Changes In Petrophysical Properties Research Articles

Estimation of seismic attenuation for reservoirs mapping and inverse Q‐filtering: An application on land seismic data

AbstractSeismic attenuation values based on inverse Q‐filtering are useful in enhancing seismic data resolution for quantitative interpretation. However, as field attenuation estimations are often contaminated by noise and overburden effects, an inverse Q‐filter may reduce the signal‐to‐noise ratio. To help reservoir mapping with seismic resolution enhancement, we use well logs and surface seismic data to estimate the attenuation of a depth interval composed of several carbonate oil reservoirs onshore the Arabian Peninsula. From applying a prestack Q inversion on τ–p gathers from a large 3D seismic survey, the estimated effective attenuation values show spatially coherent patterns, likely corresponding to changes in the petrophysical and fluid properties. The patterns of the estimated effective attenuation values correlate considerably with the anticline geometry, with higher values on its crest than its flanks. The small percentage of the negative effective attenuation values indicates that the apparent attenuation and overburden effects are insignificant. At well locations, the effective attenuation values show a good correlation with the average porosity of the main reservoir, verifying that they are related to changes in petrophysical properties in the analysis interval. We propose an extended inverse Q‐filter with two additional parameters to make it more robust and flexible and test it with synthetic and field datasets. We estimate a multiple‐scattering attenuation value from well logs and use it in the proposed filter for the field dataset. The compensation for the multiple‐scattering effects suppresses the overprints of the data components unrelated to the target interval, hence benefitting the quantitative data interpretation.

The role of salt dissolution on the evolution of petrophysical properties in saline-lacustrine carbonate reservoirs: Pore structure, porosity–permeability, and mechanics

The invasion of low-salinity fluids into saline-lacustrine reservoirs may induce salt dissolution, resulting in significant changes in petrophysical properties. Accurate characterization of these changes due to salt dissolution is critical for efficient drilling and development. Here, low-salinity core flooding experiments and several core analysis technologies were employed to investigate the changes in pore structure, porosity–permeability evolution, and mechanical properties. Ion compositions in outlet fluids analyzed in real-time to determine the dissolved salt mineral compositions, mainly include halite (1.66 %), anhydrite (7.32 %), and glauberite (0.98 %). Salt dissolution produces many dissolution pores, especially with sizes ranging from 0.01 to 10 μm, broadens the seepage channels, and enhances the connectivity between pores, which significantly improves rock porosity close to 1.80 times the initial one and permeability to 70 times initial one. During the salt dissolution process, the stability of clay minerals is weakened to induce dispersion, migration, and swelling behavior, leading to the reduction of porosity and permeability. A semi-empirical power law relating expressing porosity and permeability for saline-lacustrine reservoirs is proposed, allowing for salt dissolution and clay swelling during the invasion process of low-salinity fluids. Although salt dissolution could slightly relieve stress sensitivity damage by 56.54 %, it significantly weakens rock strength by reduction of elastic modulus (43.51 %) and compressive strength (61.54 %). The positive and negative effects of salt dissolution on petrophysical properties are critical to guide the development of formation damage control measures for saline-lacustrine reservoirs.

Fluid–Rock Interactions in Geothermal Reservoirs, Germany: Thermal Autoclave Experiments Using Sandstones and Natural Hydrothermal Brines

As renewable energy, geothermal can contribute substantially to the energy transition. To generate electricity or to harvest heat, high-saline fluids are tapped by wells of a few kilometres and extracted from hydrothermal reservoirs. After the heat exchanger units have been passed by, these fluids are reinjected into the reservoir. Due to the pressure and temperature differences between the subsurface and the surface, as well as the cooling of the fluids in the power plant, unwanted chemical reactions can occur within the reservoir, in the borehole, and within the power plant itself. This can reduce the permeability of the reservoir as well as the output of the geothermal power plant. This study aims to simulate real subsurface reactions using batch and leaching experiments with sandstone or sandstone powder as solid phase, and deionised water or natural brine as liquid phase. It is demonstrated that fluid composition changes after only a few days. In particular, calcite, aragonite, clay minerals, and zinc phases precipitate from the natural brine. In contrast, in particular minerals containing potassium, arsenic, barium, and silica are dissolved. Due to the experimental set-up, these mineral reactions mainly took place on the surface of the samples, which is why no substantial changes in petrophysical properties could be observed. However, it is assumed that the observed reactions on the reservoir scale have a relevant influence on parameters such as permeability.

Laboratory simulation of salt weathering under moderate ageing conditions: Implications for the deterioration of sandstone heritage in temperate climates

AbstractSalt weathering is a significant process affecting the deterioration and conservation of stone‐built heritage in many locations and environments. While much research has focused on the impact of salt weathering under arid or coastal conditions with characteristic climatic conditions and salt types, many sites found to be experiencing salt‐induced deterioration, such as sandstone rock‐hewn cave temples in Gansu Province, China and sandstone buildings in the northern UK, experience high humidities, moderate temperature ranges, and different salt types. To evaluate the impact of salt weathering on sandstone‐built heritage under such mild humid environmental conditions, a lab simulation experiment was designed. The experiment was carried out on three types of sandstone (used in the northern UK and Gansu Province, China) and utilized a realistic diurnal humidity and temperature cycle (85% RH/16°C + 60% RH/22°C), and three widespread damaging salts, that is, Na2SO4, MgSO4, and the mixture of Na2SO4–MgSO4. The nature and extent of deterioration was monitored by photography, weight loss, and the changes in petrophysical properties measured using hardness, ultrasonic pulse velocity (P‐wave velocity), water absorption coefficient by capillarity, open porosity, and apparent density. All three sandstones were found to be susceptible to MgSO4 and the mixture of Na2SO4–MgSO4, but weakly affected by Na2SO4 under mild humid environmental conditions.

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

AbstractChanges in permeability can impact geological processes, geohazards, and geothermal energy production. In hydrothermal systems, high‐temperature heat sources drive fluid convection through the pore network of reservoir rocks. Additionally, thermal fluctuations may induce microfracturing and affect the mineralogical stability of the reservoir rock, thus modifying the fluid pathways and affecting permeability and strength. This study describes the results of thermal heating events lasting several hours on a “moderately altered” plagioclase‐clinochlore‐calcite‐quartz andesite and a “highly altered” plagioclase‐clinozoisite‐quartz‐clinochlore andesite from the Rotokawa Geothermal Field, New Zealand. We use a low thermal gradient (~1.2 °C/min) in an H2O‐saturated, 20‐MPa pressure environment to constrain changes in petrophysical properties associated with transitory thermal phenomena between 350 and 739 °C. As the treatment temperature increases, the mass reduces, while porosity and permeability increase. These effects were greater in the “moderately altered” andesite than in the “highly altered” andesite. Microfracturing is responsible for these changes at lower temperatures (e.g., ≤400 °C). At higher temperatures (e.g., >400 °C), microfracturing remains partially responsible for these rock property changes (e.g., higher permeability); however, these changes are also a product of clinochlore, quartz, and (when present) calcite reacting out of the altered andesite, and increasing porosity. We propose that at temperatures >400 °C, volumetric phase changes associated with heat‐driven reactions in a wet environment can contribute to microcracking and porosity/permeability changes. Our data support observations where high‐temperature conditions at the margins of magma bodies can be associated with substantial increased permeability and decreased strength.

Experimental investigation of changes in petrophysical properties and structural deformation of carbonate reservoirs

Abstract To examine the effect of pressure on pore structure and petrophysical properties of carbonate rock, the porosity, permeability, CT scanning, SEM and elastic wave velocity of two carbonate core plug samples from an oilfield in Southwest Iran were analyzed under cyclic pressure. One of the plugs was calcite and the other was dolomite with anhydrite nodules. The cyclic pressure exerted on the samples increased from 13.79 MPa to 27.58 MPa in six steps, and the variations in petrophysical properties of the two samples at different pressure loading and unloading steps were counted and analyzed. The results show that the calcite sample decreases in porosity and permeability with the increase of pressure, which is consistent with the results from compression and shear wave velocity tests. In the dolomite sample, the decreasing trend was not observed; fluctuations of compressive and shear velocities were observed during the loading stage, which may be due to different geometries of the pores and the porosity variation in the sample. Understanding the variation of carbonate petrophysical properties with pressure is helpful for optimizing reservoir development scheme.

Experimental investigation of changes in petrophysical properties during CO2 injection into dolomite-rich rocks

Carbon dioxide may be injected into an underground geological structure for mere geo-sequestration purposes or as a means of enhanced hydrocarbon recovery. During such an operation, CO2 is expected to dissolve in the in-situ fluids (primarily consisting of brine) generating a reactive in-situ solute (i.e. carbonated brine). Subsequently, a series of consecutive chemical reactions may occur between the solute and the host rock. While these reactions are generally known from a qualitative perspective, to what extent they may impact on the host formation’s petrophysical properties requires extensive evaluation on a case by case basis. Due to the presence of highly reactive minerals in their composition, carbonate rocks (e.g. dolostone) present a more complex system to evaluate in terms of the above mentioned chemical reactions.This experimental study has been carried out to evaluate changes in the petrophysical properties of a number of heterogeneous dolostone samples after undergoing carbonated brine flooding under in-situ reservoir conditions. In this study, the core-flood experiments are complemented by pre- and post-flood porosity, permeability and NMR (nuclear magnetic resonance) measurements, X-ray CT scanning and X-ray Diffraction (XRD) and Energy-Dispersive X-ray (EDX) analysis. Overall, a slight increase in the porosity was observed in most samples, most likely, caused by the dissolution of dolomite (CaMg(CO3)2), calcite (CaCO3) and/or anhydrite (CaSO4). The results also show an increase in the permeability of some samples which again could be attributed to dissolution of the minerals. The X-ray CT images show signs of excessive dissolution of minerals and the creation of dissolution patterns (i.e. wormholes). On the other hand, reductions in permeability and porosity by 57% and 12%, respectively, were also observed in a sample. This is believed to be due to the combined effects of the mineral precipitation and mechanical compaction mechanisms dominating over the mineral dissolution. A small shift in the pore size distribution of the samples towards smaller pore sizes was also observed which is believed to have been caused by mechanical compaction.

An Analysis on mCSEM Responses for Sandstone Reservoirs with Respect to Changes in Petrophysical Properties

An Analysis on mCSEM Responses for Sandstone Reservoirs with Respect to Changes in Petrophysical Properties

New hydrocarbon trap models for the diagenetic transformation of opal-CT to quartz in Neogene siliceous rocks

Neogene siliceous rocks are known to act as both reservoirs and seals. However, the role of diagenetic alteration in the development of hydrocarbon reservoirs, seals, and traps in such formations is poorly understood. This study proposes two new models for hydrocarbon traps involving siliceous rock reservoirs with well-developed matrix porosity. The models are based on observations from the Yurihara and Toyotomi fields in Japan to evaluate changes in petrophysical properties associated with confining pressure and diagenetic transformations from opal-CT to quartz. When this diagenetic transformation boundary is located at depths greater than approximately 1000 m (3300 ft), the overlying opal-CT porcelanite layer forms a seal and the underlying clay-poor quartzose porcelanite forms a reservoir, facilitating the development of a hydrocarbon trap. The quartzose porcelanite containing less than 6% Al2O3 (an indication of clay content) can act as a reservoir even 1000 m (3300 ft) below the boundary. When the boundary is located at depths shallower than approximately 500 m (1650 ft), the overlying opal-CT porcelanite is unable to form a seal because of lower confining pressures—although clay-poor quartzose porcelanite continues to be a good reservoir. However, clay-rich quartzose porcelanite intercalated with the reservoir can act as a seal. When the quartzose porcelanite contains greater than 15% Al2O3, it has the potential to seal at the depth of the boundary. The requirement for a seal drops to greater than 8% Al2O3 at 1000 m (3300 ft) below the boundary. Such traps have not been reported outside Japan; therefore, these models are likely to be useful for hydrocarbon exploration in siliceous rock formations elsewhere, such as in the Monterey Formation of California.

Well-log inversion and modeling—A tool for understanding ambiguity and sensitivity of seismic data to changes in petrophysical properties

The estimation of petrophysical properties (i.e. porosity, lithology, and pore fluids) is commonly done by first estimating rock properties (i.e., S- and P-wave velocities and density) from prestack seismic data. Then by some empirical or theoretical transform, the rock properties are converted to the desired petrophysical properties. There are risks associated with this procedure. Among other things, what sensitivity does a rock property show to changes in petrophysical properties or what is the possibility of having more than one combination of petrophysical properties yielding the same rock-property value (ambiguity)?

Application of Lévy Random Fractal Simulation Techniques in Modeling Reservoir Mechanisms in the Kuparuk River Field, North Slope, Alaska

Summary Incorporating a suitable level of heterogeneity into reservoir simulations is necessary for accurate prediction of production rates and final recoveries. Spatial correlation of petrophysical properties, particularly permeability extrema, exerts a profound influence on flow underlying reservoir displacement and depletion processes. Common modeling techniques are founded on Gaussian assumptions for statistical distributions. Such Gaussian-based approaches can inadequately model the permeability extrema that can dominate reservoir performance. However, optimal reservoir management strategies at the Kuparuk River Field require that significant efforts be made to correctly model reservoir behavior. This study utilizes a new method, Lévy fractal simulation, for interpolating permeability at a former gas injection area now being targeted for oil production. The main producing interval is a diagenetically and mineralogically complex clastic unit. The diagenetic complexity causes difficulties in the lateral modeling of large changes in petrophysical properties observed in near-vertical wells, particularly permeability. Prior efforts at modeling the movement of gas, at typical interwell scales, have met with limited success. In this study, the Lévy technique employs automatic calibration with log and core data for the interwell interpolation of the spatially complex reservoir properties. The Lévy fractal simulations preserve the sharp jumps in reservoir properties observed at stratigraphic boundaries and within reservoir subzones. The spatially correlated petrophysical properties are consistent with geologic experience. A fine-scale permeability model incorporating well conditioning data was built using the Lévy fractal interpolation technique. This model encompassed not only the gas injection area but drillsite patterns immediately adjacent. The model preserves the geometry of the reservoir units so that truncation and onlap relationships are preserved. The permeability extrema in the model are characterized by lateral continuities extending over many grid-blocks away from control locations. Porosity was modeled using sequential Gaussian simulation in which well porosity logs were used as the primary conditioning data, and the modeled permeability used as secondary conditioning data. The fine-scale model was then used as input in an upscaled dynamic simulator built to test reservoir mechanisms. The model was also useful for prognosing porosity and permeability at proposed well locations. Early drilling results indicate that substantial quantities of producible oil remain in the former gas injection area.

Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitization

Segments of the root to the Caledonian collision zone are exposed along the west coast of Norway, allowing the study of processes and petrophysical conditions at the deepest crustal levels. P-T conditions prevailing in these crustal root zones correspond to eclogite facies. Eclogitization is associated with marked changes in petrophysical properties, notably a density (10–15%) and Vp increase which may give a mantle signature to an eclogitized crust. The observed ductility enhancement may be caused by transformation plasticity, reduction in grain size, formation of new mineral assemblages and presence of fluid. In the Western Gneiss Region and the Bergen Arcs, eclogites form along shear zones, interpreted as fluid pathways with sharp boundaries to their metastable protolith. This makes eclogite shear zones potential deep crustal reflectors. Dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available. Timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the P-T space, but rather on the introduction of fluids into the system. Eclogites are not only passive recorders of P-T-t paths illustrating very deep subduction of crust (> 100 km); rapid changes in petrophysical properties make fluid-induced eclogitization a dynamic process that influences geodynamics. Stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn-eclogitic pseudotachylytes in the metastable protolith. Deformation, including seismicity, enhances fluid infiltration and further eclogitization. Dense and rheologically weak eclogites may delaminate and sink into the mantle. In the absence of fluids, crustal doubling may occur without metamorphic re-equilibration. The evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to P and T.