Low Poisson's Ratio Research Articles

Fluid Property Discrimination in Dolostone Reservoirs Using Well Logs

AbstractThe Ordovician Majiagou Formation is one of the main gas‐producing strata in the Ordos Basin, China. The identification of hydrocarbon‐bearing intervals via conventional well logs is a challenging task. This study describes the litholog of Ma 5 (Member 5 of Majiagou Formation) dolostones, and then analyzes the responses of various conventional well logs to the presences of natural gas. The lithology of the gas bearing layers is dominantly of the dolomicrite to fine to medium crystalline dolomite. Natural gas can be produced from the low resistivity layers, and the dry layers are characterized by high resistivities. Neutron‐density crossovers are not sensitive to the presences of natural gas. In addition, there are no significant increases in sonic transit times in natural gas bearing layers. NMR (nuclear magnetic resonance) logs, DSI (Dipole Sonic Imager) logs and borehole image logs (XRMI) are introduced to discriminate the fluid property in Majiagou dolostone reservoirs. The gas bearing intervals have broad NMR T2 (transverse relaxation time) spectrum with tail distributions as well as large T2gm (T2 logarithmic mean values) values, and the T2 spectrum commonly display polymodal behaviors. In contrast, the dry layers and water layers have low T2gm values and very narrow T2 spectrum without tails. The gas bearing layers are characterized by low Vp/Vs ratios, low Poisson's ratios and low P‐wave impedances, therefore the fluid property can be discriminated using DSI logs, and the interpretation results show good matches with the gas test data. The apparent formation water resistivity (AFWR) spectrum can be derived from XRMI image logs by using the Archie's formula in the flushed zone. The gas bearing layers have broad apparent formation water resistivity spectrum and tail distributions compared with the dry and water layers, and also the interpretation results from the image logs exhibit good agreement with the gas test data. The fluid property in Majiagou dolostone reservoirs can be discriminated through NMR logs, DSI logs and borehole image logs. This study helps establish a predictable model for fluid property in dolostones, and have implications in dolostone reservoirs with similar geological backgrounds worldwide.

Preparation of nanotwinned cBN cutting edge by combining mechanical lapping and ion beam polishing

Nanotwinned cubic boron nitride (nt-cBN) is a promising tool material for ultra-precision cutting of ferrous metals. However, high hardness and excellent thermal stability make it difficult to obtain sharp cutting edge by using conventional methods. In this study, the factors influencing the material removal modes and edge sharpness in mechanical lapping process have been investigated, and a novel method combining mechanical lapping and ion beam polishing is proposed for achieving the sharp nt-cBN cutting edge. The results show that the material removal in mechanical lapping is mainly contributed by the transgranular ductile failure at the nanometer scale. Besides, high material hardness with a low Poisson's ratio and small impact force on the cutting edge are beneficial for obtaining sharp cutting edge. Therefore, the ion beam polishing process is adopted to sharpen the mechanically lapped cutting edge. By using a steel baffle plate with straight profile and eliminating the clearance between the baffle plate and rake/flank face with resin filler, the edge radius is observed to decrease from micrometre scale to about 60 nm. The proposed method represents a promising strategy for the preparation of ultra-sharp cutting edge with low cost and high accuracy.

Structural Heterogeneity in Source Zones of the 2018 Anchorage Intraslab Earthquake and the 1964 Alaska Megathrust Earthquake

AbstractStructural heterogeneities in subduction zones can affect slip behaviors of the megathrust faults and the generation of intraslab earthquakes. In this work we study the 3‐D seismic structures (Vp, Vs, and Poisson's ratio) in and around the source zones of the 2018 Anchorage intraslab earthquake (Mw 7.1) and the 1964 Alaska megathrust earthquake (Mw 9.2). The Anchorage earthquake occurred in an anomalous zone within the subducting Yakutat/Pacific plate with a higher Poisson's ratio than the normal slab. Above the source zone, the overriding North American plate shows a low Vs and a high Poisson's ratio. These features indicate that strong dehydration occurs in the source zone and released fluids ascend into the overlying crust. Two areas with long‐term slow slip events in the Upper and Lower Cook Inlet predominantly exhibit a high Poisson's ratio in the lowermost portion of the crust and the cold nose of the mantle wedge, whereas a low Poisson's ratio zone is revealed between them, suggesting that their segmentation is possibly related to localized slab‐releasing fluids. In the Prince William Sound, the rupture of the 1964 Great Alaska earthquake initiated beneath a high‐V and high Poisson's ratio zone of the overlying crust and the large slips occurred beneath a low‐Vs and high Poisson's ratio zone, suggesting that lateral heterogeneities of the overriding plate may have played an important role in the nucleation and rupture processes of the Great Alaska earthquake.

Analytical orientation criteria for drilling and completion-induced fracture initiation considering fluid infiltration from the wellbore

Analytically-developed criteria are presented for the orientation of fracture initiation in porous-permeable rocks considering fluid infiltration from non-perforated and perforated wellbores. For non-perforated wellbores, this is a fully-exact analytical solution representing the initiation of drilling-induced tensile fractures (DITFs). For perforated wells, a set of analytical approximations are derived for fracture initiation representing completion-induced hydraulic fractures (CIHFs). DITF orientation is used to constrain the in-situ stress tensor, especially in the case of transverse DITFs, which are rarer than longitudinal DITFs. Results indicate that transverse CIHF initiation only occurs over a wellbore pressure-at-breakdown window, while longitudinal CIHF initiation occurs at comparatively higher wellbore pressures. Transverse CIHF initiation however, occurs more frequently than transverse DITFs, because the presence of perforations aids transverse fracture initiation. Low Biot's poroelastic coefficient and high Poisson's ratio values also aid transverse fracture initiation. The region of the in-situ stress states where transverse initiation is promoted can be shown in dimensionless plots. Case studies are run on seven prolific shale plays (Barnett, Bakken, Fayetteville, Haynesville, Niobrara, Marcellus, and Vaca Muerta) using breakdown pressures from the literature. Transverse CIHF initiation is found to be a highly likely outcome, as opposed to results from past studies that neglected pore pressure effects. Based on the findings of this study, recommendations are made to practitioners to enhance their chances on achieving transverse CIHF initiation in the field. These involve slow injection rates, oriented perforation strategy and strong bonding between the casing, cement and the surrounding rock to prevent fracturing fluid leakages.

Experimental and finite element modelling evaluation of cement integrity under diametric compression

With the increased exploration activity in unconventional and extreme operational environment, well integrity is becoming a challenging problem. Cement sheath, being an important barrier element, should be designed properly for adaptation of complicated loads surrounding the wellbores. This experimental and modelling study evaluates casing-cement-formation system under diametrically compressive load.Digital image correlation (DIC) technique was employed to examine the strain distribution and failure in experimental setup. A 3D numerical model was developed and the comparison between simulation and experimental results indicated a reasonable match. Using validated model, the effect of cement elastic properties on stresses in cement was studied. The stress distribution in the cement-formation system shows that casing-cement interface is the weakest point in the system, thus it has the highest risk of failure. The failure initiates at orientation parallel (θ = 0°) to the applied load followed by in perpendicular (θ = 90°) direction. Hoop stress is more sensitive to Young's modulus and Poisson's ratio than radial stress, so it is more critical for the failure assessment. In general, flexible cement with low Young's modulus and high Poisson's ratio has low risk of failure.

Elastic and magnetic properties of Fe3P up to core pressures: Phosphorus in the Earth's core

Phosphorus (P) is considered a possible light element alloying with iron (Fe) in the Earth's core due to its siderophile nature and the ubiquity of P-bearing iron alloys in iron meteorites. The sequestration of P by liquid metals during the core formation possibly results in the relatively low concentration of P in the bulk silicate Earth. In this study, we performed single-crystal and powder X-ray diffraction, synchrotron Mössbauer spectroscopy and nuclear resonant inelastic X-ray scattering measurements in diamond anvil cells to investigate the elastic and magnetic properties of Fe3P under high pressures. Our X-ray diffraction results suggest that there is no structural phase transition up to 111 GPa. However, a volume collapse was observed at 21.5 GPa in Fe3P, ascribed to a magnetic transition as evidenced by synchrotron Mössbauer spectroscopy results. Fitting the volume-pressure data by the Birch-Murnaghan equation of state gives bulk modulus KT0=162.4(7) GPa, its first pressure derivative KT0′=4.0 (fixed) and zero-pressure volume V0=370.38(6) Å3 for the magnetic phase and KT0=220(7) GPa, KT0′=4.0 (fixed) and V0=357(1) Å3 for the non-magnetic phase. Sound velocities of Fe3P were determined up to 152 GPa by nuclear resonant inelastic X-ray scattering, demonstrating that Fe3P bears a low shear velocity and high Poisson's ratio at core pressures compared to Fe and Fe3S. When forming a solid solution Fe3(S,P) with Fe3S at core pressures, Fe3P may favorably influence the elastic properties of Fe3(S,P) to match the seismic observations of the inner core.

Structure of the Sediment and Crust in the Northeast North China Craton from Improved Sequential H-k Stacking Method

AbstractH-k stacking method is a standard receiver-function method to detect crustal thickness. But this method can not be applied in low-velocity sedimentary basins. To solve this problem, we propose an improved sequential H-k stacking method. The improved method needs two sequential stacks. Firstly, sediment structure is calculated using converted waves and multiples on the bottom boundary of sediments. Secondly, the sedimentary results are applied to calculate the crustal structure. Theoretical calculations and “recovery tests” indicate that the improved method can obtain accurate estimates in sedimentary basins. With the teleseismic data of North China Craton, the structure of sediments is thick in the depression and thin in the uplifted area, which is consistent with Deep Seismic Sounding results. The crust to the west of the North-South Gravity Lineament is relatively thick and has a low average Poisson ratio, whereas the east is relatively thin and has a high average Poisson ratio. This result and the structural feature from data regression imply that the eastern crust of the North China Craton has experienced wide extension, which reflect the crustal response to the severe destruction and deformation in that area compared to the western crust.

Fracture and strength of a TiZr-based metallic glass at low temperatures

With decreasing the testing temperature, a TiZr-based metallic glass (MG) shows obvious increments in yield strengths, tension-compression strength asymmetry, and deviations of shear-fracture-angle, implying that the MG displays increased resistance to plastic shearing but decreased critical stress for brittle normal fracture. These results well explain the ductile-to-brittle transition of MGs caused by low temperature. Besides, the low-temperature failure criterion was established and applied to various other MGs. It is demonstrated that the MGs with lower Poisson's ratio may be more sensitive to suffer low-temperature induced brittle fracture.

Variations in Crustal and Uppermost Mantle Structures Across Eastern Tibet and Adjacent Regions: Implications of Crustal Flow and Asthenospheric Upwelling Combined for Expansions of the Tibetan Plateau

AbstractWe present new, high‐resolution constraints on crustal and uppermost mantle structure of the southeastern, eastern, and northeastern Tibetan plateau and adjacent regions by combining extensive data from three regional‐scale dense seismic arrays. Our results show significant differences in the crustal and uppermost mantle structure within the Songpan‐Ganzi terrane. The southern part has a thick crust and high Poisson's ratio, while the crustal thickness becomes less and Poisson's ratio is nearly normal in the northeast. Prominent low‐velocity anomalies appear beneath both the southern and northeastern Songpan‐Ganzi terrane at depths of 20–40 km, but they are not interconnected. Seismic velocities of the uppermost mantle are slow in the northeastern Songpan‐Ganzi terrane relative to the south. We further find that the crust of the central Qilian orogen is thickened but with remarkable low Poisson's ratio. Low velocities are visible in both the mid‐lower crust and uppermost mantle, similar to the northeastern Songpan‐Ganzi terrane. For comparison, the crustal low velocities are less pronounced beneath the central Qilian orogen. High Poisson's ratio and the mid‐lower crustal anomaly of the southern Songpan‐Ganzi terrane strongly indicate partial melting of the crust. Localized asthenospheric upwelling, however, can account for the coincident anomalies in both the mid‐lower crust and uppermost mantle beneath the northeastern Songpan‐Ganzi terrane and central Qilian orogen. We conclude that outward expansion of the Tibetan plateau has evolved by a combination of crustal flow on its southeastern margin and mantle upwelling likely induced by removal of thickened lithosphere on its easternmost and northeastern edges.

Influences of lithology on in-situ stress field in low permeability reservoirs in Bonan Oilfield, Bohai Bay Basin, China

Abstract The differences of rock mechanical properties were analyzed based on triaxial compression test in low permeability reservoirs of the Bonan Oilfield. Through the analysis of reservoir mechanics, the influence mechanisms of different mechanical properties of rocks on reservoir in-situ stress were studied. By means of stress ellipse and finite element simulation, the influence rules of different mechanical properties of rocks on in-situ stress field were discussed. For the low permeability reservoirs of the Bonan Oilfield, the coarser rock has a larger Young's modulus value and a lower Poisson's ratio. The rock mechanical parameters and stress-strain relationship of sandstone facies and mudstone facies are different. Different rocks have different mechanical properties, which cause extra stress at the lithological contact interface, and the existence of extra stress affects the reservoir in-situ stress. Without considering the influence of structural features on the in-situ stress field, the reservoir in-situ stress is controlled by the magnitude of extra stress and the angle between lithological contact surface and boundary stress.

Influencing factors and fracability of lacustrine shale oil reservoirs

Abstract Shale oil reservoirs need fracturing in order to achieve commercial productivity. A key factor influencing the fracturing process is shale fracability, which is strictly dependent on rock fabric. In this study, we focused on the lacustrine shale oil reservoirs located in Dongying Sag and in the Northern Jiangsu Basin. Through X-ray mineral diffraction, rock mechanics, and P- and S-wave velocity tests, we analyzed the mineral components and mechanics of different shale lithofacies, as well as the influence of these factors on shale fracability. Based on the results of these analyses, we proposed a new method for the evaluation of lake shale oil reservoir fracability. The experimental results showed that the macrostructure, mineral content, lithology, and the presence of microfractures in the shale are the main factors affecting its brittleness. Shales with lamellar and sheet-like structures have relatively high Young's modulous or lower Poisson's ratios; under these conditions, netted fractures are easily formed during fracturing. The fracture toughness of carbonate minerals in shales is strong. An appropriate amount of carbonate minerals seemed to increase the shale brittleness; however, an excessive amount increased its cohesiveness, hindering the initiation and propagation of fractures. Felsic minerals are characterized by high brittleness, a characteristic that favors shale fracability; on the other hand, clay minerals and organic matter are highly plastic, a characteristic that hinders fracturing transformations. Due to their macrostructure and composition, lamellar clayey limestones (dolomite), lamellar clayey siltstones, layered calcareous (dolomitic) siltstones, sheet-like clay-containing siltstones, and flaggy carbonate-containing siltstones are expected to be easily fractured. In addition, the presence of microfractures seemed to reduce the overall fracture toughness of the rock: they penetrated artificial fractures, enhancing rock fracability. Taking into account the fracture toughness of carbonate minerals and the plastic effect of organic matter, we proposed a new formula for the calculation of the brittleness index, based on the mineral component method. The brittleness indices obtained through the proposed formula were linearly correlated with those calculated through the rock mechanics parameters method. Overall, our results indicate that the silty lithological successions occurring in the study area should be the easiest to fracture, followed by the calcareous (dolomitic) and clayey or organic-rich lithological successions. This study is of guiding significance for evaluating the fracability of different lithofacies in lacustrine shale oil reservoirs. Especially, the new proposed brittleness index containing the plastic effect of organic matter can correctly evaluate the actual brittleness of organic-rich shale.

Characterization of quartz in the Wufeng Formation in northwest Hunan Province, south China and its implications for reservoir quality

Quartz is one of the most important and common minerals in marine shales. This is especially true for the Wufeng-Longmaxi and Niutitang Formations in south China. However, the type, origin and formation mechanism of the quartz in these marine shales are as yet unclear. Thirteen samples of Ordovician shale from the Wufeng Formation in northwest Hunan Province were collected for this study. The quartz within the Wufeng shale was characterized through a series of experiments employing scanning electron microscopy with cathodoluminescence, nitrogen physisorption, X-ray diffraction, and geochemical analysis. The results were then used to assess the implications of the quartz component for reservoir quality. Three quartz types were identified based on scanning electron microscopy and cathodoluminescence imaging. Type I quartz has a relatively large grain size and exhibits bright luminescence, indicative of a terrigenous origin, whereas Type II and Type III quartz is authigenic and has dull or no luminescence. Type II quartz has small grain sizes and may be mainly biogenic in origin. This type of quartz may have formed during early diagenesis, with the silica being sourced from the dissolution of siliceous organisms. Type III quartz generally appears as dark rims on Type I quartz grains; the silica may have been derived from clay mineral alteration, with quartz-formation occurring during middle or late diagenesis. The authigenic quartz (Types II and III quartz) likely formed as a cement and would be expected to generate brittle behavior. The shale with a high content of authigenic quartz has a high Young's modulus and low Poisson's ratio. The biogenic authigenic quartz (Type II quartz) was pore-filling and formed during shallow burial. It would thus have been able to resist further compaction and provide a rigid framework within which the primary pores in the shale could be preserved, controlling the distribution of organic matter and the development of organic pores. The biogenic authigenic quartz has positive relationships with porosity and pore volume and specific surface area. Shale that was high in biogenic quartz had high porosity and adsorption capacity. The characterization of quartz in marine shales has significance for shale gas exploration and development and provides a new angle for evaluating the fracability and storage capacity of shale gas reservoirs.

Feasibility analysis on the pilot test of acid fracturing for carbonate reservoirs in Halfaya Oilfield, Iraq

AbstractThe oil reservoir of Mishrif Formation is of strong heterogeneity vertically and laterally, and its single‐well production rate after conventional acidizing is low and declines fast. Therefore, we intend to carry out the pilot test on diverting acid fracturing so as to improve the stimulation effect of acid fracturing by enlarging the lateral and vertical stimulation range. In this paper, a series of experimental studies (eg, rock mechanics, acid rheology, acid‐rock reaction kinetics, and full‐diameter core etching diversion) were carried out to deal with the reservoir characteristics and stimulation demand of Mishrif Formation, the principal oil reservoir in Halfaya Oilfield in Iraq, the Middle East. It is shown that calcite content of Mishrif Formation reservoir is higher than 95%, indicating the strong acid dissolution. As for rock mechanics, the characteristics of “low Young's modulus and high Poisson's ratio” are presented, and the stress‐strain curves show certain elastic‐plastic characteristics. Additionally, a large number of wormholes appear on the surface of the full‐diameter cores after acid etching, so it is necessary to consider the serious filtration of acidizing fluid near the wellbore. The mechanical strength of rock plates decreases after acid etching, so it is recommended to adopt and optimize the closed acidizing technology so as to avoid excessive etching. Furthermore, the self‐support flow conductivity of etched fractures is weak, so it is recommended to adopt the sand fracturing or the combined stimulation technology of acid fracturing + sand fracturing to improve the reservoir stimulation effects.

Stress redistribution of dynamic loading incident with arbitrary waveform through a circular cavity

SummaryIn actual geotechnical and civil engineering, dynamic stress concentrations around cavities generated by wave sources widely exist. In this study, based on the complex variable theory and Fourier transform method, the expression of the dynamic stress concentration factor (DSCF) around a circular cavity in infinite homogeneous media subjected to transient waves with arbitrary waveform is obtained. The relationships between both steady‐state and transient DSCF and their waveform parameters are investigated quantitatively. The results indicate that a relatively large tensile stress is generated with low Poisson's ratio under steady‐state incidence. Under the condition of transient incidence, the position of the wave peak has a minor effect on the DSCF in the case of small wavenumber, but it has a significant effect in the opposite case. It is found that when the wavenumber is high, such as 0.5, the stress response lags behind the stress wave. In addition, the closer the wave peak to the center of the waveform, the greater the potential damage of the transient incidence.

Influence of the Characteristics of Cement Plug on well Integrity for Offshore Abandonment

With the development of offshore oil fields into the middle and late stages, more and more oil wells are facing the problem of disposal. The reliability of the plugging between the cement plug and the wellbore is directly related to the safety of offshore operations and the problem of marine ecological damage caused by oil and gas leakage. Starting from the cement failure mechanism between the cement plug and the casing, consider the heterogeneity of the formation and the magnitude of the ground stress. The finite element software is used to simulate the interaction between the cement plug and the casing with different characteristics, and then judge the risk of sealing failure. Furthermore, the influence of cement plug properties on casing stress and cementation strength is obtained. Study show that the use of high strength, low modulus of elasticity, low Poisson's ratio cement is conducive to prevent cement plug tensile failure. When the local layer is hard, the method of reducing the elastic modulus of the cement plug can be adopted; and when the local layer is soft, the method of appropriately increasing the elastic modulus of the cement plug should be adopted. It has certain guidance and reference significance for the safe disposal of offshore wellbore.

Predicting gas content in high-maturity marine shales using artificial intelligence based seismic multiple-attributes analysis: A case study from the lower Silurian Longmaxi Formation, Sichuan Basin, China

Abstract Gas content is the most important index used to evaluate shale gas reservoirs. Compared to the North American marine shales, the marine shales of China are older, characterized by high thermal maturity, and have experienced multiple stages of tectonism. There is no clear positive correlation between the gas content and TOC content for marine shales in China; therefore, it is not feasible to indirectly predict the gas content by predicting TOC content, as has been extensively done for the North American shales. In this study, the geophysical responses of reservoirs with high gas contents, the factors that influence the gas content have been determined based on well log interpretation and seismic rock physics analysis. The seismic attributes and elastic parameters that are sensitive to the gas content have also been investigated. Additionally, a pre-stack seismic inversion has been carried out through a combination of well log and 3D seismic data to obtain the data volumes of the seismic elastic parameters, which are highly sensitive to variations in gas content. A seismic multiple-attribute analysis based on neural networks of AI (artificial intelligence) technology has been applied to determine the type and the combination of elastic parameters for computing the gas content data volume. This kind of analysis has been carried out based on pre-stack inversion of seismic data. The results suggest that the lower Silurian Longmaxi Formation, a high-quality gas-rich reservoir, is characterized by its low density, low Poisson's ratio and low velocity. A combination of six parameters or attributes optimally predicts gas content. The studied block, W202, generally has a high gas content. Two classes of sweet spots have been predicted in the study area based on these results. The application of neural networks has provided a precise prediction for the spatial distribution of gas saturation in the shale reservoir, serving as a basis for locating and designing horizontal wells for shale gas development.

Elastomer-like deformation in high-Poisson's-ratio graphene allotropes may allow tensile strengths beyond theoretical cohesive strength limits

Phenomenological theories of deformation in brittle solids generally envisage fracture under tension as the breaking of atomic bonds perpendicular to the fracture plane, thereby ignoring the role of bond rotation. Using density functional theory calculations, we found that during the early stage of the uniaxial tension of graphene allotropes with high Poisson's ratios, bond rotation effectively lessens bond stretch and increases the fracture strain. Specifically, in the deformation of an allotrope, Gr10, with a Poisson's ratio of 0.8, bond rotation results in an S-shaped stress-strain curve, akin to those of elastomers. Moreover, the tensile strength (σth) and the Young's modulus (E) of Gr10 exceed the theoretical cohesive strength limit σth≈E/10, reaching σth≈E/1.7. However, a universal relationship between bond lengths and charge density distribution along bond paths was found to be suitable for all carbon-carbon covalent bonds. Consequently, all carbon-carbon bonds obey a common shape of bond-force vs. bond-strain curve, with bond strength, S, and bond stiffness, K, following S≈K/9; hence, σth≈E/10 remains valid for the low-Poisson's-ratio graphene allotropes whose deformation is dominated by bond stretch. Overall, we suggest the trade-off between bond stretching and bond rotation can be utilized to enhance the fracture strain of two-dimensional carbon structures.

The Calculation of Coal Rock Fracture Conductivity with Different Arrangements of Proppants

An accurate evaluation of coal rock fracture conductivity is an important prerequisite for predicting the productivity of CBM wells. Coal rock is soft and fragile, with low elastic modulus and high Poisson ratio. In the process of fracturing flowback, the contact deformation between proppant and fracture wall will affect the fracture conductivity when the proppant is embedded in the coal rock; thus the calculation method of plate fracture conductivity is no longer suitable for the evaluation of coal rock. Based on the contact deformation theory of elastic mechanics, a method for calculating contact deformation of proppant in fracture is proposed. Considering the effect of the deformation and embedded depth of proppant and the tortuosity of pore flow channel between proppant particles on fracture conductivity, a model for calculating fracture conductivity of coal rock fractures under three kinds of proppant arrangement (Model 4-1, Model 3-1, and Model 2-1) is established. Comparison of calculation results of theoretical model and experiments confirmed that the arrangement of proppant in coal rock fracture is closest to Model 3-1, and the influence of mechanical parameters of coal rock and proppant on fracture conductivity is calculated and analyzed by this theoretical model. The study shows that the coal rock fracture conductivity is affected little by Poisson’s ratio of coal rock and proppant, which is greatly influenced by the elastic modulus of them, and the effect of particle size of proppant is especially significant.

Integrated reservoir characterization of a Utica Shale field

We performed an integrated multidisciplinary study for reservoir characterization of a Utica Shale field in eastern Ohio covered by a multiclient 3D seismic data set acquired in 2015. Elastic seismic inversion was performed in-house for effective reservoir characterization of the Utica Shale, which covers the interval from the top of Upper Utica (UUTIC) to the top of Trenton Limestone. Accurate, high-fidelity inversion results were obtained, including acoustic impedance, shear impedance, density, and VP/VS. These consistent inversion results allow for the reliable calculation of geomechanical and petrophysical properties of the reservoir. The inverted density clearly divides the Point Pleasant (PPLS) interval as low density from the overlying UUTIC Shale interval. Both Poisson's ratio (PR) and brittleness unmistakably separate the underlying PPLS from the overlying Utica interval. The PPLS Formation is easier to hydraulically fracture due to its much lower PR. Sequence S4 is the best due to its higher Young's modulus to sustain the open fractures. The calculated petrophysical volumes indisputably delineate the traditional Utica Shale into two distinctive sections. The upper section, the UUTIC, can be described as having 1%–2% total organic carbon (TOC), 3.5%–4.8% porosity, 10%–24% water saturation, and 40%–58% clay content. The lower section, PPLS, can be described as having 3%–4.5% TOC, 5%–9% porosity, 2%–10% water saturation, and about 15%–35% clay content. Both sections exhibit spatial variation of the properties. Nevertheless, the underlying PPLS is obviously a significantly better reservoir and operationally easier to produce.

Thermal Effects of Liquid/Supercritical Carbon Dioxide Arising From Fluid Expansion in Fracturing

Summary Liquid/supercritical carbon dioxide (L/SC-CO2) fracturing is an emerging technology for shale gas development because it can effectively overcome problems related to clay swelling and water scarcity. Recent applications show that L/SC-CO2 fracturing can induce variations in temperature. Understanding of this phenomenon is rudimentary and needs to be carefully addressed to improve the understanding of CO2 thermodynamic behavior, and thus helps to optimize CO2 fracturing in the field. In this paper, we develop a numerical model to assess the impact of thermal effect on fracture initiation during CO2 fracturing. The model couples fluid flow and heat transfer in the fracture, and is verified by a peer-reviewed solution and observation in laboratory experiments. The velocity, pressure, and temperature are calculated at various time to demonstrate the thermodynamic behavior during fracture initiation. A pseudo shock wave is observed, associated with a compression wave and an expansion wave, which finally leads to an increase in temperature in the new fracture and a decrease in temperature in the initial fracture. The thermal stress is derived to investigate the difference between hydraulic fracturing and CO2 fracturing. The results show that thermal stress, resulting from CO2 fracturing initiation, is comparable to the rock strength, which will help induce microfractures, and thus promote the fracture complexity. The formation pressure after CO2 fracturing is also calculated to evaluate the pressure-buildup potential. This work highlights the importance of CO2 expansion during and after fracturing. It is one of the unique features that differs from hydraulic fracturing. For field-design recommendations, to enhance the thermal effect of CO2 fracturing, it is a good strategy to pump CO2 at high pressure and low temperature into the reservoirs with high Young's modulus, low Poisson's ratio, low permeability, and high geothermal temperature (or large depth). This paper does not address the dynamics of fracture propagation under the influence of thermal effect. Rather, it intends to demonstrate the potential of the thermal effect of CO2 fluid in assisting the fracture propagation, and the importance of incorporating the compressibility of CO2 into fracture modeling and operation design. Failing to account for this thermal effect might underestimate the fracture complexity and stimulated reservoir volume.