Nazmul Haque Mondol

Anisotropy of experimentally compressed kaolinite-illite-quartz mixtures

The anisotropy of physical properties is a well-known characteristic of many clay-bearing rocks. This anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling. Preferred orientation of clay minerals is an important factor causing anisotropy in clay-bearing rocks such as shales and mudstones that are the main cap rocksofoilreservoirs.Thepreferredorientationofclaysdepends mostly on the amount of clays and the degree of compaction. To studytheeffectoftheseparameters,wepreparedseveralsamples compressingattwoeffectiveverticalstressesamixtureofclays illite and kaolinite and quartz silt with different clay/quartz ratios. The preferred orientation of the phases was quantified with Rietveld analysis on synchrotron hard X-ray images. Pole figures for kaolinite and illite display a preferred orientation of clay platelets perpendicular to the compaction direction, increasing in strength with clay content and compaction pressure. Quartz particles have a random orientation distribution. Aggregate elastic properties can be estimated by averaging the singlecrystal properties over the orientation distribution obtained from the diffraction data analysis. Calculated P-wave velocity anisotropy ranges from 0% pure quartz sample to 44% pure clay sample, highly compacted, but calculated velocities are much higher than measured velocities. This is attributed to uncertainties about single-crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in themodel.Inthiswork,wepresentaneffectivemethodtoobtain quantitative data, helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties ofclay-bearingrocks.

Experimental compaction of clays: relationship between permeability and petrophysical properties in mudstones

ABSTRACT This study determines the relationship between permeability and other petrophysical properties in synthetic mudstones as a function of vertical effective stress. Six brine-saturated clay slurries consisting of smectite and kaolinite were compacted in the laboratory under both controlled pore pressure and proper drained conditions. Porosity, permeability, bulk density, velocity (both Vp and Vs) and rock mechanical properties were measured constantly under increasing vertical effective stress up to 50 MPa. The results show that smectite-rich clays compact significantly less and have lower bulk density, velocity, permeability, bulk and shear modulus but higher Poissons ratio compared to kaolinite-rich clays at the same effective stress. Kaolinite aggregates compacted to about 26% porosity at 10 MPa effective stress corresponding to about 1 km burial depth in a normally compacted basin, whereas a pure smectite aggregate has a porosity of about 46% at the same stress. The permeability of kaolinite aggregates varies between 0.1 mD and 0.001 mD, while that of smectite aggregates varies from 0.004 mD to 0.00006 mD (60 nD) at stresses between 1 MPa and 50 MPa. Permeabilities in clays show a logarithmic decrease with increasing effective stress, bulk density, velocity or decreasing porosity. At the same porosity or bulk density, permeabilities differ up to five orders of magnitude within the smectite–kaolinite mixtures. Applications of the Kozeny–Carman equation for calculating permeability based on porosity in mudstones will therefore produce highly erroneous results. The relationships between Vp, Vs, bulk and shear modulus to permeability also vary by up to four orders of magnitude depending on the clay compositions. Velocities or rock mechanical properties will therefore not be suitable to estimate permeability in mudstones unless the mineralogy and textural relationships are known. These experimental results demonstrate that smectite content may be critical for building up pore pressure in mudstones compared to kaolinite. The results help to constrain compaction and fluid flow in mudstones in shallower parts of the basins (<80–100°C) where mechanical compaction is the dominant process. These results may also have implications for waste disposal and engineering practice, including structural design and slope stability analysis.

Elastic properties of clay minerals

Clay minerals are the most abundant materials in sedimentary basins. The most common—like kaolinite, illite, chlorite, and smectite—are found in various amounts in mudstones and are also often found in clastic and nonclastic reservoir rocks. Their presence alters the elastic behavior of reservoir rocks significantly as a function of mineral type, volume and distribution. Thus, two sandstones with the same clay amount might have different elastic properties due to differences within the clay population. The elastic properties of clay minerals are therefore important in rock physics modeling to understand the seismic and sonic log responses of shaley sequences and clay-bearing reservoir rocks.

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf

Sonic velocity, density, and resistivity log data from 60 wells on the Norwegian Shelf have been used to investigate velocity-depth trends in sedimentary rocks as a function of sediment composition, porosity, pore-pressure, burial-history, and compaction processes. A first-order linear velocity-depth trend line has been estimated from published velocity data. The data analyzed in this study, however, show significant variations from this trend line, indicating that no general velocity-depth function can be used when performing more accurate analyses like depth conversion of seismic data, pore-pressure prediction, or basin modeling. Lower Tertiary smectitic sediments from the northern North Sea and Haltenbanken are characterized by relatively low velocities compared to the overlying Pliocene and Pleistocene sediments, causing a distinct velocity inversion. A significant velocity increase at a burial depth corresponding to 70–100C was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales. Overpressured Jurassic sediments from Haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference. The reduced velocities may be a direct response to lower effective stresses and, thus, reduced elastic compaction. Low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy. The high velocity/depth ratio of Barents Sea sediments (after correcting for Tertiary exhumation) is explained by the burial history of the area, the subsequent thermal exposure of the sediments over time, and thus, the amount of quartz cementation.

Synthetic mudstone compaction trends and their use in pore pressure prediction

A general way of predicting pore pressure in sedimentary basins is to use relationships of sonic travel time and/or seismic interval velocity versus depth / effective stress in mudstones. Pore pressure is then estimated from the divergence from generalized compaction trends. The key to a successful conversion of mudstone compaction trends to pore-pressure prediction is to characterize the mudstones as functions of lithology and textural variations and to establish relationships between porosity/density/velocity/ permeability versus effective stress. Sedimentary basins are awfully heterogeneous as functions of sedimentary facies, tectonic development, and diagenetic history. Fluid transport in sedimentary basins is therefore controlled largely by the heterogeneity of the basin fill. Overpressure generation is the function of fluid flux generated by compaction (porosity loss) and the permeability along the most permeable pathways to the surface. Generation of fluids from kerogen and dehydration of minerals may add to this fluid fluxes, but permeability is the most critical parameter determining the development of overpressure. A series of laboratory measurements were conducted to investigate the relationships between velocity/porosity/density/permeability versus effective stress for pure smectite, kaolinite, and silt, and their mixtures. Experimental compaction has shown that the permeability of clay minerals vary by a factor of 10 4 to 10 5 for the same porosity. The smectite-rich mudstones have very low permeability compared to others and have potentiality to develop overpressure even at shallow or moderate burial depth. Calculations assuming vertical flow show that fracture pressure will be reached if the effective permeability of the shale forming the seal is less than 0.1-0.01 nD (nanodarcies). The experimental results of relationships between velocity/porosity/ density/permeability versus effective stress of well-characterized mudstones were compared to the field data which demonstrated a close match, confirming that experimental data can be useful to distinguish mudstones for pore-pressure prediction.

Porosity and permeability development in mechanically compacted silt-kaolinite mixtures

Summary Prediction of porosity and permeability in mudstones are challenging because of the uncertainties associated with textural and mineralogical composition. This study investigates the development of porosity and permeability in thirteen mechanically compacted brine-saturated synthetic mudstones consisting of silt and kaolinite mixtures. The drained uniaxial mechanical compaction tests were performed under room temperature and atmospheric pressure. Total porosity and vertical permeability (parallel to the direction of applied stress) were measured continuously for each sample under increasing effective stress up to 50 MPa. Results show that mineralogy is a dominant factor controlling the development of porosity and permeability in silt-clay mixtures. Kaolinite-dominated samples have much lower porosity and permeability compared to silt-dominated samples. At 20 MPa effective stress that corresponds to about 2 km depth of burial of normally compacted basin, the pure kaolinite sample was compacted to about 20% porosity whereas the pure silt sample had retained 35% porosity. The permeability of pure kaolinite and silt samples were about 0.003 and 0.2 mD respectively at the same effective stress. The porosity and permeability reduction were not systematic with increasing kaolinite content in silt aggregates or vice versa. The porosity and permeability difference between pure silt and pure kaolinite aggregates were relatively less at low effective stresses but increased significantly with increasing effective stress. At high effective stresses the permeability was 2-3 orders of magnitude lower in pure kaolinite than in pure silt. Calculating permeability in mudstones from porosity without considering mineralogy will therefore introduce significant error in permeability estimation. Our experimental results provide valuable constraints on porosity-stress/depth, permeability-stress/depth and porosity-permeability relationships for shallow mudstones that will have practical use for basin modelling, pore pressure prediction and fluid flow modelling in shallower part of the basins (<80-100 0 C) where mechanical compaction is the dominant process.

Lateral changes in reservoir properties of the Stø Sandstone in the Snøhvit field, SW Barents Sea

The Sto sandstone being the main reservoir in the Barents Sea area is moderate to well sorted and mineralogically mature. This formation is thickest in the southwestern part of the Snohvit field and gradually thinning eastward. The main objective of this study is to find out this variation using rock physics analysis. Two wells (7120/6-2S and 7121/5-1) from the field were used in this study to investigate lateral rock property variation within the Sto sandstone reservoir. Stress-dependent mechanical compaction varies because of mineralogy and textural difference from east to west despite similar effective stress regime during burial. Chemical compaction also plays a significant role which depends on the dissolution of quartz grains at stylolites and pressure solution of grain-to-grain contact and available specific surface area to precipitate quartz cement. More stylolites generated in shalye sandstone in the east compared to clean sandstone in the west suggested higher cementation in the eastern wells compared to the west. It can be concluded that both mechanical and chemical compaction processes resulted in rock property variations in the same reservoir rock within the field.

Introduction to Geomechanics: Stress and Strain in Sedimentary Basins

At shallow depths in sedimentary basins there are soft clays and loose silts and sands, unless there are carbonates or carbonate cemented layers. At greater depths they are transformed by diagenetic processes to hard clay stones, shales, siltstones and sandstones. Sedimentary rocks continuously undergo physical and chemical changes as a function of burial depth, temperature and time, and important hydro-mechanical parameters change during burial, erosion and uplift. An understanding of these processes is important in order to predict the magnitude and distribution of sediment properties and stresses in the basin (strain). The in-situ stress condition affects the rock response to changes in the stress field due to drilling and petroleum production.

Experimental Compaction of Kaolinite Aggregates - Effects of Grain Size on Mudrock Properties

SUMMARY This study investigates the effects of grain size on petrophysical and acoustic properties of experimentally compacted synthetic mudstones. Four brine-saturated kaolinite aggregates sorted by the grain size were compacted in the laboratory under vertical effective stress up to 50 MPa. Results show that the kaolinite aggregates compacted differently and have atypical acoustic and petrophysical properties as a function of grain size. The maximum compaction was observed in a mixture which contained mixed grain sizes (composite aggregates) whereas the minimum compaction was found in the mixture containing the smallest grain size (<2 microns). The composite aggregate compacted to 27% porosity at 20 MPa effective stress while the aggregates containing the <2 micron grains retained porosity of about 34% at same effective stress. This could be explained by the distribution of effective stress over a larger number of grain contacts in the fine-grained mixture compared to coarser-grained mixtures. The finest grained sample has the lowest density, permeability and velocity compared to the other mixtures. To our knowledge this is the first time grain size related changes in physical properties during compaction has been demonstrated in kaolinite aggregates. These results will have practical application in rock physics, seismic and well log interpretation.

Prestack simultaneous inversion to predict lithology and pore fluid in the Realgrunnen Subgroup of the Goliat Field, southwestern Barents Sea

AbstractAn integrated multidisciplinary workflow has been implemented for quantitative lithology and fluid predictions from prestack angle gathers and well-log data within the Realgrunnen Subgroup in the Goliat Field, southwestern Barents Sea. We have first performed a qualitative amplitude-variation-with-angle (AVA) attribute analysis to assess the spatial distribution of lithology and fluid anomalies from the seismic data. A simultaneous prestack elastic inversion was then carried out for quantitative estimates of the P-impedance and VP/VS ratio. Probability density functions, a priori lithology, and fluid class proportions extracted from well-log training data are further applied to the inverted P-impedance and VP/VS seismic volumes. The AVA qualitative analysis indicates a class IV response for the top of the reservoir, whereas anomalies from the AVA attribute maps agree largely with the clean sand probabilities predicted from the Bayesian facies classification. The largest misclassification in the li...