Clean Sandstones Research Articles

Is it the grain size or the characteristic pore size that controls the induced polarization relaxation time of clean sands and sandstones?

There is a wide range of evidence to suggest that permeability can be constrained through of induced polarization measurements. For clean sands and sandstones, current mechanistic models of induced polarization predict a relationship between the low‐frequency time constant inferred from induced polarization measurements and the grain diameter. A number of observations do, however, disagree with this and indicate that the observed relaxation behavior is rather governed by the so‐called dynamic pore radius Λ. To test this hypothesis, we have developed a set of new scaling relationships, which allow the relaxation time to be computed from the pore size and the permeability to be computed from both the Cole‐Cole time constant and the formation factor. Moreover, these new scaling relationships can be also used to predict the dependence of the Cole‐Cole time constant as a function of the water saturation under unsaturated conditions. Comparative tests of the proposed new relationships with regard to various published experimental results for saturated clean sands and sandstones as well as for partially saturated clean sandstones, do indeed confirm that the dynamic pore radius Λ is a much more reliable indicator of the observed relaxation behavior than grain‐size‐based models.

Modeling squirt dispersion and attenuation in fluid-saturated rocks using pressure dependency of dry ultrasonic velocities

Modeling dispersion and attenuation of elastic waves in fluid-saturated rocks due to squirt flow requires the knowledge of a number of geometrical parameters of the pore space, in particular, the characteristic aspect ratio of the pores. These parameters are usually inferred by fitting measurements on saturated rocks to model predictions. To eliminate such fitting and thus make the model more predictive, we propose to recover the geometrical parameters of the pore space from the pressure dependency of elastic moduli on dry samples. Our analysis showed that the pressure dependency of elastic properties of rocks (and their deviation from Gassmann’s prediction) at ultrasonic frequencies is controlled by the squirt flow between equant, stiff, and so-called intermediate pores (with aspect ratios between [Formula: see text]). Such intermediate porosity is expected to close at confining pressures of between 200 and 2000 MPa, and thus cannot be directly obtained from ultrasonic experiments performed at pressures below 50 MPa. However, the presence of this intermediate porosity is inferred from the significant linear trend in the pressure dependency of elastic properties of the dry rock and the difference between the bulk modulus of the dry rock computed for spherical pores and the measured modulus at 50 MPa. Moreover, we can infer the magnitude of the intermediate porosity and its characteristic aspect ratio. Substituting these parameters into the squirt model, we have computed elastic moduli and velocities of the water-saturated rock and compared these predictions against laboratory measurements of these velocities. The agreement is good for a number of clean sandstones, but not unexpectedly worse for a broad range of shaley sandstones. Our predictions showed that dispersion and attenuation caused by the squirt flow between compliant and stiff pores may occur in the seismic frequency band. Confirmation of this prediction requires laboratory measurements of elastic properties at these frequencies.

Experimental Verification of the Petroelastic Model in the Laboratory – Fluid Substitution and Pressure Effects

The poroelastic model is a major component in the workflows for the interpretation of time-lapse (or 4D) seismic data in terms of fluid repartition and/or pressure variation during the exploitation of reservoirs. This model must take into account both the fluid substitution effect and the pressure variation effect on the measured seismic parameters (velocities, impedance). This paper describes an experimental verification in the laboratory of this model. Regarding fluid substitution, Biot- Gassmann model is the most popular model. This model assumes that the shear modulus is independent of the nature of the saturating fluid, as long as this latter is not viscous and give the expression of the variations of the bulk modulus of the rock due to fluid substitution as function of the parameters of the rock frame and of the saturating fluids. The experimental validation, dealing with these two items, demonstrates on various samples of sandstone and limestone that the shear modulus of the rock is independent of the not too viscous saturating fluid. This is verified even with viscous fluids (viscosity as large as 10<sup>4<sup/> cP) if the differential pressure, that is to say the difference between the confining pressure and the pore pressure, is high (closure of the mechanical defects); the bulk modulus of the crystal constituent of mono-mineral rocks (limestone, clean sandstone) is close to tabulated values; under fixed differential pressure but variable pore and confining pressures, the variation of the rock bulk modulus can be explained by the nonlinearity of the fluid bulk modulus. These three types of experimental results constitute unambiguous corroborations of Biot-Gassmann theory. Regarding pressure effects, the relevant parameter is the differential pressure <i>P<i/><sub><i>diff<i/><sub/> = <i>P<i/><sub><i>c<i/><sub/> – <i>P<i/><sub><i>p<i/><sub/>, that is to say the difference between the confining pressure P<sub><i>c<i/><sub/> and the pore pressure P<sub><i>p<i/><sub/>. More precisely, this means that P-wave and S-wave velocities only depend on the differential pressure <i>P<i/><sub><i>diff<i/><sub/> = <i>P<i/><sub><i>c<i/><sub/> − <i>P<i/><sub><i>p<i/><sub/>, and not in an independent way on P<sub><i>c<i/><sub/> and on P<sub><i>p<i/><sub/>. Increasing the differential pressure P<sub><i>diff<i/><sub/> tends to stiffen the rock by closing the mechanical defects (grain contacts, microcracks, microfractures...). The consequence on velocities and attenuations is variable according to the relative abundance of these mechanical defects in the rock sample. Limestones are often weakly pressure dependent, whatever the pressure level. This is due to the ease with which mechanical defects can be cemented by carbonate crystals. Consolidated sandstones are often sensitive to the differential pressure P<sub><i>diff<i/><sub/> and the unconsolidated geomaterials (sands) are very pressure sensitive. The pressure dependence of the velocities is often well approximated by a power law. The exponent of this power law, often called the Hertz exponent, is a good way to quantify the pressure sensitivity of the rock velocities.

Mineral Compositional Trends and Their Correlations with Petrophysical and Well-Logging Parameters Revealed by Quanta + Bestmin Analysis: Miocene of the Carpathian Foredeep, Poland

AbstractThis study uses the data from Miocene rocks of the Carpathian Foredeep to test the performance of the computer programs QUANTA and BESTMIN in aiding the interpretation of geophysical log data. These programs were designed to help trace trends in the mineral composition of rocks, the chemical composition of minerals, and the effects of these data on petrophysical and geophysical logging parameters. Chemical and X-ray diffraction data for 65 samples of shales, sandstones, and carbonates taken from cored wells in the molasse basin of the Carpathian Foredeep were processed. Compositional differences were detected between rocks sourced from the platform and rocks sourced from the Carpathians. Quartz, K-feldspar, and zircon were more abundant in the coarse-grained rocks (sandstones), while calcite, ankerite, siderite, pyrite, barite, halite, celestite, apatite, anatase, chlorite, 2:1 minerals, and organic matter were more abundant in the fine-grained rocks (shales). Plagioclase reached its maximum in coarse shales. Ankerite, chlorite, and dioctahedral 2:1 minerals had more Fe in the coarse-grained rocks. The dioctahedral 2:1 minerals in fine-grained rocks had a greater concentration of smectitic layers. This information permitted the precise calculation of grain density, porosity, adsorbed water, and some geophysical logging parameters. It also permitted the calibration of well-log response, in particular, the macroscopic neutron absorption cross-section (Σa) combined with the photoelectric absorption factor (P{e}) or with Pe + Ca (calcium content, measurable in wells by spectroscopic techniques) with porosity and cation exchange capacity (CEC). The NaCl concentration in the pore waters was found to range from the values typical for seawater in shales to the freshwater level in clean sandstones.

Model of sand formations for joint simulation of elastic moduli and electrical conductivity

This paper presents an approach for the joint simulation of elastic-wave velocities (compressional and shear waves) and electrical conductivity for clastic rocks using a unified microstructural model. We treat clastic formations as porous composite materials containing two components: solid grains and pores completely saturated with fluids (gas–water–oil mixture). The effective shapes of both components correspond to triaxial ellipsoids. For calculating the effective properties of such a composite, we apply the self-consistent effective media approximation (EMA) method. This method treats all components equally without introducing any host and requires having knowledge of their shapes. To determine the pore and grain effective aspect ratios, we have used a set of experimental data that included the empirical regression equations published for the elastic-wave velocities in clean sandstone formations and Archie’s law for the electrical conductivity. By applying the inversion procedure that consists of minimizing the difference between the experimental and EMA-predicted data, the aspect ratios of grains and pores as a porosity function have been obtained. Based on this relation and knowing individual property components (elastic and electrical conductivity) for grains and conductivity for pore saturating fluids, we can calculate effective properties of the clean sandstone formations. We demonstrate that to simplify the simulation of effective properties, grains and pores can be approximated by spheroids. However, in this case, the relative errors that the inversion process yields are higher in comparison with those of the three-axis ellipsoids. Additionally, we have determined the aspect ratios of components for several velocity equations (including different confinement pressures) and various cementation factors ‘m’ in Archie’s law. We have modelled electrical conductivity for clastic formations with different water mineralization and oil saturation. The effective conductivity obtained by using our approach reflects a good fit with the published experimental data.

Prediction of lithofacies and reservoir quality using well logs, Late Cretaceous Williams Fork Formation, Mamm Creek field, Piceance Basin, Colorado

Understanding the controls and distribution of reservoir quality is important for the economic success of tight-gas reservoirs in which diagenesis interacts with primary depositional variations in environment and texture to exert a strong control on pore networks, rock mechanical properties, and natural fractures. In the Upper Cretaceous Williams Fork Formation of the Piceance Basin, framework grain composition is a major control on compaction and the occurrence of authigenic phases. Alteration of volcanic grains in the upper Williams Fork led to grain-coating clay precipitation. Ferroan dolomite cement is found only in the deeper marine-influenced intervals in which dolostone fragments are present. This study shows that careful petrographic assessment of lithofacies heterogeneity can be upscaled by correlation with log properties to yield tools for field-scale reservoir quality prediction. Twelve lithofacies are identified based on sand-grain populations, cement types, and clay matrix content. Sandstones of the highest reservoir quality are those with grain-coating clays that inhibit quartz cementation; these sandstones can be identified based on high-density porosity log values. Sandstones with the poorest reservoir qualities are tightly cemented with carbonate and quartz cement or are rich in clay matrix. Carbonate-cemented intervals are identified by low-density porosity. Clay matrix–rich samples have high gamma-ray and low-density porosity values. The presence of abundant potassium feldspar in the upper intervals results in high gamma-ray readings even in the clean (clay matrix–free) sandstone.

Geophysical investigation of barite deposit in Tunga, Northeastern Nigeria

Gravity and electromagnetic methods were combined to characterize known veins and possibly unveil other concealed veins in Tunga, Northeastern Nigeria. Barite occurs in the study area as vein deposits hosted in clean sandstone rocks. Laboratory analyses of samples show average specific gravity of the variety to vary between 3.16 and 4.24 while the host sandstone rock is 2.63. The study is focused on an area of 0.5 km2 measuring 1.0 km in the strike and 0.5 km in the dip (designated X1 to X6 in northwest-southeast azimuth) which appears as the most promising terrain in barite prospectivity. Relatively high Bouguer anomalies were recorded over regions that are presumably underlain by barites. Barite mineralization potential varies from high with gravity values of between 177.507 and 181.072 mGals around western flanks of Traverses X2 and X3 to medium values (174.681 and 176.801 mGals) around central areas of Traverse X3. Areas of low Bouguer anomaly were presumably barren (X1, X4, X5, and X6). The variations in the Bouguer anomaly values are presumably influenced by the density contrast between the barite and the host rocks. Areas of high gravity values exhibit very low conductivity values (< 10 mS/m) and are presumed underlain by dense materials of low conductivity to which barite mineralization can be associated. Thus, in this study, areas of high gravity values accompanied by low conductivities on the western/southwestern flank of the prospect around Traverses X2 and X3 are associated with barite mineralization. The trend of mineralization in the area is southwest-wards in the direction of River Benue. Thus, higher barite mineralization potential probably exists beneath River Benue and its southern flank in Tunga area. Key words: Barite, vein, bouguer anomaly, conductivity, mineralization.

Water pollution in relation to mineral exploration: a case study from Alayi-Ovim area of Southeastern Nigeria

Water samples from rivers, streams, springs, and shallow wells in Alayi-Ovim area of southeast Nigeria have been analyzed for Pb, Ca, Mg, Fe, Mg, PO(4), NO(3), CO(3), SO(4), Cl, and pH. The analyses were carried out using atomic absorption spectrometer and Hach Direct Reading Equipment. Results of the analyses from the area conform to the WHO (1995) standards for drinking water. However, the results show relative enrichment of Ca, pH, Mg, CO(3), and Cl. Low values were obtained for Fe, SO(4), and NO(3). While the Cl and Pb enrichment in the area north of Alayi-Ovim axis is attributed to proximity to the lead-zinc and chloride-rich formations of the Turonian Eze-Aku and the Albian Asu River; the Ca, Mg, SO(4), and CO(3) enrichment in Southern part of Alayi-Ovim is due to the limestone-bearing Late Maastrichtian Nsukka Formation. Furthermore, the very low values of less than 5ppm for these characters in water in the central region correlate well with the relatively clean Maastrichtian quartz arenite Ajali Sandstone Formation. The Pb-Zn and Cl incursions into the water system from the Older Albian Asu River/Turonian Eze-Aku Formations in the northern part of Alayi-Ovim area and the leaching of Mg, and Ca into the water system in the Maastrichtian limestone area in the south thus constitute geochemical indices for chemical pollution and mineral exploration for brine and dolomitic limestone in the area.

Dielectric spectroscopy measured on dry, oil-, freshwater- and brine-wetted rocks using an insulating layer method and its relations to petrophysical parameters

Dielectric measurements, in the frequency range from 100 Hz to 15 MHz, were carried out using an insulating film method, on shaly and clean sandstone samples, dry, oil-impregnated, partially or fully saturated with fresh water or solutions of NaCl, at different concentrations. The results show that two kinds of polarization relating to interface dominate: the inner surface polarization (ISP) at the pore surface in rocks and the outer surface polarization (OSP) on the outer surface of the rock sample. Both ISP and OSP are of the Debye type and can be fitted very well using the Cole–Cole equation. The dielectric spectra of ISP reflect the length scale of pore size or throat size, whereas the relaxation strength depends heavily on the inner pore surface area. Clay contributes to dispersion in rocks, mainly through increasing the inner surface. When the effect of water content is minimized, the dielectric properties measured on brine-wetted rock samples show great salinity dependence. The increase in salinity shifts the relaxation spectra to the high frequency end; it also enhances the relaxation strength. In the frequency range studied, no dispersion in dry and oil-saturated samples has been observed, which underscores the key role played by water in dielectric polarization in rocks.

Pressure effects on the joint elastic-electrical properties of reservoir sandstones

We conducted a laboratory study of the joint elastic-electrical properties of sixty-three brine-saturated sandstone samples to assess the likely impact of differential pressure (confining minus pore fluid pressures) in the range 8–60 MPa on the joint interpretation of marine seismic and controlled-source electromagnetic survey data. The samples showed a wide range of petrophysical properties representative of most sandstone reservoirs. We found that a regression equation comprising both a constant and an exponential part gave a good fit to the pressure dependence of all five measured geophysical parameters (ultrasonic P- and S-wave velocity, attenuation and electrical resistivity). Electrical resistivity was more pressure-sensitive in clay-rich sandstones with higher concentrations of low aspect ratio pores and micropores than in clean sandstones. Attenuation was more pressure-sensitive in clean sandstones with large open pores (macropores) than in clay-rich sandstones. Pore shape did not show any influence on the pressure sensitivity of elastic velocity. As differential pressure increases, the effect of the low aspect ratio pores and micropores on electrical resistivity becomes stronger than the effect of the macropores on attenuation. Further analysis of correlations among the five parameters as a function of pressure revealed potentially diagnostic relationships for geopressure prediction in reservoir sandstones.

Thermophysical study of sandstone reservoir rocks

Thermal conductivity of both dry and fully water (brine solution, 5 g/l NaCl concentration) saturated rocks have been measured for 35 sandstone core samples obtained from both Hungary and Egypt. Measurements are executed in order to look for thermal anisotropy. The rock porosity, permeability and density were measured beside thermal conductivity, while some correlations were performed in order to differentiate among clayey laminated, flaser bedding and clean sandstone reservoirs. There is a very close correlation among thermal conductivity, rock porosity, bulk density, gas permeability and ultrasonic wave velocities for non laminated clean sandstones, while in banded sandstone reservoirs these relations show more than one trend giving a strong impression indicating the presence of more than one grain–fabric system and/or complex diagenesis. The relationship among the measured thermal conductivity for the core axis, the top and bottom are close and homogeneous for isotropic clean non laminated Szolnok sandstones, while they are vary in a wide range for banded and laminated Bahariya sandstone samples. The measured specific heat capacity indicates that some banded sandstone samples of the Bahariya Formation have bound water appears at 100 °C and giving low specific heat capacity value. This bound water can acted on all petrophysical parameters especially electrical properties. The prediction of specific heat capacity and heat flow from the reservoir temperature is possible using the calculated multi-regression and linear regression models.

Stochastic simulations of dependent geological variables in sandstone reservoirs of Neogene age: A case study of Kloštar Field, Sava Depression

The research presented herein is the first attempt to perform geostatistical simulations on three geologic variables, porosity, thickness, and depth to reservoir, in the Croatian Pannonian basin. The data were collected from a reservoir of Lower Pontian age in Klostar Field, located in the western part of the Sava Depression. All three variables were analyzed using sequential Gaussian simulations (SGS). Information regarding present-day depth, thickness, and locations of areas with higher porosity values were used to reconstruct paleo-depositional environments and the distribution of different lithotypes, ranging from medium-grained, to mostly clean sandstones and to pure, basin marls. Estimates of present-day thickness and depth can help to define areas of gross tectonic displacement and the role of major faults that have been mapped in the field. However, since mapping of the raw data (including porosities) does not allow the reconstruction of paleo-depositional environments, sequential indicator simulations (SIS) were applied as a secondary analytical tool. For this purpose, several cutoff values for thickness were defined in an effort to distinguish the orientation of depositional channels (main and transitional). This was accomplished by transforming porosities to indicator values (0 and 1) and by applying a non-linear “indicator kriging” technique as the “zero” map for obtaining numerous indicator realizations by SIS. In the SGS and the SIS approaches, the simulations encompassed 100 realizations. A representative realization was then selected using purely statistical criteria, i.e., two realizations were almost always chosen in accordance with the order of calculation. The 1 st and 100 th realizations were selected for each variable in the SGS and SIS and five “indicator kriging” maps were chosen for the thicknesses cutoffs.

Laboratory measurements of porosity, permeability, resistivity, and velocity on Fontainebleau sandstones

The relations among the resistivity, elastic-wave velocity, porosity, and permeability in Fontainebleau sandstone samples from the Ile de France region, around Paris, France were experimentally revisited. These samples followed a permeability-porosity relation given by Kozeny-Carman’s equation. For the resistivity measurements, the samples were partially saturated with brine. Archie’s equation was used to estimate resistivity at 100% water saturation, assuming a saturation exponent, [Formula: see text]. Using self-consistent (SC) approximations modeling with grain aspect ratio 1, and pore aspect ratio between 0.02 and 0.10, the experimental data fall into this theoretical range. The SC curve with the pore aspect ratio 0.05 appears to be close to the values measured in the entire porosity range. The elastic-wave velocity was mea-sured on these dry samples for confining pressure between 0 and [Formula: see text]. A loading and unloading cycle was used and did not produce any significant hysteresis in the velocity-pressure behavior. For the velocity data, using the SC model with a grain aspect ratio 1 and pore aspect ratios 0.2, 0.1, and 0.05 fit the data at [Formula: see text]; pore aspect ratios ranging between 0.1, 0.05, and 0.02 were a better fit for the data at [Formula: see text]. Both velocity and resistivity in clean sandstones can be modeled using the SC approximation. In addition, a linear fit was found between the P-wave velocity and the decimal logarithm of the normalized resistivity, with deviations that correlate with differences in permeability. Combining the stiff sand model and Archie for cementation exponents between 1.6 and 2.1, resistivity was modeled as a function of P-wave velocity for these clean sandstones.

Bioturbation influence on reservoir quality: A case study from the Cretaceous Ben Nevis Formation, Jeanne d'Arc Basin, offshore Newfoundland, Canada

The delineation Ben Nevis L-55 well, located in the Hebron-Ben Nevis field, offshore Newfoundland, targeted the Cretaceous Ben Nevis Formation in the petroleum-rich Jeanne d'Arc Basin. This case study focuses on the bioturbated net-pay horizons and assesses the importance of animal-sediment interactions in controlling the porosity and permeability of sandstone reservoir intervals. Our data reveal that bioturbation can either reduce permeability and porosity by as much as approximately 33% or enhance it by up to 600%, dependent on burrow type and behavior of the trace-making organism. The net-pay interval in the cored interval of Ben Nevis L-55 is characterized by Ophiomorpha-dominated ichnofabrics. The action of bioturbators can be classified in terms of sediment mixing, sediment cleaning, sediment packing, and pipe-work-building strategies. Bioturbation has the potential to (1) increase isotropy or uniformity of grain size by destroying sedimentary laminae through burrow homogenization, or (2) decrease isotropy by selectively sorting grains into burrow lining and fill by grain size, and through creation of open-burrow systems filled with later sediments of differing character to the host sediment. The petrophysical characteristics of the reservoir facies are highly dependent on trace fossil morphology, presence or absence of burrow linings, nature of burrow fills, burrow size, and bioturbation intensity. Mudstone-rich facies and ichnofabrics containing mudstone-filled and/or lined burrows (e.g., Ophiomorpha and clusters of Chondrites) have the net effect of permeability reduction. In contrast, permeability enhancement is documented from muddy sandstone facies with clean sand-filled burrows (e.g., Thalassinoides) and clean sandstones with burrow-mottled or diffuse to massive textures.

Application of the BISQ model to clay squirt flow in reservoir sandstones

An understanding of the variations of compressional wave velocity and attenuation in sandstones as a function of pore‐fluid properties is important for the identification and monitoring of hydrocarbon reservoirs. In order for seismic inversion to be well constrained a rock physics model that describes accurately the observed behavior is required; a number of such models have been proposed but have not been extensively tested against laboratory data. The unified Biot and squirt flow model was applied to data obtained from ultrasonic experiments conducted on a wide range of sandstone types when saturated with different pore fluids with viscosities ranging from 0.3 to 1000 cP. This model was seen to match well all the observed velocity and attenuation values as opposed to the Biot model, which was only successful for clean sandstones saturated with low viscosity fluids. This has led to the conclusion that a squirt flow mechanism is important at the grain scale. In addition, an increase in the dynamic uniaxial strain modulus of the dry frame with increasing pore‐fluid viscosity was observed. In the absence of free gas or grain contact microcracks (the laboratory data were collected at 60 MPa effective pressure), this can be linked to decreasing local flow and decreasing relaxation in the soft pore space of the sandstone, the extent of which is in turn connected to the clay content and pore structure of the sandstone. These results imply that at ultrasonic frequencies the squirt‐flow is due to the presence of clay.

Relationships between induced polarization relaxation time and hydraulic properties of sandstone

We investigated electrical and physical–chemical properties of six sandstone samples with contrasting mineralogical characteristics and with hydraulic conductivity varying in a wide range. The electrical data were obtained from time domain spectral induced polarization (IP) measurements. We inverted the IP decays to relaxation time distributions, and then compared the modal relaxation times with the dominant pore throat diameters obtained from the Mercury Injection Capillary Pressure (MICP) data. We found a positive logarithmic relationship between the relaxation time and the pore throat diameter. Also, we found the normalized chargeability (an integral IP parameter) to be positively correlated with the clay content. These two results suggest that the polarization of our sandstones is controlled by the pore throat distribution, and by the clay content. The logarithmic relationship contradicts previous theories, and is not universal. Adopting an approach of Kruschwitz and her co-workers, we calculated the effective diffusivity from IP and MICP data, and we found the effective diffusivity values ranging from 2.9 × 10−13 to 1.6 × 10−10 m2 s−1. High diffusivity values, typical of surface diffusion, were obtained for clean sandstones. Low diffusivity values were obtained for clayey sandstones, and they were one to two orders of magnitude lower than those characteristic of the surface diffusion. We proposed two mechanisms to explain the ‘slow’ diffusion: (1) the effect of surface tortuosity of pore throats filled with clay minerals and (2) the effect of pore geometry. These two effects represent an obstacle in assessing the pore throat diameter and hydraulic conductivity of sandstones with large specific surface and clay content on the basis of spectral IP measurements. However, we believe that the sandstones featuring ‘slow’ diffusion can be discriminated based on the integral polarization parameters, and that the relaxation time remains a valuable parameter for assessing hydraulic properties of clean sandstones.

Innovative Use of Kaolinite in Downhole Scale Management: Squeeze-Life Enhancement and Water Shutoff

Summary Applied research has been undertaken to examine the potential of kaolinite combined with a kaolinite-fixation agent to increase squeeze lifetime through alteration of near-wellbore surface characteristics and mineralogy, and to provide water-shutoff control. With respect to enhancing squeeze lifetime, it is documented that kaolinite increases the quantity of inhibitor adsorbed. Conversely, clean sandstone with low clay content commonly provides a poor substrate for adsorption. Furthermore, in reservoirs that experience near-wellbore formation damage because of kaolinite mobilization, it has been shown that the use of a fixation agent as part of a squeeze treatment can increase squeeze lifetime. Using these facts, research has assessed the feasibility of injecting micro-crystalline kaolinite (average particle size 2 μm) combined with the fixation agent and scale inhibitor as a means of mechanically altering near-wellbore mineralogy and surface-property characteristics within clean, high-permeability sandstones. The testing has been designed to mimic the squeeze procedure used in the field for performing such a job and involves no additional steps to those used in a normal squeeze (i.e., preflush, main treatment, overflush). The paper presents the results of coreflood experiments that demonstrate "proof of concept," along with examples of potential field applications. A further concept, born from the initial idea, was the use of kaolinite and fixation agent for efficient, low-cost, environmentally friendly water shutoff. There are several available products for water shutoff, but the disadvantage of these is that they are not acceptable for use in Norway because of poor environmental characteristics. Hence, there was a need to fill this gap by developing water-shutoff technology to meet country-specific environmental legislation. The paper provides details of coreflood testing where an excess of kaolinite has been used to form an internal and external filter cake that is attached to the wellbore face and within the near-wellbore region using the fixation agent. The paper draws on data from StatoilHydro-operated fields in order to highlight the potential of this innovative approach to downhole scale management and water control.

Compaction trends for shale and clean sandstone in shallow sediments, Gulf of Mexico

Compaction depth trends are important in drilling, basin modeling, and seismic exploration for several purposes: (1) to de-tect overpressure and hydrocarbon zones and distinguish them from seismic velocity anomalies; (2) to calculate interval velocities and depth conversion involving seismic data and Earth models; (3) to predict seismic signatures of sand-shale interfaces as a function of depth; and (4) to recognize over-compacted zones due to uplift. Several authors have studied the effects of compaction on the porosity of sands and shales (e.g., Magara, 1980; Ramm and Bjorlykke, 1994). The effects of compaction on velocity-depth trends have been provided by different authors (e.g., Al-Chalabi, 1997; Faust, 1951; Japsen, 2000). However, porosity and velocity depth trends in the shallow section are not well established. The main challenge in computing such trends is the paucity of well-log data in the shallow subsurface. Figure 1, a typical well log from the Gulf of Mexico, lacks measurements in the shall...

Troll West Oil Field - A Giant Gas Field Becomes the Largest Oil Field on the Norwegian Continental Shelf

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 112616, "Troll West Oilfield Development-How a Giant Gas Field Became the Largest Oil Field in the NCS Through Innovative Field and Technology Development," by Richard Dyve Jones, StatoilHydro, and Erland Saeverhagen, SPE, Arve K. Thorsen, SPE, and Sveinung Gard, SPE, Inteq, prepared for the 2008 IADC/SPE Drilling Conference, Orlando, Florida, 4-6 March. The paper has not been peer reviewed. The Troll West oil field has been developed with more than 110 horizontal subsea wells including 53 multilateral (ML) wells. The thin oil rim was developed through extensive use of multilateral drilling and wells containing up to 7 horizontal branches. The process of drilling the ML wells and the benefit/risk evaluation for the ML process are discussed. Introduction The Troll field is offshore Norway on the Norwegian Continental Shelf (NCS) in 300-m water depth, as shown in Fig. 1. The field was discovered in 1979 and covers an area of 750 km2. Drilling of the production wells began in 1994, and production began September 1995. Oil production to September 2007 was 188.3×106 m3 of oil, all from horizontal wells. The 250 horizontal sections have drilled 900 000 m, including 560 000 m of reservoir section. Field Description and Challenges Gas and oil are found mainly in the Sognefjord formation and in the underlying Fensfjord formation. The oil in the Troll West is from a 22- to 26-m thick oil column under a small gas cap. In the Troll West gas province, there is a 12- to 14-m-thick oil column and a gas column up to 200 m thick. The Troll reservoir sands are of two distinct types, with large grading between. The C sand is a clean coarse sandstone, with permeability ranging from 1 to more than 10 darcies. The M sand is micaceous, with a finer grain size than the C sand, and, in general, has permeabilities in the range of 1 to 100 md. The sorting in this sand is poorer than in the C sand; therefore, the permeability is less. Recovery Strategy. Oil production is through long horizontal wells drilled just above the oil/water contact (OWC) in the thin oil zone. The main recovery strategy is pressure depletion, but with simultaneous expansion of the gas cap above and the water zone below the oil. In the Troll West oil province, some of the gas produced has been injected back into the reservoir to optimize the oil production. One important aspect is to recover the oil quickly, because less oil can be extracted when the pressure declines in Troll East. Therefore, limits were placed on gas extraction from Troll East.

Facies in the Unayzah Formation and the Basal Khuff Clastics in subsurface, northern Kuwait

ABSTRACT This is the first reporting of the Unayzah Formation from a northern Kuwait deep well where a 304 ft of core was cut in the upper part of the formation. The core consists of about 202 ft of the Unayzah Formation, 74 ft of the Basal Khuff Clastics and 28 ft of the Khuff Formation. Within the Unayzah Formation six major lithofacies were interpreted: (1) high-angle cross-bedded sandstone; (2) fine-grained sandstone; (3) normally graded coarse pebbly sandstone; (4) medium- to fine-grained sandstone and siltstone; (5) red mudstone and siltstone; and (6) black shale. In the Basal Khuff Clastics, four lithofacies were recognised: (1) fine- to medium-grained clean sandstone; (2) muddy bioturbated sandstone and siltstone; (3) dark grey to black carbonaceous shale with sand interbeds; and (4) interbedded sandstone, siltstone and dolomite. These lithofacies can be subdivided into four broad palaeoenvironments; ?eolian, braided fluvial, flood plain, and coastal plain. The possible eolian facies are represented by high-angle, cross-bedded sandstones of likely dune affinity. Braided fluvial facies consist of pebbly sandstones, which may grade normally to fine-grained sandstone and siltstone. Flood-plain environments are interpreted for the red mudstone, shale, and siltstone, which consist of prominent root casts and other subaerial exposure features. This facies is prominently repeated three times within the cored zone. Coastal plain environments are represented by tidal channels consisting of fine-grained and clean sandstone, crevasse splay and interfluve sediments consisting of bioturbated sandstone, siltstone and shale and by dark grey to black shale of coastal marsh in origin. The core did not cover the lower contact of the Unayzah Formation. The upper contact of the formation with the Basal Khuff Clastics is placed at the top of the last oxidised red zone. Due to the limited palynofloral information, the age of the cored interval is not definitive but most likely is of Kazanian to Tatarian (Wordian to mid-Wuchiapingian). The Basal Khuff Clastics and the overlying Khuff Formation is equivalent to the OSPZ6 biozone and the Unayzah Formation could be in part equivalent to the OSPZ5 biozone. The sediments belonging to the Basal Khuff Clastics mark the onset of a major transgression that developed into a platform where carbonates of the overlying Khuff Formation were deposited.