Overpressure Evolution Research Articles

Behaviour study of monitored embankment founded on compressible soil

This paper presents continuous monitoring results of an instrumented embankment behaviour built on compressible soil. In order to better understand of the embankment behaviour and its foundation, geotechnical investigations, measurements of the monitoring data, interstitial overpressure evolution as well as numerical modelling with soft soil model are analysed. The findings highlight various factors resulting in the failure of the infrastructure. The loading program has proven to be incompatible, which subsequently resulted in an excess of pore pressures. The numerical modelling results have illustrated clearly the behaviour of the embankment particularly, the horizontal displacements and the interstitial overpressure.

Tracking fluid pressure buildup from focal mechanisms during the 2003–2004 Ubaye seismic swarm, France

Recent studies have shown that the Ubaye seismic swarm that occurred in the French southwestern Alps in 2003–2004 was triggered by fluid overpressures. This contribution provides additional constraints on the temporal and spatial changes in fluid overpressure during this swarm. The orientations of the double‐couple nodal planes of an extended set of 74 focal solutions, spanning the whole 2003–2004 episode, are compared with the regional stress field. Based on a Mohr‐Coulomb analysis, these comparisons provide estimates of fluid pressures along seismic fault planes. We show that the fluid overpressures required to reactivate the cohesionless fault planes vary through time, with values close to 35 MPa at the inception of the swarm. Overpressures then increase up to 55 MPa during the burst of seismic activity and lastly decrease down to 20 MPa at the end of the crisis. We also show that the fluid overpressures are developed as patches along two parallel faults bordering a releasing bend structure characterized by low to null overpressure. The development of moderate fluid overpressure at the swarm inception enables the reactivation of normal, transtensional, and strike‐slip faults while the development of larger fluid overpressures during the burst of seismic activity progressively enables the reactivation of further misoriented normal, transtensional, and transpressional faults. In order to reconcile the spatial and temporal evolution of the fluid overpressures and the seismic activity, we propose that creep compaction could be the process allowing the successive development of fluid overpressure and the migration of seismicity.

Evolution of continental formation pressure in the middle part of the Western Sichuan Depression and its significance on hydrocarbon accumulation

Abstract Based on the distribution features of present formation pressure, analysis on logging data, and genetic mechanisms of abnormal pressure, the overpressure evolution was reconstructed using numerical basin simulation and other quantitative analysis methods. The phase differences of pressure increasing mechanisms, enhancement differences among different mechanisms, and the control of geopressure evolution on gas migration and accumulation were discussed. The causes of overpressure in the Xujiahe Formation include under-compaction (from early Late Triassic to the end of the Jurassic), hydrocarbon generation (from end of the Early Jurassic to the Neogene) and tectonic compression (since the Cretaceous). It is suggested that tectonic compression and hydrocarbon generation are the principal factors of the present overpressure in the Xujiahe Formation. Overpressure transmission is the main cause of overpressure in the Jurassic strata which occurred during the intense tectonic activity periods since the Cretaceous. The overpressure is the main force for hydrocarbon migration and overpressure-related microfractures are the major pathways for gas migration. Fault transport and overpressure driving caused the formation of Jurassic secondary gas reservoirs with distant sources. The areas with high pressure coefficient in the Jurassic and high-overpressure zones in the Xujiahe Formation are optimum targets for exploration.

Flame acceleration of premixed methane/air explosion in parallel pipes

The explosion propagation characteristics in parallel pipes have been studied for the first time in two different types of pipe. It was found that flame speed and overpressure in two branches of equal length were close when the ignition was at the pipe head. The explosion violence was strengthened after the flame and blast wave were superimposed. When the ignition was acted in the corner of one branch, the peak overpressure near the meeting point was higher than that at the two adjacent points, while the flame speed showed a downtrend. When the parallel pipe was not full of gas, the peak overpressure in the two branches followed similar trends and showed an obvious downtrend. However, when the blast wave reached the meeting point, the peak overpressure also showed an obvious upward trend until the end of the pipe. The flame accelerated in the two different branches of unequal length, but it slowed down when traveling to the meeting point, and the peak overpressure evolution was contrary to the flame speed. The results suggested that the violence of the underground gas explosion near the meeting point was more serious, such that the equipment and people in this area should be paid more attention.

Overpressure and earthquake initiated slope failure in the Ursa region, northern Gulf of Mexico

We use fluid flow and slope stability models to study the evolution of overpressure and slope stability in the Ursa region, northern Gulf of Mexico. Our predictions match measured overpressures (pressures in excess of hydrostatic) from Integrated Ocean Drilling Program Expedition 308 Site U1324 above 200 m below seafloor (mbsf) but overpredict deeper (200–610 mbsf) overpressures by 0.4–1.1 MPa. Modeled overpressure at Site U1322 matches measurements for the entire section (0–240 mbsf) with exception of the measurement at 240 mbsf. Slope stability models that integrate modeled overpressure, vertical stress, and effective stress during deposition predict slope failure at 61 ka on the eastern end of the region. This failure corresponds to the base of a mass transport deposit that has been interpreted as a retrogressive failure initiated by high overpressure. Overpressure alone could not drive failure of a second mass transport deposit (MTD2) that has its base along the 27 ka horizon. With an earthquake acceleration model coupled with our slope stability model, we predict that horizontal acceleration from a magnitude 5 earthquake within 140 km of the Ursa region at 27 ka would initiate the failure that created MTD2; the same earthquake at 20 ka would have to be within 40 km for failure. This magnitude and maximum rupture distance are consistent with seismicity near the Ursa region. We therefore propose that in some cases, overpressure drives failure on low‐angle slopes; however, earthquakes, even on passive margins, may play a critical role in initiating slope failure in sediments weakened by overpressure.

Effect of Overpressure on Rock Frame and Its Implications for Overpressure Evolution

Overpressure is a hot topic in the study of sedimentary basins. It is important in generation, maturation migration, and accumulation of hydrocarbon, but the effects of overpressure on rock frame have not been investigated. In this study, experiments were carried out to study the effects of overpressure on rock frame structures using five core samples from the Junggar basin, Northwest China. The deformations and velocities for the samples were measured at different effective pressures related to non-equilibrium compaction and fluid expansion overpressure mechanisms. The results show that the effect of overpressure on rock frames gradually increases when the effective pressure drops down to a certain value (called critical pressure). Moreover, non-equilibrium compaction mechanism has more effects on rock frames than fluid expansion mechanism under the same effective pressure. Furthermore to study rock frame structural changes, we use Kuster and Toksöz's expressions to simulate the effective aspect ratios of inclusions α (penny shapes) for different effective pressures. The results show that the α decreases dramatically when the effective pressure decreases from the critical pressure. Changes of α can be interpreted as responses to the rock frame changes when grains conform one another by rotating and self-adjusting. However, different mechanisms of overpressure have different effects on rock frames. The rock frame can be affected more easily by overpressure in shallow regions generated by non-equilibrium compaction mechanism. Once this kind of rock frames are preserved after overpressure releases to a normal hydrostatic pressure, they can be identified by their specific rock frame characters. This method provides a new way to study overpressure release and fluid migration and accumulation.

Overpressure distribution and pressuring mechanism on the southern margin of the Junggar Basin, Northwestern China

On the basis of the pressure data ob- tained from the southern margin of the Junggar Basin, Northwestern China, the distribution and evolution of overpressures in high permeable sandy and in low-permeability shale formations are characterized. The evolution of overpressures in various structural processes, especially in erosion and mechanical de- formation, is quantified by numerical modeling stud- ies. The modeling results show that anomalies of high pressure observed in boreholes are likely a combination of several effects, i.e. 1) the compaction caused by overloading, 2) the compaction caused by lateral tectonic stressing, 3) lateral transmission of hydraulic pressure associated with fold formation, and 4) vertical hydraulic connection caused by opening faults crossing formations with different initial excessive pressures. The main mechanisms of overpressures and their distributions are defined.

Modeling fluid transfer along California faults when integrating pressure solution crack sealing and compaction processes

Several models pertaining to earthquake cycles imply intermittent fluid flow through fault. During the interseismic period, increase in fluid pressure from hydrostatic to lithostatic values is a crucial parameter in mechanisms leading to earthquakes. To achieve such pressures, geodynamic processes (gouge compaction, fluid flow) and changes in permeability are required. Previous models have postulated that changes in permeability (by self‐healing) are faster than the effects of geodynamic processes. We consider the different mechanisms and rates of crack sealing near active fault on the examples of uplifted Californian faults. We find that natural crack sealing is normally not achieved by a rapid self‐healing process. Pressure solution, with mass transfer from solution cleavage to cracks, appears to be a more important mechanism for crack sealing and creep during postseismic deformation. The geometry of transfer path and experimental data have been used to model crack sealing rates by pressure solution which are estimated to be rather slow, similar to the recurrence time of some earthquakes. Such slow changes in permeability may be crucial factors in controlling the increase in fluid pressure and, consequently, the mechanism of critical failure in faults. Then, numerical modeling of fluid pressure and transfer around active faults has been performed integrating a slow change in permeability by crack sealing, gouge compaction, and fluid flow from depth. This modeling shows various location and evolution of fluid overpressure during the interseismic period depending on these processes and allows one to estimate the amount of fluid transferred from depth during interseismic periods.

Fluid pressure evolution during the earthquake cycle controlled by fluid flow and pressure solution crack sealing

Recent studies of active Californian faults allow us to investigate the mechanisms of fluid flow and crack sealing along faults and to model fluid pressure evolution during earthquake cycles. The model is first based on the observation that fluids flow from depth along active faults (stables isotopes and traces elements analyses). The model is also based on the study of mechanisms of deformation of rocks near and within active faults at depth. Sampling was performed in faults that have been recently uplifted. Studies on thin sections of rocks produce evidence of the mechanisms of crack sealing and compaction of fractured zones as transient processes after each earthquake. We found that pressure solution creep and crack sealing are likely to control deformation and permeability change during inter-seismic period. Crack sealing and compaction processes are modeled to give the kinetics of these processes. Then numerical modeling of fluid pressure and transfer around active faults have been performed by integrating slow changes in permeability by crack sealing, gouge compaction and fluid flow from depth. This modeling shows that location and evolution of fluid overpressure varies during the inter-seismic period depending on the heterogeneity of the faults (nature of minerals and fluids, spacing between micro-fractures, thermodynamic and kinetics parameters).

The influence of faults and intraplate stresses on the overpressure evolution of the Halten Terrace, mid-Norwegian margin

The Halten Terrace is a structural element of the Meso-Cenozoic mid-Norwegian margin. The pore fluid pressure distribution in the faulted Jurassic formations on the Halten Terrace is characterized by significant lateral variations. In general, the fluid overpressure increases stepwise across faults from east to west, from zero (hydrostatic fluid pressure) to about 30 MPa. Fault-bounded pressure cells can therefore best explain the fluid pressure distribution. The results of analyses of log-derived porosities indicate that the high overpressure in the westernmost pressure cell was built up recently. However, despite the high sedimentation rates during Plio-Pleistocene, the high overpressure cannot be explained by local mechanical compaction. Alternative explanations for the high overpressure proposed by other authors are based on pore fluid volume increase (e.g. hydrocarbon generation). We propose that the high overpressure is caused by fluid flow from the deep Rås Basin to the western part of the Halten Terrace, through fractures in the Mesozoic, deep seated Klakk Fault Complex. Opening of fractures in this fault zone by seismic and static mechanisms is possible in the present-day intraplate stress field, which is characterized by a NW–SE oriented maximum horizontal stress direction. During Miocene, the maximum horizontal stress was E–W oriented, which implies a stress rotation during Pliocene. The E–W orientation of the maximum horizontal stress has impeded the initiation and opening of fractures in the N–S striking Klakk Fault Complex during Miocene. Fluid flow from the Rås Basin through faults of the Klakk Fault Complex can therefore have occured since Pliocene. Thus, the rotation of the intraplate stress directions can explain why the build-up of overpressure on the western part of the Halten Terrace occured recently, as indicated by the results of porosity analyses. Understanding the overpressure evolution of the Halten Terrace is important for exploration in that area, as hydrocarbons have been found in the hydrostatic pressure cells, whereas they are absent in the high overpressure cells.

Basinal fluid flow models related to Zn-Pb mineralization in the southern margin of the Maritimes Basin, Eastern Canada

The carbonate-hosted Zn-Pb deposits in the Lower Windsor Group, Nova Scotia, are located along the southern margin of the Maritimes basin. Previous studies suggest that the deposits formed from basinal brines expelled from the basal part of the Maritimes basin (Horton Group), possibly related to tectonic events in late stages of the basin history, but it remains unclear where the specific source regions were and which mechanism is responsible for fluid expulsion from the source regions to the sites of mineralization. This paper presents two numerical models to address these problems. The first model, based on a stratigraphic profile ranging from the center to the southern margin of the Maritimes basin, simulates the distribution and evolution of fluid overpressures due to sediment compaction. The second model simulates the temperature distribution in a marginal sub-basin under the assumption that fluid flow was driven by topographic relief related to an uplift (highlands) proximal to the ore deposits.Modeling of fluid overpressure evolution indicates that, if the Horton Group sediments were laterally confined by basement highs or faults, fluid pressures approaching or exceeding loading pressures would be easily built up within the Horton Group after deposition of the overlying Windsor evaporites. Strong overpressures in the Horton Group rocks are predicted not only in the central part of the Maritimes basin but also in shallower sub-basins close to the sites of mineralization. In contrast, if the Horton Group rocks are assumed to be laterally continuous across the Maritimes basin, fluid pressures in the Horton Group would remain near hydrostatic and strong overpressures would be built up only within the evaporite layer. In both cases, the geothermal gradients would not be significantly disturbed by the sediment compaction-driven fluid flow.Modeling of topography-driven flow indicates that the geothermal gradients are only slightly disturbed if rock permeabilities are inherited from the compaction model but could be strongly disturbed if higher permeabilities are assigned to the basal aquifer, and high-permeability zones (conduits) are assumed to cut through the evaporite layer and link the highlands (recharge area), basal aquifer, and discharge area. The temperature at the site of mineralization could be increased relative to the background temperature, but under steady-state conditions it could not have reached the ca. 250 degrees C indicated by fluid inclusions in the deposits. Such high temperatures could be reached transiently if the conduits and basal aquifer had permeabilities higher than about 0.1 D.The numerical modeling results suggest that sediment compaction-driven fluid flow could not have been responsible for mineralization because it cannot satisfy the thermal conditions at the deposits. Topography-driven flow may satisfy the thermal conditions at the deposits under the assumption of high-permeability conduits and a basal aquifer. Whether or not such conditions existed in the southern margin of the Maritimes basin needs further study. Based on the modeling results of fluid overpressure development, we favor a model in which the main-stage ore-forming fluids were derived from the basal part of individual sub-basins proximal to the deposits and were driven by sudden release of overpressures, probably triggered by tectonic events. Topography-driven flow may have been dominant in the postore stage, i.e., after the dissipation of overpressures. This model agrees with other geochemical studies that indicate separate source regions for different deposits, high fluid flow rates, and involvement of lower salinity fluids after the main-stage mineralization.

Geological Models for the Next Decade: What Do We Need to Reduce Hydrocarbon Exploration Risk?

Several main areas of quantitative geologic model development, needed over the next decade, are outlined using illustrative examples to emphasize need. In order of importance these areas are: (i) motion of multiple faults in 3-D tied to multi-phase fluid-flow and overpressure evolution in sediments: (ii) dynamical primary and secondary hydrocarbon migration in 3-D; (iii) self-consistent salt and sediment dynamical interactions; (iv) turbidite and sequence stratigraphic models; (v) self-consistent dynamical tectonic and sediment evolution models; (vi) integrated economic and geological relative importance and risking methods. As vital adjuncts, each area of model development should include the capabilities: (a) of forward and inverse modes of operation; (b) of being able to provide quantitative measures of uniqueness, resolution, precision and sensitivity in relation to intrinsic model assumptions, parameters in the model, numerical limits, and in respect of quality, quantity and sampling frequency of present-day data used as control information; (c) of yielding quantitative probabilistic assessments of the range of results in view of the indeterminancy of point (b); (d) of culling needed data from a common data base; (e) of being built and used in a user-friendly, technology-transferable mode. No crystal ball is perfect, so this present view of the next decade's needs in geologic modeling should be continuously up-dated and modified as future information becomes available, which will influence the values, worth, and priorities of the presently estimated future needs in geologic model requirements.