Topic
Geologic overpressure
About: Geologic overpressure is a research topic. Over the lifetime, 28 publications have been published within this topic receiving 554 citations.
Papers
TL;DR: In this paper, the authors define high overpressure as pore pressure that approaches the overburden stress, i.e., pore fluid pressure greater than normal pressure, and define the effective stress as the portion of the total stress carried by the rock grains.
Abstract: Normal pressure is pore fluid pressure that equals the hydrostatic pressure of a column of formation water extending to the surface. Overpressure is pore fluid pressure greater than normal pressure. However, no standard definition exists for what constitutes high overpressure. What can be said is that high overpressure often means trouble. For an explorationist, it could mean blown reservoir seals; for a driller, it could mean excessive time spent fighting formation fluid influxes and/or drilling fluid losses.
A practical upper limit for pore pressure is the overburden stress. Pore pressures in this range are on the verge of opening fractures that can vent fluid and bleed off pressure like a pressure relief valve. Therefore, criteria for defining high overpressure are sometimes expressed in terms of a percentage of the overburden stress, say, pore pressure greater than 90% of the overburden stress. In this article, high overpressure will be defined simply as pore pressure that approaches the overburden stress.
All but one potential cause of overpressure can produce high pressure. Fortunately, the mechanism that cannot generate high pressure is the most common cause of overpressure. Therefore, detecting high overpressure basically boils down to determining where extraordinary overpressure mechanisms may be encountered.
Overpressure detection is based on the premise that pore pressure affects compaction-dependent geophysical properties such as density, resistivity, and sonic velocity. Shales are the preferred lithology for pore pressure interpretation because they are more responsive to overpressure than most rock types. Consequently, overpressure detection centers around shale deformation behavior.
For stress ranges of practical interest, shale compaction is controlled by the difference between total applied stress and pore fluid pressure. This difference, termed the effective stress , represents the portion of the total stress carried by the rock grains. Figure 1 illustrates the effective stress concept with laboratory data for …
190 citations
TL;DR: In this paper, the authors examined the porosity change in porosity during rapid loading by trench turbidites and subsequent underthrusting in the Nankai Trough accretionary complex.
Abstract: Subduction complexes provide an opportunity to examine the interactions of deformation and fluid flow in an active setting. Ocean Drilling Program Leg 190 investigated the relationship between deformation, physical properties, and fluid flow in the toe of the Nankai Trough accretionary complex. With three sites (two from Leg 190, one from a previous leg) penetrating the decollement zone at various stages of development along the same transect, it is now possible to examine the change in porosity during rapid loading by trench turbidites and subsequent underthrusting. Results indicate inhibited dewatering and probable overpressure development seaward of the frontal thrust. Comparison of a reference site porosity versus depth curve to data from a site located within the protothrust zone indicates an overpressure ratio, λ * , of ∼0.42, where λ * = [(pore pressure - hydrostatic pressure)/(lithostatic pressure - hydrostatic pressure)]. These overpressures suggest that the hemipelagic sediments have insufficient permeability for fluid escape to keep pace with the rapid loading by turbidite deposition within the trench. At a site 1.75 km farther arcward, an excess pore pressure ratio of λ * = ∼0.47 was estimated, reflecting the additional loading due to recent thickening by the frontal thrust.
149 citations
TL;DR: In this article, a one-dimensional hydrodynamic model simulating the evolution of pressure and porosity in a progradational deltaic system is presented. But the model is applied to the upthrown and downthrown sides of the major growth fault in the Eugene Island (EI) 330 field (offshore Louisiana).
Abstract: The complex pressure and porosity fields observed in the Eugene Island (EI) 330 field (offshore Louisiana) are thought to result from sediment loading of low-permeability strata. In this field, fluid pressures rise with depth from hydrostatic to nearly lithostatic, iso-pressure surfaces closely follow stratigraphic surfaces which are sharply offset by growth-faulting, and porosity declines with effective stress. A one-dimensional hydrodynamic model simulates the evolution of pressure and porosity in this system. If reversible (elastic) compaction is assumed, sediment loading is the dominant source of overpressure (94%). If irreversible (inelastic) compaction and permeability reduction due to clay diagenesis are assumed, then thermal expansion of pore fluids and clay dehydration provide a significant component of overpressure (>20%). The model is applied to wells on the upthrown and downthrown sides of the major growth fault in the EI 330 field. Assuming that sediment loading is the only pressure source and that permeability is a function of lithology and porosity, the observed pressure and porosity profiles are reproduced. Observation and theory support a conceptual model where hydrodynamic evolution is intimately tied to the structural and stratigraphic evolution of this progradational deltaic system.
96 citations
TL;DR: In this article, two types of vertical overpressure configuration can be identified by electronic logs and mud pressure, based on the calibration with the test pressure, and some important implications for hydrocarbon exploration can be drawn.
59 citations
TL;DR: In this paper, a single pressure equation for fluid expulsion and overpressure in low permeable rock has been studied and compared, and it is shown that the rate of change of porosity can be expressed in at least two different ways for dehydration reactions and oil generation.
55 citations
Related Topics (5)
Performance Metrics
| Year | Papers |
|---|---|
| 2016 | 1 |
| 2013 | 5 |
| 2012 | 2 |
| 2011 | 2 |
| 2010 | 1 |
| 2008 | 1 |