INTERPRETING PORE PRESSURE IN MARINE MUDSTONES WITH PORE PRESSURE PENTROMETERS, IN SITU DATA, AND LABORATORY MEASUREMENTS
Open Access
- Author:
- Long, Hui
- Graduate Program:
- Energy and Geo-Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 03, 2007
- Committee Members:
- Peter B Flemings, Committee Chair/Co-Chair
Derek Elsworth, Committee Chair/Co-Chair
Abraham S Grader, Committee Member
Chris J Marone, Committee Member
Demian Saffer, Committee Member
John T Germaine, Committee Member - Keywords:
- Overpressure
mudstones
penetrometer
IODP
consolidation
permeability
compression index
URSA - Abstract:
- Pore fluid pressure plays an important role in deformation and mass transfer in the Earth’s crust. However, it is extremely challenging to directly measure pore fluid pressures in low-permeability mudstones. We developed a new pressure penetrometer, the Temperature-Two-Pressure (T2P) probe, which allows the pore fluid pressure to be accurately inferred from partial dissipation records. We used the strain path method to simulate pore pressure generation and dissipation due to penetration of penetrometers with various geometries. Our theoretical analyses suggest that one of the key controls on soil behavior is the undrained rigidity index. The step geometry of the T2P enables that the pore fluid pressure and hydraulic diffusivity of the penetrated sediment to be estimated independently within a very short monitoring time by comparing the dissipated pressures at the tip and shaft pressure ports. The measured data suggested the proposed approach can provide reliable and rapid estimates of pore fluid pressure from partial dissipation records. However, this approach requires high quality dissipation data and accurate soil model. Modeling results show that the tip pressure dissipation of a tapered probe initially follows that of its needle probe, starts to depart from its needle probe when the pressure front coming from its overlying shaft reaches the tip pressure port, and converges to the pressure dissipation of its overlying shaft when the narrow pressure pulse caused by its needle probe decays away. During the transition, it forms a “bench” on the tip pressure dissipation curve. We related the excess pore pressure ratio on the “bench” to a single parameter, the undrained rigidity index. This allows the in situ pressure to be estimated from partial dissipation data without knowing detailed soil properties. In addition, we proposed a new extrapolation approach, inverse square root of time extrapolation, based on the model results. It can provide pore fluid pressure with desirable accuracy for soft marine sediments with low undrained rigidity index. On the other hand, we conducted extensive uniaxial consolidation tests on whole core samples to obtain the consolidation properties of the sediments, and use them to predict pore fluid pressure from porosity profiles. The results suggest that the compression index linearly decreases with in situ void ratio. This implies that a local virgin compression curve cannot validly be extrapolated over a large range in effective stress. This effect is particularly important at shallow depth where void ratio decreases rapidly. The relationship of compressibility index versus void ratio can be obtained from a single consolidation test by compressing the soil over a large range in effective stress. A virgin compression curve can then be constructed based on this relationship to predict pore fluid pressure. In the Ursa Basin, this new approach successfully predicted pressures interpreted from the penetrometer measurements within the non-deformed sediments. The mass transport deposits appear to be more compacted than the non-deformed sediments. The virgin compression curve based on the assumption of uniaxial strain underpredicts the in situ pressure in the mass transport deposits.