Browsing by Subject "Pore pressure"
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Item Compressibility and permeability of Gulf of Mexico mudrocks, resedimented and in-situ(2014-05) Betts, William Salter; Flemings, Peter Barry, 1960-Uniaxial consolidation tests of resedimented mudrocks from the offshore Gulf of Mexico reveal compression and permeability behavior that is in many ways similar to those of intact core specimens and field measurements. Porosity (n) of the resedimented mudrock also falls between field porosity estimates obtained from sonic and bulk density well logs at comparable effective stresses. Laboratory-prepared mudrocks are used as testing analogs because accurate in-situ measurements and intact cores are difficult to obtain. However, few direct comparisons between laboratory-prepared mudrocks, field behavior, and intact core behavior have been made. In this thesis, I compare permeability and compressibility of laboratory-prepared specimens from Gulf of Mexico material to intact core and field analysis of this material. I resediment high plasticity silty claystone obtained from Plio-Pleistocene-aged mudrocks in the Eugene Island Block 330 oilfield, offshore Louisiana, and characterize its compression and permeability behavior through constant rate of strain consolidation tests. The resedimented mudrocks decrease in void ratio (e) from 1.4 (61% porosity) at 100 kPa of effective stress to 0.34 (26% porosity) at 20.4 MPa. I model the compression behavior using a power function between specific volume (v=1+e) and effective stress ([sigma]'v): v=1.85[sigma]'v-⁰̇¹⁰⁸. Vertical permeability (k) decreases from 2.5·10-¹⁶ m² to 4.5·10-²⁰ m² over this range, and I model the permeability as a log-linear function of porosity (n): log₁₀ k=10.83n - 23.21. Field porosity estimates are calculated from well logs using two approaches; an empirical correlation based on sonic velocities, and a calculation using the bulk density. Porosity of the resedimented mudrock falls above the sonic-derived porosity and below the density porosity at all effective stresses. Measurements on intact core specimens display similar compression and permeability behavior to the resedimented specimens. Similar compression behavior is also observed in Ursa Basin mudrocks. Based on these similarities, resedimented Gulf of Mexico mudrock is a reasonable analog for field behavior.Item Compression and permeability behavior of natural mudstones(2011-12) Schneider, Julia, 1981-; Flemings, Peter Barry, 1960-; Mohrig, David; Cardenas, Meinhard B.; Day-Stirrat, Ruarri J.; Germaine, John T.Mudstones compose nearly 70% of the volume of sedimentary basins, yet they are among the least studied of sedimentary rocks. Their low permeability and high compressibility contribute to overpressure around the world. Despite their fundamental importance in geologic processes and as seals for anthropogenic-related storage, a systematic, process-based understanding of the interactions between porosity, compressibility, permeability, and pore-size distribution in mudstones remains elusive. I use sediment mixtures composed of varying proportions of natural mudstone such as Boston Blue Clay or Nankai mudstone and silt-sized silica to study the effect of composition on permeability and compressibility during burial. First, to recreate natural conditions yet remove variability and soil disturbance, I resediment all mixtures in the laboratory to a total stress of 100 kPa. Second, in order to describe the systematic variation in permeability and compressibility with clay fraction, I uniaxially consolidate the resedimented samples to an effective stress equivalent to about 2 km of burial under hydrostatic conditions. Scanning electron microscope images provide insights on microstructure. My experiments illuminate the controls on mudstone permeability and compressibility. At a given porosity, vertical permeability increases by an order of magnitude for clay contents ranging from 59% to 34% by mass whereas compressibility reduces by half at a given vertical effective stress. I show that the pore structure can be described by a dual-porosity system, where one rock fraction is dominated by silt where large pores are present and the majority of flow occurs and the other fraction is dominated by clay where limited flow occurs. I use this concept to develop a coupled compressibility-permeability model in order to predict porosity, permeability, compressibility, and coefficient of consolidation. These results have fundamental implications for a range of problems in mudstones. They can be applied to carbon sequestration, hydrocarbon trapping, basin modeling, overpressure distribution and geometry as well as morphology of thrust belts, and an understanding of gas-shale behavior.Item Dynamics of dilative slope failure(2013-12) You, Yao; Flemings, Peter Barry, 1960-; Mohrig, DavidSubmarine slope failure releases sediments; it is an important mechanism that changes the Earth surface morphology and builds sedimentary records. I study the mechanics of submarine slope failure in sediment that dilates under shear (dilative slope failure). Dilation drops pore pressure and increases the strength of the deposit during slope failure. Dilation should be common in the clean sand and silty sand deposits on the continental shelf, making it an important mechanism in transferring sand and silt into deep sea. Flume experiments show there are two types of dilative slope failure: pure breaching and dual-mode slope failure. Pure breaching is a style of retrogressive subaqueous slope failure characterized by a relatively slow (mm/s) and steady retreat of a near vertical failure front. The retreating rate, or the erosion rate, of breaching is proportional to the coefficient of consolidation of the deposit due to an equilibrium between pore pressure drop from erosion and pore pressure dissipation. The equilibrium creates a steady state pore pressure that is less than hydrostatic and is able to keep the deposit stable during pure breaching. Dual-mode slope failure is a combination of breaching and episodic sliding; during sliding a triangular wedge of sediment falls and causes the failure front to step back at a speed much faster than that from the breaching period. The pore pressure fluctuates periodically in dual-mode slope failure. Pore pressure rises during breaching period, weakens the deposit and leads to sliding when the deposit is unstable. Sliding drops the pore pressure, stabilizes the deposit and resumes breaching. The frequency of sliding is proportional to the coefficient of consolidation of the deposit because dissipation of pore pressure causes sliding. Numerical model results show that more dilation or higher friction angle in the deposit leads to pure breaching while less dilation or lower friction angle leads to dual-mode slope failure. As a consequence, pure breaching is limited to thinner deposits and deposits have higher relative density.Item Experimental studies in hydraulic fracture growth : fundamental insights and validation experiments for geomechanical models(2019-01-23) Al Tammar, Murtadha Jawad; Sharma, Mukul M.; Ravi-Chandar, Krishnas; Olson, Jon E.; Prodanović, Maša; Espinoza, David N.Novel experimental capabilities to study hydraulic fracturing in the laboratory are developed and utilized in this research. Fracturing experiments are conducted using two-dimensional (2-D) test specimens that are made from synthetic, porous materials with well-characterized properties. Fracture growth during the experiments is captured with clear, high resolution images and subsequent image processing using Digital Image Correlation (DIC) analyses. First, we investigated the problem of a hydraulic fracture induced in a soft layer bounded by harder layers. The experiments reveal a clear tendency for induced fractures to avoid harder bounding layers. This is seen as fracture deflection or kinking away from the harder layers, fracture curving between the harder bounding layers, and fracture tilt from the maximum far-field stress direction. In addition, when a fracture is induced in a relatively thin layer, the fracture avoids the harder bounding layers by initiating and propagating parallel to the bounding interfaces. Fracture propagation parallel to the bounding layers is also observed in relatively wide layers when the far-field stress is isotropic or very low. Complex fracture trajectories are induced in layered specimens when the far-field differential stress is low or intermediate. In a second set of experiments, we used homogeneous specimens with multiple fluid injection ports. It is clearly shown that injection-induced stresses can appreciably affect hydraulic fracture trajectories and fracturing pressures. We show that induced hydraulic fractures, under our laboratory conditions, are attracted to regions of high pore pressure. Induced fractures tend to propagate towards neighboring high pore pressure injection ports. The recorded breakdown pressure in the fracturing experiments decreases significantly as the number of neighboring injectors increases. The influence of an adjacent fluid injection source on the hydraulic fracture trajectory can be minimized or suppressed when the applied far-field differential stress is relatively high. Preferential fracture growth due to changes in pore pressure in field applications as compared to our laboratory observations is also discussed. In a third set of experiments, we show that the breakdown pressure of test specimens can be reduced markedly with low injection rates, cyclic borehole pressurization, and/or constant pressure injection. This is largely related to the extent of pressurized region around the borehole caused by fluid leakoff in dry specimens and possible specimen weakening by fluid contact. The breakdown pressure can also be reduced by notching the specimen borehole when the injection fluid is allowed to flow and leak off along the borehole notch. In a fourth set of experiments, we compared fracture growth induced by a viscous liquid and a gas which are glycerin and nitrogen, respectively. The experiments show that fractures propagate through test specimens in a gradual manner when induced by glycerin at various injection rates. By contrast, nitrogen injection induces fractures that grow much more rapidly, which we attribute to its compressible nature and ultralow viscosity. The breakdown pressure is also shown to be markedly lower for nitrogen fractures compared to glycerin fractures. Moreover, an experimental evidence of fluid lag when fractures are induced with viscous fluids is demonstrated. Lastly, experiments were conducted to examine the behavior of an induced hydraulic fracture as it approaches a cemented natural fracture. We show a tendency for the induced hydraulic fracture to cross thick natural fractures filled with softer materials than the host rock and to be diverted by thick natural fractures with harder filling materials. The induced hydraulic fracture also tends to cross hard natural fractures when the natural fractures are relatively thin. In addition, the induced hydraulic fracture from the injection port is shown to be diverted by a thin, hard natural fracture that is placed relatively close to the injection port but crosses the same natural fracture when placed farther away from the injection port. These observations, and numerous others, documented in this dissertation provide fundamental insights on various aspects of hydraulic fracture propagation. Our extensive set of laboratory observations are also very useful in validating numerical hydraulic fracturing simulators due to the small-scale, 2-D nature, and characterized properties of the test specimens used in the experiments.Item Failure mechanics, transport behavior, and morphology of submarine landslides(2010-12) Sawyer, Derek Edward; Flemings, Peter Barry, 1960-; Mohrig, David; Lavier, Luc; Hornbach, Matthew; Shipp, R. Craig; Nikolinakou, MariaSubmarine landslides retrogressively fail from intact material at the headwall and then become fluidized by strain weakening; the final deposits of these flows have low porosity, which controls their character in seismic reflection data. Submarine landslides occur on the open slope and also localized areas including margins of turbidite channel-levee systems. I develop and quantify this model with 3-D seismic reflection data, core and log data from Integrated Ocean Drilling Program Expedition 308 (Ursa Basin, Gulf of Mexico), flume experiments, and numerical modeling. At Ursa, multiple submarine slides over the last 60 ky are preserved as mass transport deposits (MTDs). Retrogression proceeded from an initial slope failure that created an excavated headwall, which reduced the horizontal stress behind the headwall and resulted in normal faults. Fault blocks progressively weakened until the gravitational driving stress imposed by the bed slope exceeded soil strength, which allowed the soil to flow for more than 10 km away from the source area. The resulting MTDs have lower porosity (higher bulk density) relative to non-failed sediments, which ultimately produces high amplitude reflections at the base and top of MTDs. In the laboratory, I made weak (low yield strength) and strong flows (high yield strength) from mixtures of clay, silt, and water. Weak flows generate turbidity currents while moving rapidly away from the source area. They create thin and long deposits with sinuous flow features, and leave behind a relatively smooth and featureless source area. In contrast, strong flows move slowly, do not generate a turbidity current, and create blocky, highly fractured source areas and short, thick depositional lobes. In Pleistocene turbidite channels of the Mississippi Fan, deep-seated rotational failures occurred in the flanking levees. The rotational failures displaced material into the channel from below where it became eroded by turbidity flows. This system achieved a delicate steady state where levee deposition and displacement along the fault into the channel was balanced by erosion rate of turbidity flows. This work enhances our understanding of geohazards and margin evolution by illuminating coupled processes of sedimentation, fluid flow, and deformation on passive continental margins.Item Gas hydrate reservoirs of the deepwater Gulf of Mexico : characterization and consequences(2021-12-03) Meazell, Patrick Kevin, II; Flemings, Peter Barry, 1960-; Covault, Jacob; Mohrig, David; Summa, LoriGas hydrate is found in cold, high-pressure, marine sediments around the world. Hydrate is important as a carbon sink, a natural geohazard, and a valuable economic resource. I use classic sedimentologic analyses, well log analysis, X-ray CT, seismic stratigraphy, pore pressure estimation, and basin modeling to elucidate the geologic conditions within highly-saturated, natural gas hydrate reservoirs in the deepwater northern Gulf of Mexico. I begin with the characterization of the channel-levee hydrate reservoir in GC-955 with grain size experiments, lithofacies mapping. Hydrate is found in thin-bedded layers of sandy silt that increase in net-to-gross and mean grainsize downhole. I use these results to interpret deposition of overbank sediment gravity flows from a deepwater bypass channel as it becomes increasingly confined by the levees it builds. Next, I use 3D seismic data to identify the relationship between similar channel-levee systems and venting seafloor gas mounds in the Terrebonne Basin of the Walker Ridge protraction area. I estimate the pore pressures, and show that below the hydrate phase boundary, free gas in the levees builds to a critical pressure and creates hydraulic fractures to the seafloor. I describe a conceptual model by which the venting process perturbs the hydrate stability zone, leading to further venting from shallower positions and the formation of distinct rows of gas mounds on the seafloor. Finally, I combine geomechanical properties of the GC-955 reservoir with the structure of the Terrebonne Basin system to show that the pressure estimates are well within reason. Together, these studies provide new insights into where hydrate is found, and how hydrate systems can both control and in turn be controlled by fluid flow, pressure, and stress in the deepwater environmentItem New approaches to model and analyze diagnostic fracturing injection tests(2019-08-12) Wang, HanYi; Sharma, Mukul M.; Cramer, Dave; Foster, John; Sepehrnoori, Kamy; McClure, MarkOver the past two decades, Diagnostic Fracture Injection Tests (DFIT), which have also been referred to as Injection-Falloff Tests, Fracture Calibration Tests, MiniFrac Tests in the literature, have evolved into a commonly used and reliable technique to evaluate reservoir properties, fracturing parameters and obtain in-situ stresses. However, the established methods for modeling analyzing DFIT data make two simplifying assumptions: (1) Carter’s leak-off and, (2) Constant fracture compliance (or stiffness) during fracture closure. Both assumptions are violated during fracture closure and this is why G-function based models and subsequent related work can lead to an incorrect interpretation of closure pressure and are not capable of consistently fitting both before and after closure data coherently. In addition, current after-closure analysis relies on impulse solutions with short-term injection assumption and assumes that all injected fluid leaks off into formation. In reality, fluid continues leaking into formation with variable fracture-wellbore system storage, and some injected fluid stay inside wellbore due to pressurization and some stay inside fracture because of residual fracture width. Thus, without considering variable system storage and the associated fluid leak-off, the shortterm impulse solution may introduce significant errors in our interpretation of afterclosure data. In this dissertation, a global DFIT model is developed, which not only preserves the physics of unsteady-state reservoir flow behavior, elastic fracture mechanics, material balance, but also accounts for variable fracture stiffness/compliance and fracture pressure dependent leak-off during fracture closure. For the first time, the before-closure and afterclosure data can be analyzed coherently in a single mathematical model. With our new global DFIT model, we investigated how different mechanisms impact pressure fall-off behavior in a coupled manner. This had been oversimplified in existing literature. In addition, based on this global model, we present a new method to estimate the minimum in-situ stress and to history match DFIT data globally using forward modeling and inverse modeling approaches. New workflows have been proposed to analyze the DFIT data to obtain pore pressure, formation permeability, fracture surface roughness properties and the associated un-propped fracture conductivity. In addition, the normalized fracture conductivity as a function of effective normal stress can be obtained from the DFIT data. Unique pressure fall-off signatures that are associated with naturally fractured reservoirs are presented and discussed, along with representative field cases. Finally, a modified DFIT, rapid injection and fall-off test (RIFT) is proposed to estimate in-situ stress and pore pressure in a single test, without having to shut-in these wells for weeks. RIFT can be used to estimate the minimum in-situ stress in naturally fractured reservoirs with strong pressure-dependent permeability, where DFIT often fails to identify the closure pressure. In addition, the cumulative flow-back volume in the wellbore storage regime on a stiffness plot provides an estimate of the effective fracture volume.Item Pore pressure within dipping reservoirs in overpressured basins(2013-08) Gao, Baiyuan; Flemings, Peter Barry, 1960-A systematic study of how mudstone permeability impacts reservoir pore pressure is important to understand the regional fluid field within sedimentary basins and the control of sediment properties on subsurface pressure. I develop a 2D static model to predict reservoir overpressure from information estimated from the bounding mudstones and structural relief. This model shows that close to a dipping reservoir, the mudstone permeability is high in the up-dip location and low in the down-dip location. This characteristic mudstone permeability variation causes the depth where reservoir pressure equals mudstone pressure (equal pressure depth) to be shallower than the mid-point of the reservoir structure. Based on the 2D static model, I constructed a nomogram to determine the equal pressure depth by considering both farfield mudstone vertical effective stress and reservoir structural relief. I find the equal pressure depth becomes shallower with decreasing vertical effective stress, increasing reservoir structural relief, and increasing mudstone compressibility. Pressure predicted by the static model agrees with pressure predicted by a more complete model that simulates the evolution of the basin and is supported by field observations in the Bullwinkle Basin (Green Canyon 65, Gulf of Mexico). This study can be applied to reduce drilling risk, analyze trap integrity, and facilitate safe and efficient exploration.Item Stress, porosity, and pore pressure in fold-and-thrust belt systems(2018-10-12) Gao, Baiyuan; Flemings, Peter Barry, 1960-; Nikolinakou, Maria; Saffer, Demian; Lavier, Luc; Cardenas, BayaniI use forward geomechanical modeling to study the mechanical and fluid flow behaviors in compressional regions such as fold-and-thrust belts and accretionary wedges. Under drained conditions, lateral tectonic loading increases the mean effective stress and deviatoric stress and drives the sediments to shear-failure as the sediment approaches the deformation front (or trench location). The shear-induced porosity-loss accounts for about one third of the total porosity-loss during tectonic loading under drained conditions. There are four characteristic stress regions in my model: far-field, transition, critical state wedge, and footwall. In the transition zone, the shear-stress ratio varies significantly and the stress state changes from uniaxial-strain compression state to critical state. Increasing the basal friction coefficient leads to a higher surface slope angle and more porosity loss in the footwall whereas increasing the sediment internal strength leads to a lower surface angle and more porosity loss in the hanging wall. Fluid flow has a great impact on stress and compression in fold-and-thrust belts. My models show that the hanging-wall overpressure is greater than the footwall near trench but lower than the footwall overpressure towards the inner wedge. The high hanging-wall overpressure near trench is cause by the rapid increase of total mean and deviatoric stress. A significant finding is that the high overpressure near trench reduces the vertical effective stress and causes the décollement to be weak near the frontal wedge. Low permeability and high convergence rate promote overpressure generation and enlarge the overpressure-weakened decollement region. This study has broad impacts on the earthquake studies, faults stability analysis, and topics associated fluid flow transport in compressional margins.