The principal objective of this work is to characterize the elastic anisotropic properties of clay-rich rocks for consolidation conditions simulating compaction at depths of 0-2 km in young sedimentary basins of interest for hydrocarbon exploration. To realize this objective, researchers will develop an entirely new laboratory apparatus capable of directly measuring the anisotropic properties of clay-rich rocks subjected to stress states simulating confinement at depth. The ultrasonic phased-array compaction cell that will be designed and fabricated at Lawrence Berkeley National Laboratory (LBNL) will employ a P-wave ultrasonic phased-array and a series of resonant P- and broadband S-wave transducers to recover the five transverse isotropic (TI) elastic constants of clay-rich rocks during compaction, without the need for sample unloading, recoring, or reorienting.
Lawrence Berkeley National Laboratory
There is a growing concern that anisotropy in the overlying reservoir strata, when left uncorrected for, can hinder efforts to image reservoir structure and estimate reservoir properties using seismic methods. While clay-rich rocks are estimated to comprise 75% of sedimentary basins, only a limited number of experimental anisotropy studies have been performed on such rocks, especially for higher porosity clay-rich sediments/rocks encountered in the first two kilometers of depth. New data on the stress-dependent anisotropic properties of compacted clay-rich rocks (i.e., c11, c33, c55, c66, c13 or can be used directly to achieve proper focusing in anisotropic imaging and to provide amplitude corrections for AVO and may provide valuable information on overpressure that can be used in borehole stability analysis.
Despite the growing data set on shale anisotropy present in the literature, anisotropy data for less-consolidated and diagenetically altered clay-rich rocks (25-45% porosity, 0-2 km depth) is presently sparse (especially ). The project performers will use the ultrasonic phased-array compaction cell to fill this gap by developing a new data set describing the magnitude and character of the anisotropy of shallow (0-2 km depth) clay-rich rocks from sedimentary basins that are of interest for hydrocarbon exploration. Using the knowledge gained from these experiments and microstructural analysis of the compacted samples (from X-ray diffraction, SEM, and possibly neutron diffraction), the researchers propose to develop stress-dependent effective-medium models describing the influence of compaction-induced texture on the anisotropic properties of clay-rich rocks.
This project has successfully developed a state-of-the-art laboratory device for measuring the anisotropic elastic constants of clay-rich rocks during compaction. This device employs an ultrasonic compressional wave phased-array and resonant P-wave and broadband shear wave transducers. The project performers have used this device to measure the evolution of transverse anisotropy in compacting clay-rich sediments provided by industry partners from reservoirs in West Africa. In the final year of the project, additional anisotropy measurements will be collected on clay-rich core provided by Chevron and BP. This data will be used to develop a stress-dependent effective medium model that describes the evolution of transverse anisotropy in clay-rich rocks for use in seismic imaging and AVO efforts.
The financial impact of properly incorporating anisotropy in seismic imaging and analysis can be measured as the cost of a dry hole versus the value of a discovery well. With the typical cost of a well at $5-10 million, knowledge of the anisotropic properties of the overburden required for proper time-to-depth conversion can be significant, particularly in sedimentary basins. This project represents a collaborative effort between LBNL and the oil and gas industry to quantify the magnitude and character of the anisotropy of clay-rich rocks at depths of 0-2 km. This effort will utilize expertise in laboratory rock physics and ultrasonic phased-arrays that resides at LBNL to develop a new laboratory technology anisotropy measurement and to apply this technology to develop a new data set quantifying the stress-dependent anisotropic properties of clay-rich rocks.
The project is nearing completion. Data collection and processing is complete. A draft of the paper describing the phased array compaction cell and its utility for measuring the anisotropic properties of soft sediments (to be submitted to Geophysics for publication) is finished. Technology transfer of this device and the findings of this research to Industry (Chevron) are complete.
Two journal submissions are in preparation: one describing the ultrasonic phased-array compaction cell, and a second presenting the anisotropy measurements and analyses on marine sediments provided by industry partners.