‐ and S‐wave velocities and corresponding petrophysical (porosity and density) properties of seven unconsolidated natural sands with different mineralogical compositions and textures are reported. The samples were compacted in a uniaxial strain configuration from 0.5 up to 30 MPa effective stresses. Each sand sample was subjected to three loading cycles to study the influence of stress reduction on acoustic velocities and rock physical properties with the key focus on simulating a complex burial history with periods of uplift. Results show significant differences in rock physical properties between normal compaction and overconsolidation (unloaded and reloaded). The differences observed for total porosity, density, and P‐ and S‐wave velocities are attributed to irrecoverable permanent deformation. Microtextural differences affect petrophysical, acoustic, elastic and mechanical properties, mostly during normal consolidation but are less significant during unloading and reloading. Different pre‐consolidation stress magnitudes, stress conditions (isotropic or uniaxial) and mineral compositions do not significantly affect the change in porosity and velocities during unloading as a similar steep velocity–porosity gradient is observed. The magnitude of change in the total porosity is low compared to the associated change in P‐ and S‐wave velocities during stress release. This can be explained by the different sensitivity of the porosity and acoustic properties (velocities) to the change in stress. Stress reduction during unloading yields maximum changes in the total porosity, P‐ and S‐wave velocities of 5%, 25%, and 50%, respectively. These proportions constitute the basis for the following empirical (approximation) correlations: Δϕ ∼ ±5 ΔVP and ΔVP ∼ ±2ΔVS. The patterns observed in the experiments are similar to well log data from the Barents Sea. Applications to rock physics modelling and reservoir monitoring are reported in a companion paper.