Formation Fluid Pressure Research Articles

Efficient sealing of cemented sheath: An oil-responsive self-healing latex with toughness enhancement for cement composites

The integrity of the cemented sheath is important to the safe exploration of oil and gas resources. Sealing failure of cement sheath will result in formation fluid channeling and pressure carryover in the annulus. The both toughening and oil-responsive self-healing latex (TOS) was prepared to achieve efficient sealing of cemented sheath, and prolong its service life. The structure and oil swelling behavior of TOS, its toughening and self-healing effects on hardened cement paste (HCP) were analyzed by comprehensive characterizations. Results demonstrated that TOS possessed excellent stability and an oil swelling rate of 1235 %. Cement specimens by curing 28d containing 8 % TOS exhibited a 27.8 % increase in flexural strength, a 48.7 % reduction in elasticity modulus and a 33.8 % reduction in brittleness coefficient. The addition of TOS can effectively improve the toughness of cement. The average pore size of specimens containing 8 % TOS admixture decreased to 9.17 nm. This indicates that TOS enhanced the structural density of cement materials. The crack about 103μm in the cracking specimens with 8 % TOS was almost healed, and the corresponding permeability was reduced 99.1 % after curing in oil. Adsorption and film formation of TOS can resist the cracking of cement sheath, and the oil swelling expansion of film in cement matrix can repair the cracked structure.

Effect of Formation Pressure on Pore Structure Evolution and Hydrocarbon Expulsion in Organic-Rich Marine Shale

Exploring the relationship between formation pressure and shale pore evolution is helpful for the enrichment and development of marine shale gas accumulation theory. The thermal evolution experiment was carried out on the Xiamaling Formation (Pr3x) lowly matured marine shale, which has a similar sedimentary environment to the Longmaxi Formation (S1l) highly matured marine shale. Comparative experiments of open and semi-closed pyrolysis and multiple pore structure characterization techniques, including CO2 and N2 physisorption, mercury intrusion porosimetry, and field emission scanning electron microscopy, were conducted. The marine shale pore evolutionary model under formation pressure is proposed by characterizing pore evolution, and hydrocarbon expulsion and retention for shales under and without formation fluid pressures. The results show that the existence of formation pressure increases the percentage of quartz and reduces the content of clay minerals. The change in formation pressure has no obvious effect on the maturity evolution of shale samples. With the increase of formation pressure, the pore morphology of shale gradually changes from narrow slit pores to ink bottle-shaped pores. The retained hydrocarbons in shale mainly occupy the mesopore space, and the existence of formation pressure promotes hydrocarbon expulsion, especially the hydrocarbon expulsion in the mesopore. In addition, formation pressure improves pore connectivity, especially in the high-over mature stage of shale. With the increase of formation pressure, the micropore volume decreases slightly, the mesopore volume increases significantly, and the macropore volume changes have two stages.

Monitoring the 2021 Mw 8.2 Alaska Earthquake by an Offshore Seismic and Fluid Pressure Observation Network and Implications for Ocean‐Crust Dynamic Coupling

AbstractGround shaking caused by earthquakes is accompanied by seafloor and sub‐seafloor formation fluid pressure variations in offshore areas, but there have been few collocated observations of these signals. In this work, we report seismic and high‐sampling‐rate fluid pressure records of the 2021 Mw 8.2 Alaska earthquake by the Ocean Networks Canada (ONC) NEPTUNE observatory at an epicentral distance of ∼2,200 km in the northeast Pacific Ocean. The system comprises observatory nodes in various tectonic environments, with each node including buried broadband seismometers, seafloor pressure sensors, and, at two nodes, borehole pressure sensors. Seismic and tsunami waveforms of the Mw 8.2 earthquake were documented in detail. Seismic seafloor pressure variations (Psf) were dominated by Rayleigh waves of periods between 5 and 50 s, with peak amplitudes of 3–4 kPa at most sites. Waveform similarity and the linear scaling between Psf and vertical ground acceleration indicate forced acceleration of the water column being dominant in governing Psf during long‐period surface‐wave arrivals, with an additional component of elastic oscillation occurring at higher frequencies (>0.1 Hz) causing extra pressure signals. Analysis of formation pressure variations due to various types of ocean loading of distinctly different frequencies (e.g., tides, tsunami, and infragravity waves) shows stable one‐dimensional vertical loading efficiencies that depend on lithology at each borehole site, with loading response being strongly influenced by the presence of free gas at shallow depths within the Cascadia accretionary prism. Inter‐site comparisons of seismic and seafloor pressure waveforms demonstrate a key role of sediment thickness in the amplification of surface wave amplitudes.

Seismic Formation Fluid Pressure Observations Reveal High Anisotropy of Oceanic Crust

AbstractFractures and faults in igneous oceanic crust affect hydrothermal circulation and hydration of the oceanic plates. Seismic observations have provided information about lithospheric fabrics, but direct measurements of oceanic crust’s elastic properties were not made. We report determinations of compressibility of the upper igneous crust (of 3.6 Ma) of the Juan de Fuca plate, using observed formation fluid pressure oscillations in sealed boreholes associated with passing surface waves from distant earthquakes. We determine an azimuthal variation of formation‐matrix compressibility by a factor of ∼5, with the crust being most compressible in the plate‐spreading direction across the structural fabric inherited from crustal creation. This is equivalent to a seismic compressional wave speed anisotropy of ∼50%–60%, much greater than that of standard seismic measurements (typically <20%). This likely reflects a previously unresolved degree of fracturing of the uppermost oceanic crust, consistent with existing observations that suggest a high degree of hydraulic permeability anisotropy.

Some issues of ensuring the stability of the walls of directional wells and preventing the absorption of process fluids

Most of the main types of complications in the process of drilling wells, such as collapses, taluses, collapses of the walls, the formation of caverns, etc., are associated with external mechanical and hydrodynamic effects on the walls of the wellbore. Therefore, ensuring the stability of the borehole walls is one of the urgent and difficult technological formation fluid pressure on mechanical processes in rocks when they are opened with a directional well, especially of horizontal wells. This article provides solutions to problems associated with hydraulic fracturing of a well and the condition of its prevention, calculation of generalized stresses in the rock formation for inclined well. As a result of this calculations, the necessary data are obtained for making a decision on the density of the drilling fluid to drilling the considered interval of rocks, as well as for making other technological decisions. The given calculation formulas make it possible to fully evaluate the effect of the formation fluid pressure on the mechanical processes in the rocks when they are opened by a horizontal well. Keywords: hydraulic fracturing; blade bit; steel ball-shaped toothed bit; polycrystalline diamond bit; laser drilling; impact rope drilling; rotary drilling.

Theoretical preconditions for modeling wellbore stability and predicting hydraulic fracturing

One of the complex technological tasks in the process of drilling is to ensure the stability of the wellbore walls, as well as their modeling for further forecasting the state of the wellbore and the likelihood of hydraulic fracturing. This is due to the fact that most of the complications and factors affecting the equilibrium state of the wall are associated with external influences. The article discusses the mechanical and partially hydraulic aspects of solving the described problems associated with modeling the stability of the wellbore walls and predicting hydraulic fracturing. As a result of calculations, the necessary data are obtained for making a decision on the density of the drilling fluid for drilling the considered interval of rocks. The assumed model of the porous rock and the given calculation formulas make it possible to fully evaluate the influence of the formation fluid pressure on the mechanical processes in the rocks when they are opened by a well. Keywords: hydraulic fracturing; blade bit; steel ball-shaped toothed bit; polycrystalline diamond bit; laser drilling; impact rope drilling; rotary drilling.

Seismicity and Stress Associated With a Fluid‐Driven Fracture: Estimating the Evolving Geometry

AbstractA coupled approach, combining the theory of rate‐ and state‐dependent friction and methods from poroelasticity, forms the basis for a quantitative relationship between displacements and fluid leak‐off from a growing fracture and changes in the rate of seismic events in the region surrounding the fracture. Poroelastic Green's functions link fracture aperture changes and fluid flow from the fracture to changes in the stress field and pore pressure in the adjacent formation. The theory of rate‐ and state‐dependent friction provides a connection between Coulomb stress changes and variations in the rate of seismic events. Numerical modeling indicates that the Coulomb stress changes can vary significantly between formations with differing properties. The relationship between the seismicity rate changes and the changes in the formation stresses and fluid pressure is nonlinear, but a transformation produces a quantity that is linearly related to the aperture changes and fluid leak‐off from the fracture. The methodology provides a means for mapping changes in seismicity into fracture aperture changes and to image an evolving fracture. An application to observed microseismicity associated with a hydrofracture reveals asymmetric fracture propagation within two main zones, with extended propagation in the upper zone. The time‐varying volume of the fracture agrees with the injected volume, given by the integration of rate changes at the injection well, providing validation of the estimated aperture changes.

In Situ Permeability and Scale Dependence of an Active Accretionary Prism Determined From Cross‐Borehole Experiments

AbstractPermeability controls fluid flow in the Earth's crust and affects a wide range of processes including advective transport and pore pressure generation. However, in situ measurements of permeability are few, especially in active tectonic settings or at scales relevant to regional flow. We analyze formation fluid pressure records from oceanic boreholes in the Nankai accretionary prism offshore southwest Japan, focusing on unexpected responses to drilling operations conducted at boreholes ~100 m to the northeast. We develop a 2‐D numerical model of transient fluid flow and conduct a parametric grid search to define hydraulic diffusivity. A value of 0.19–0.46 m2/s (corresponding to a permeability of 9.8 × 10−13 to 2.4 × 10−12 m2) yields the best fit to observed pressure responses. Together with laboratory measurements on core samples and drillstrem tests reported in previous studies, our analysis indicates a strong scale dependence of permeability, likely reflecting the presence of permeable faults and fractures.

A Numerical Model for Caprock Analysis for Subsurface Gas Storage Applications

In considering a site for gas storage, it will be important to evaluate the effects of gas storage on the formation, so as to minimize the risk of a breach occurring in the system. Gas injection will result in an increase in formation fluid pressure, especially around the injection source, which in turn results in redistribution of the stress field. The induced deformations within the reservoir can potentially result in a damage zone within the caprock formation. This mechanical failure may involve shear along many of the existing fractures or creation of new fractures that reduce the sealing properties of the caprock system. The main objective of this paper is to develop a model to estimate the growth and extension of cracks in the caprock. In order to achieve this, the smeared crack approach is used to model the process of cracking in the caprock. Smeared cracking is a continuum approach for damage mechanics which is based on the idea that a crack is modeled by modifying the strength and stiffness of the material. The main model presented in this paper has three sub-models, which are the reservoir model, the caprock model and the smeared crack model. The reservoir model is a simplified coupled hydro-mechanical model that numerically simulates the radial fluid flow and analytically estimates the associated stress and strain within the reservoir. The results of the reservoir model are used as boundary conditions for the caprock model that estimates the stress and strain within the sealing caprock due to the deformation of the reservoir. Using the calculated stress and strain, the smeared crack model predicts the growth and extension of cracks within the caprock. The caprock is assumed to be initially crack free and impermeable. The developed model is then used to study the Yort-e-shah aquifer caprock in Iran to predict the growth and extension of cracks.

Studying empirical correlation between drilling specific energy and geo-mechanical parameters in an oil field in SW Iran

Multiplicity of the effective factors in drilling reflects the complexity of the interaction between rock mass and drilling bit, which is followed by the dependence of parameters and non-linear relationships between them. Rock mass or, in other words, the formation intended for drilling, as the drilling environment, plays a very essential role in the drilling speed, depreciation of drilling bit, machines, and overall drilling costs. Therefore, understanding the drilling environment and the characteristics of the in-situ rock mass contributes a lot to the selection of the machines. In this work, a 1D geo-mechanical model of different studied wells is built by collecting the geological data, well logs, drilling data, core data, and pressure measurements of the formation fluid pressure in various wells. Having the drilling parameters of each part of the formation, its specific energy is calculated. The specific energy index can be used for predicting the amount of energy consumed for drilling. In order to find the relationship between the drilling specific energy (DSE) and its effective parameters, the multivariate regression model is used. Modeling DSE is done using the multivariate regression, which contains the parameters rock characteristics, well logs, and a combination of these two features. 70% and 30% of the data are, respectively, selected as the training and test for validation. After analyzing the model, the correlation coefficients obtained for the training and test data were, respectively, found to be 0.79 and 0.83. The parameters uniaxial compressive strength (UCS), internal friction angle, and fluid flow are among the most important factors found to affect DSE.

Research advances in the formation poredynamics of sedimentary basins

In the 21st century, the geodynamics is developing towards quantitative researches. However, due to the irreversible geological processes, it was very difficult to recover the geological process. In particular, the restoration of geological parameter evolution process at the microscopic scale has become a major scientific problem in geology presently. Thereby, a concept of the formation poredynamics is revised and proposed, and the formation poredynamics is a fundamental discipline which focus on the mechanical characteristic of porous media, the pore evolution law, the dynamic genesis and the seepage property of pore fluid during the burial process of clastic rocks. Moreover, it is a new interdiscipline of underground diagenetic dynamics and pore fluid dynamics, and also is as an important part of sedimentary basin dynamics. Research advances were made in both basic theory and applied research. The advances in the basic theory include: (1) the static equilibrium principle of the formation pore, (2) the porosity evolution mechanism and quantitative model of sandstone during the burial diagenetic process, (3) the compaction characteristic and the porosity evolution quantitative model of mudstone, (4) the theoretical relationship between the underground pore fluid temperature and the pore fluid pressure, (5) the influence of the tectonism-induced additional geostress on the pore fluid pressure, and (6) the relationship between the mudstone compaction and the vitrinite reflectance (Ro) of organic matter. The advances in the applied research include: (1) the geotemperature-geopressure system division of the sedimentary basin and the interpretation of the hydrocarbon distribution dynamic, (2) the modification of the strata pressure prediction model, (3) the construction of the reservoir critical properties and the reservoir dynamics evaluation system, (4) the simulation of the evolution process of the formation fluid pressure, (5) the numerical simulation and physical experimental simulation on the sandstone hydrocarbon charging dynamics, and (6) the dynamic process analysis of the hydrocarbon accumulation in tight sandstone. Through the integration between the pore genesis evolution and the pore fluid dynamic evolution, the formation poredynamics is one of the representative discipline branches that the geological dynamics research had developed toward the underground microscopic scale in recently 20 years, and it also is an inevitable result from the quantitative development of the formation and distribution mechanisms of sedimentary mineral deposits. Based on the formation poredynamics research, eight important research achievements are summarized, and the geological researches are extended from the macroscopic scale to the microscopic scale, to find out the pore parameter evolution law under control of the formation pore evolution during the burial process, and update and improve exploration and production application technologies.

Time Optimizing near the Pay Zone

Well control techniques are used in oil and gas drilling operations to control bottom hole pressure and avoid any fluid influx from formation to the well. These techniques are highly important near the pay zone in term of time. Controlling formation fluid pressure and thereby the formations behavior in a predictable fashion will help toward more optimized environmental friendly drilling operation. Time consumed to control the formation fluid pressure will range between few hours to many days. This paper discusses hydrostatic pressure distribution and changes near the pay zone for one oil blocks in Kurdistan, in the northern part of Iraq. Obtaining homogeneous increase in some drilling fluid properties will help the engineer to better interpret sampling of the lithological columns and reduce potential hole problems and operation time.

Fluid evolution in the Dabei Gas Field of the Kuqa Depression, Tarim Basin, NW China: Implications for fault-related fluid flow

A series of fault-related folds developed in the Dabei Gas Field of the Kuqa Depression, western China form major traps for hydrocarbon accumulation. The Bashijiqike (K1bs) sandstone reservoirs in the different fault-related folds of the Dabei Gas Field display similar excess fluid pressure, formation water type and salinity, and as well as light oil and gas properties. The light oil and gas in the K1bs sandstone reservoir of the traps share the same source and were generated at a thermal maturity level of 1.4–1.6% Ro and 1.7–2.3% Ro, respectively. To investigate the fluid evolution in the fault-related fold traps, an integrated fluid inclusion analysis was performed including petrography, fluorescence spectroscopy, microthermometry, Laser Raman spectroscopy and thermodynamic modeling. The fluid evolution recorded by the fluid inclusions in the different fault-related folds indicates that the formation fluid was characterized by low salinity at normal hydrostatic pressure from approximately >9–5 Ma. From 5 to 3 Ma the formation water salinity and fluid pressure increased rapidly. The formation water attained the highest salinity at approximately 3 Ma to present while the pore fluid overpressure decreased with time in the reservoir. Two episodes of oil and one episode of gas charge were identified in the K1bs sandstone reservoir with the second episode of oil charge occurring around 5–4 Ma, while the gas charge occurred around 3–2 Ma. The injection of new fluids associated with oil and gas charge caused changes of the formation water salinity in the different fault-related fold traps within the gas field. The fluid evolution in the Dabei Gas Field can be used to indicate fault-related fluid flow as the thrust faults are suggested to be the primly pathways for fluid migration and the fluid flow was controlled by the reactivation of these thrust faults. The similarity of the oil and gas charge histories, formation water salinity and fluid pressure evolution in the K1bs sandstone reservoirs among the different fault-related traps in the Dabei Gas Field implies that the fault-related fluid flow process and activation of the thrust faults were concomitant since approximately 9 Ma.

Optimization of Cement Spacer Rheology Model Using Genetic Algorithm (Research Note)

The primary cement job is a critical step in successful well completion. To achieve effective cementing job, complete mud removal from the annular is recommended. Spacer and flushers are used widely to achieve this goal. This study is about weighted cement spacer systems containing a surfactant package, weighting agent and rheological modifiers. Weighted spacer systems are utilized when a high formation fluid pressure is expected inside the wellbore. A testing program is conducted in laboratory to determine the spacer fluid rheological properties in different temperatures. The measured rheological properties are estimated using the known rheological models. Each model is firstly optimized using Genetic Algorithm as an optimization tool and then the rheological properties are modeled. The performance of the genetic algorithm is then tested by comparing the real laboratory data and modeled data.

An integrated site characterization-to-optimization study for commercial-scale carbon dioxide storage

Injection of supercritical carbon dioxide (scCO2) into deep saline aquifers is considered a promising option to mitigate global climate change. At a storage site, the main objectives of carbon dioxide sequestration are to maximize the volume of scCO2 injected and minimize the leakage risk, while effectively managing formation fluid pressure buildup and the brine displaced by scCO2. An integrated characterization-to-optimization study is carried out for potential commercial-scale deep saline aquifer carbon dioxide storage proposed in western Wyoming. A three-dimensional heterogeneous reservoir model is built for which petrophysical and fluid flow parameters are populated using field characterization data and state-of-the-art laboratory measurements. The measured scCO2 relative permeability end point is low compared to previous measurements on similar sandstones, which poses a challenge for CO2 flow, formation pressure control, and storage efficiency. By carefully selecting a set of optimal well locations, perforation intervals, and bottomhole pressure constraints that lead to maximum CO2-in-place and minimal CO2 breakthrough at the producers, an injection rate ranging from 10.8 to 15.1Mt/year is achieved for a duration of 50 years. After scCO2 injection ceases, up to 62% of the total injected scCO2 can be immobilized as residual scCO2 in 1000 years. Because of the low scCO2 relative permeability end point, post-scCO2-injection chase brine operation is not found to be an effective means of enhancing residual trapping. Instead, by modulating reservoir fluid pressure, boundary conditions of the reservoir exert a more significant impact on flow. Given the same well configuration and bottomhole pressure constraints, an open reservoir with lateral background flow allows 40% additional scCO2 injection compared to a compartmentalized system without background flow. However, background flow leads to a lower trapping efficiency – after 1000 years post-scCO2-injection, only 54% of the total injected scCO2 is immobilized as residual scCO2. This research suggests that a careful engineering design can contribute to significant CO2 storage at commercial scales while enhancing storage security. Site-specific multi-phase flow data should be measured for such a design, since for the study site, chase-brine operation is not effective when scCO2 relative permeability is low.

Slow and delayed deformation and uplift of the outermost subduction prism following ETS and seismogenic slip events beneath Nicoya Peninsula, Costa Rica

Two ODP CORK (Ocean Drilling Program circulation obviation retrofit kit) borehole hydrologic observatory sites deployed in 2002 at the toe of the subduction prism off Nicoya Peninsula, Costa Rica were visited in December 2013. The five years of seafloor and formation fluid pressure data collected since the previous visit include clear signals associated with an episodic tremor and slip (ETS) event off the coast of Nicoya Peninsula in 2009, and a Mw 7.6 subduction thrust earthquake beneath the Peninsula in 2012. Formation pressure anomalies associated with the ETS event are similar to ones observed following ETS events observed previously here, as well as ones following very low frequency earthquake swarms within the Nankai accretionary prism off southwestern Japan. Positive and negative impulsive transients in the hanging wall and foot wall of the subduction thrust, respectively, suggest contractional and dilatational strain generated by local slip propagating up the thrust fault beneath the outermost prism. In the case of the 2009 event, the transients occurred roughly two weeks after the initiation of slip observed at GPS sites along the adjacent coast. At the same time, a decrease in seafloor pressure at the prism site relative to the subducting plate was observed, indicating concurrent uplift of the prism of 1.2 cm. Other events at the prism toe following ETS events closer to the coast are seen in 2006, 2007, 2008, 2010, and 2011. The time between the initiation of ETS slip constrained by GPS and the onset of the prism toe transients suggest up-dip “rupture” propagation along the seaward part of the subduction thrust at rates of a few km/day. In the case of the 2009 event, the slip at the prism toe (c. 11 cm), estimated from the 1.2 cm uplift and the local dip on the decollement (6°), is roughly a factor of 5 greater than the slip further landward estimated from GPS data by Dixon et al. (in press). In other cases, slip at the toe is less or unresolvable. Similar uplift and formation fluid pressure transients occurred two days after the 2012 Nicoya thrust earthquake, then episodically (unrelated to aftershocks) every 1–3 weeks over the next few months. Slip rates at the prism toe, estimated from the magnitude and duration of these and ETS-related uplift events, are roughly 5 cm/day. Such rates are much too slow for tsunamigenesis or seismogenesis. Total slip of the outermost prism after the 2012 earthquake, determined from the uplift accumulated until the end of the observation period and the estimated local fault dip, is roughly 75 cm, a significant fraction of the seismogenic slip of the Mw 7.6 earthquake itself (a maximum of over 3 m). Negative steps in formation pressure are observed at both the prism and subducting plate sites simultaneous with the 2012 thrust earthquake. The sign of these steps is inconsistent with the contractional deformation predicted by an elastic fault-dislocation model with slip limited to the seismogenic zone. Highly compliant (low shear modulus) sediment below the decollement, or very low-stress deformation along splay faults, may be responsible for the co-seismic dilatation observed in the outermost prism and incoming plate.

Continuous deep-seated slope failure recycles sediments and limits levee height in submarine channels

Channel-levee systems are responsible for constructing deep sea fans, among the largest sedimentary deposits on Earth. Levee height plays a key role in defining the volume and texture of the material that is deposited in the bounding levees, and thus the morphology of submarine fans. Models of channel formation and evolution generally assume that the levees aggrade in response to the cumulative overspill of turbidity flows, and that their height is controlled by these flows. In contrast, we show that levee growth in the Ursa Basin (Gulf of Mexico) is limited by the mechanical strength of the levee, not the flow behavior. While many studies document sidewall failures in levee systems, our poro-mechanical model is the first to demonstrate that collapse of levees is a large-scale, deep-seated process driven by the interaction of levee formation and high fluid pressure. Rapid deposition of a regional sand unit induced large fluid overpressure in the underlying mud, which preconditioned the system for levee failure, which then fed a large volume of sediment back into the channel-levee system. Long-lived levee failures continually reintroduced previously deposited levee material back into the channel system. This implies that a large volume of sediment is continuously recycled, which has not been previously understood. Turbidite flow models generally assume that flows progressively lose their fine-grained component due to levee overspill as they traverse the channel. In contrast, we show a mechanism by which fine-grained material can re-enter the system in large quantities, and this has significant and broad importance for models of channel and fan evolution. We also show that that levee failure introduces significant unconformities, in contrast with the common assumption that levees offer complete and high-resolution records of climate, tectonics, and sea level.

Episodic deformation and inferred slow slip at the Nankai subduction zone during the first decade of CORK borehole pressure and VLFE monitoring

With new data recovered in December 2011 and November 2012 from two Ocean Drilling Program (ODP) CORK (circulation obviation retrofit kit) borehole observatories in the toe of the Nankai subduction zone accretionary prism (Hole 808I) and in the subducting Philippine Sea plate (Hole 1173B) off Southwestern Japan, records of formation fluid pressure now span over 10yr. Over nearly this same period of time the Japanese terrestrial HiNet array has enabled detection of small earthquakes across the breadth of the accretionary prism. The records include several formation fluid pressure anomalies and concurrent local very-low-frequency earthquake (VLFE) swarms. In the subducting plate, pressure anomalies are most commonly slow negative steps that are inferred to reflect dilatational strain associated with slow slip on the subduction thrust fault in the areas of VLFE activity. In three instances, concurrent positive impulsive anomalies are observed in the prism; these are inferred to reflect contraction when slip reaches the location of Hole 808I. The spatial distribution of VLFEs suggests that slow slip may occur in patches that cumulatively span the seaward half of the subduction prism. Two anomalies occurred at times of the largest earthquakes in the region, the September 2004 Mw 7.7 off-Kii earthquake 220km to the northeast, and the March 2011 Mw 9.0 Tohoku earthquake roughly 900km to the northeast. In the subducting plate, the observed changes in pressure are abrupt and consistent in sign with the expected coseismic strain (contractional and dilatational, respectively). In the case of the Tohoku earthquake, the co-seismic pressure decrease was followed 12 days later by an additional but slow decrease, and by an impulsive but complex and long-lived increase in the accretionary prism Hole 808I. In this instance, co-seismic cross-strike stress may have triggered post-seismic slip and the deformation seen in the plate and prism. Given the great distance from the Tohoku epicenter and small change in stress estimated at the Nankai observatory sites (ca. 7kPa shear stress), triggering of local slip would require the outer subduction thrust and overlying prism in this region to be very fragile. To what degree this state might vary with time is not known, but tracking episodic slip in the seaward part of this and other subduction zones using formation-fluid-pressure and VLFE monitoring may provide valuable clues about the evolution of subduction faults through their thrust earthquake cycles.

Pressure and fluid dynamic characterisation of the Dutch subsurface

AbstractThis paper presents and discusses the distribution of fluid and leak-off pressure data from the subsurface of onshore and offshore Netherlands in relation to causes of formation fluid overpressure and the permeability framework. The observed fluid pressure conditions demonstrate a clear regional difference between the southern and the north and north-eastern part of the study area. In the southern area, formation fluid pressures are close to normal and well below measured leak-off pressures. In the north, formation fluids are overpressured and may locally even approach the measured leak-off pressures. The regional differences in fluid overpressure can, in large part, be explained by differences in geologic framework and burial history. In the south, relatively low rates of sedimentary loading and the presence of relatively permeable sedimentary units have led to the currently observed normally pressured conditions. In the northern area, relatively rapid Neogene sediment loading plays an important role in explaining the observed overpressure distributions in Cenozoic mudstones, Cretaceous Chalk and Rijnland groups, and probably also in Jurassic units. The permeability framework of the northern and north-eastern area is significantly affected by Zechstein and Triassic salt deposits and structures. These units are characterised by very low permeability and severely restrict fluid flow and pressure dissipation. This has created hydraulically restricted compartments with high overpressures (for example overpressures exceeding 30 MPa in the Lower Germanic Trias Group in the Terschelling Basin and Dutch Central Graben).

Coupled multiphase fluid flow and wellbore stability analysis associated with gas production from oceanic hydrate-bearing sediments

We conducted numerical modeling of coupled multiphase fluid-flow, thermal, and geomechanical processes during gas production from an oceanic hydrate deposit to study the geomechanical performance and wellbore stability. We investigated two alternative cases of depressurization-induced gas production: (1) production from horizontal wells in a Class 3 deposit (a hydrate layer sandwiched between two low-permeability layers); and (2) production from vertical wells in a Class 2 deposit (a hydrate layer with an underlying zone of mobile water). The analysis showed that geomechanical responses around the wellbore are driven by reservoir-wide pressure depletion, which in turn, depends on production rate and pressure decline at the wellbore. The calculated vertical compaction of the relatively soft sediments and increased shear stress caused local yielding of the formation around the well assembly for both the horizontal and vertical well cases. However, the analysis also showed that the extent of the yield zone can be reduced if using overbalanced drilling (at an internal well pressure above the formation fluid pressure) and well completion that minimizes any annular gap between the well assembly and the formation. Our further analysis indicated that the most extensive yield zone would occur around the perforated production interval of a vertical well, where the pressure gradient is the highest. In the field, such yielding and shearing of the sediments could lead to enhanced sand production if not prevented with appropriate sand control technology. Moreover, our analysis shows that the vertical compaction of the reservoir can be substantial, with subsidence on the order of several meters and vertical compaction strain locally exceeding 10%. In the field, such substantial compaction strain will require appropriate well design (such as slip joints or heavy wall casing) to avoid tensile or buckling failure of the well assembly.