Geomechanical and Acoustic Properties Measurements on Reconsolidated Mudrock Constituents at Reservoir Stresses
Daniel M. Coleff, 2014. "Geomechanical and Acoustic Properties Measurements on Reconsolidated Mudrock Constituents at Reservoir Stresses", Sedimentary Basins: Origin, Depositional Histories, and Petroleum Systems, James Pindell, Brian Horn, Norman Rosen, Paul Weimer, Menno Dinkleman, Allen Lowrie, Richard Fillon, James Granath, Lorcan Kennan
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Traditionally, mudrocks have not been studied as extensively as sandstones yet they constitute approximately two-thirds of the Earth’s sedimentary column. Shales and siltstones record the Earth’s paleogeography, depositional environments, climate, and biology. Mudrocks also act as primary source rocks leading to the formation of hydrocarbons and as seals in petroleum systems. As conventional oil and gas reserves dwindle, there is an ever-increasing interest in oil and gas shales.
Laboratory measurement of petrophysical properties results in a better characterization of petroleum system elements. Acoustic and geomechanical data gathered from mudrocks are all pertinent to the evaluation of oil and gas prospects. Variables such as mineralogy, sorting, and grain size all can affect velocity, porosity, and permeability. The reconsolidation of geo-materials to create mudrocks in the laboratory allows the geoscientist to understand the evolution of important physical properties while controlling these variables. The cost of core recovery is high and traditionally shale core has been acquired as a byproduct of coring sandstone hydrocarbon reservoirs. Laboratory-created, reconsolidated mudrock is much more uniform than naturally occurring specimens. Particle-size distributions, mineralogy, stress history, and pore-fluid chemistry are all known explicitly and can be varied individually in order to assess their influence on petrophysical properties. Because individual controls on rock properties can be isolated using these experiments, forward models for the evolution of mudrock properties during burial can be developed.
As a part of a proof of concept study to illustrate the applicability of reconsolidation techniques to exploration and production problems, four experimentally created mudrocks containing varying proportions of clay and silt constituents were fabricated and subjected to standard tri-axial strength testing and acoustic properties measurements. Using oedometer cells fitted with positive displacement pumps, driven by servo motors, and controlled by PID loops, the 100 g/l brine-saturated samples were compacted to an ultimate stress of 26.5 MPa. The samples were then extruded and loaded into standard tri-axial test cells. Axial load was increased while maintaining a constant confining stress of 20.7 MPa until the samples failed (indicated by positive volume strains).
The equipment demonstrated results consistent with published reconsolidation techniques. Acoustic measurements showed a monotonic increase in velocity with an increase in stress. P-wave velocities ranged from approximately 2400 – 2800 m/s and S-wave from 1050 – 1280 m/s. These values fall within the range of natural mudrocks. Ultimate strength values for the samples generally decreased as silt content increased from 0-25%, but increased again between 25% and 50%. This change in strength behavior may suggest that a percolation threshold was reached. Creep strains were observed, play an important role, and require further investigation.