查看更多>>摘要：The accurate evaluation of microscopic pore structure characteristics of shales is of great significance for shale oil development. In order to gain a better understanding of complicated pore structure of lacustrine shales, silty shale samples from the Upper Triassic Yanchang formation in Ordos Basin were investigated using FE-SEM, N-2 adsorption, MIP, helium pycnometry and Nano-CT. FE-SEM observations show that five typical pores (interparticle pores, intraparticle pores, dissolution-related pores, organic matter pores and microfractures) are developed in shale samples. The full-scale pore size distributions, determined by combining N2 adsorption, MIP and Nano-CT, show that the pore sizes of the samples range from 2 nm to more than 20 mu m, while the dominant pores are in the range of 20-100 nm. A certain difference exits in porosity obtained by Nano-CT, N-2 adsorption, MIP, helium pycnometry and the combination methods. The N-2 adsorption tends to underestimate the total pore volume and porosity. The MIP may overestimate or underestimate the porosity, due to the matrix compression or limited penetration of mercury, respectively. The combined porosity using multiple methods is reasonably consistent with helium pycnometry. Pore network models of these samples are extracted using the centerline algorithm. From the pore network model, the proportion of connected pores of the studied samples ranges from 15.51% to 37.45%. The coordination numbers obtained from pore network range from 1.34 to 1.84, indicating the relatively poor pore connectivity of the studied samples. Samples rich in organic matter have high proportion of connected pores in the region of interest (ROI) of 3D network. Pore throats show preferential orientation in XY plane (parallel to bedding) and then their two diagonal directions, while the Z axis of the pore structure (perpendicular to bedding) is less connected. These parameters improve the understanding on storage space and transport property of lacustrine shale oil.
查看更多>>摘要：In this study, to adjust the dew point of natural gas, two methods of Joule-Thomson and mechanical refrigeration have been analyzed in terms of technical, energy and exergy. The basis of design was a natural gas feed with an intensity of 1743 kgmole/h. The initial dew point of the feed before processing was equal to 9.65 degrees C. Comparison of the results showed that the mechanical refrigeration method had much better performance and the energy efficiency in this system was 99.84%, while this value was 97.8% for the Joule-Thomson method. Furthermore, the intensity of gas condensate production, as one of the products of dew point adjustment unit, for mechanical refrigeration that uses propane refrigerant for natural gas refrigeration was estimated as 1842 barrels/day. This value exhibited an increase of 185% compared to Joule-Thomson method. The dew points of produced gas through Joule-Thomson and Mechanical refrigeration processes were -8.4 and -24.91 degrees C, respectively. It implies that the quality of hydrocarbons dew point in the produced gas was much lower for Joule-Thomson unit compared to mechanical refrigeration. However, in terms of exergy analysis, the mechanical refrigeration method had an efficiency of 88.69%, while the exergy destruction rate for this method was 15.78% less than the Joule-Thomson process.
查看更多>>摘要：Quartz cement is a significant authigenic mineral in the tight gas sandstones of the Upper Triassic Xujiahe Formation, fourth member (Xu4 sandstones) in the western Sichuan Basin. Here we report mineralogical, petrological and geochemical data from the Xu4 sandstones to constrain the silica sources of their quartz cements. The quartz cements mainly occur as quartz grain overgrowths and pore-filling quartz cement, with major associated authigenic minerals including calcite, dolomite, illite, kaolinite, and chlorite. The homogenization temperatures of quartz cement fluid inclusions range from 49.2 degrees C to 143.4 degrees C and show a multistage continuous precipitation process. Raman spectrometry results show the presence of methane (CH4) and CO2 in quartz cement fluid inclusions. The dissolution of feldspar, the illitization of kaolinite and smectite, as well as the pressure dissolution of quartz grains provided silica sources for quartz cement. The dissolution of feldspar by organic acid (carboxylic acid) is the main silica source for quartz cement in the 80-120 degrees C interval; the illitization of smectite and kaolinite have different initial temperatures, and both consume potassium. The pressure dissolution of quartz grains results in stylolite formation. The low-temperature quartz cement (<70 degrees C) is mainly derived from the dissolution of feldspar by meteoric water. At the Late Triassic/Jurassic transition, the uplift of the western basin led to leaching by meteoric water in the upper part of the Xu4 sandstone, forming a weakly acidic fluid with a low K+/H+ ratio and giving this type of sandstone low potassium and sodium concentrations.
Alarji, HamadAlazman, AbdulazizRegenauer-Lieb, Klaus
10页
查看更多>>摘要：Acid stimulation is one of the main methodologies to enhance well productivity and increase permeability lost by near-wellbore formation damage through matrix acidizing. The desired outcome of acidizing carbonate rocks is to form deep conductive flow channels called "wormholes". The productivity enhancement depends on how deep these wormholes penetrate into the reservoir. The porous medium is characterized by three fundamental parameters which are anisotropic or isotropic porosity, permeability, and percolation. Here we test the hypothesis that effective tortuosity a key parameter for microstructure characterization in porous media has a significant effect on wormhole formation. Significant progress has been made by empirical work on carbonate acidizing to increase the stimulation efficiency, but further improvements are possible by developing a deeper understanding of the impact of effective tortuosity on wormhole initiation and development. We use Indiana limestone core flooding experiments to inject diluted hydrochloric acid (HCl) at different acid concentrations and injection rates. Effective tortuosity was calculated for the tight Indiana limestone and was compared with an earlier study on the highly porous Mount Gambier limestone to evaluate the effect of tortuosity on wormhole formation. Our results suggest that the inverse Damko center dot hler and Pe ' clet phase space, traditionally used for evaluating criteria for wormhole formation, needs to be extended by tortuosity as a third dimension to capture the effect of microstructure for the initiation of wormhole phenomenon. Incorporating this extension will improve the prediction of wormhole initiation conditions for all carbonate rock types by using a universal dimensionless phase space.
查看更多>>摘要：Hydrocarbon-water interface plays an important role in many cases and industries. However, the effect of polar oil molecules on hydrocarbon-water interfacial properties is still unclear. In this work, 9 molecular scale heptanewater interface models containing different polar oil molecules were established by molecular dynamic simulation to study the effect of polar oil molecules on hydrocarbon-water interface properties. Interfacial tension (IFT), molecular configuration, molecular orientation and interaction energy between oil and water were calculated to quantitatively characterize the heptane-water interfacial properties containing different oil components. The IFT results show that IFT decreases as the molecular polarity increases. The molecular configuration results indicate that the stronger polar oil molecules tend to adsorb to the interface while the weaker polar oil molecules tend to stay in the oil bulk space. The molecular orientation shows that the polar oil molecules possess more orderly regularity at the interface while nonpolar oil molecules can only randomly adsorb at the interface. Specifically speaking, chain polar oil molecules tend to be perpendicular to the interface while the ring structure polar oil molecules tend to be parallel to the interface. The ring oil molecules containing branched functional groups should also tend to be perpendicular to the interface under the influence of the branches. The heptanewater interaction energy results indicate that the interaction energy increases as the molecular polarity increases. Furthermore, the electrostatic energy predominates over van der Waals energy when the oil contains polar oil molecules while van der Waals energy predominates over electrostatic energy when the oil contains nonpolar oil molecules. The conclusions in this work should provide some fundamental understanding of interfacial information and some industries involving hydrocarbon-water interface.
查看更多>>摘要：In the process of producing shale gas by fracturing, a large amount of water enters shale due to shale imbibition, which affects the shale permeability because of the positive effect by cracks caused by shale swelling and the negative effect by water-blocking. However, there is still a lack of quantitative description of these two effects. In this paper, permeability tests were conducted on shale samples before and after deionized water/NaCl solution imbibition. The results show that, compared with deionized water imbibition, the decrease of shale permeability after NaCl solution imbibition is stronger and more complex because of the NaCl crystals. The permeability of shale after NaCl solution imbibition is only 0.06 times of that before imbibition, while imbibition of deionized water reduces the permeability to 0.15 times of the starting value. In deionized water imbibition, the shale swelling effect causes the increase of permeability by 2.78 times, while the water-blocking effect causes an average permeability loss as high as 99.0%. Shale swelling and water-blocking effects increase the sensitivity of shale permeability to pore pressure by 1.46 times and 1.05 times respectively. NaCl crystals can also increase the sensitivity of shale permeability to pore pressure. Except for the complex influence of NaCl solution and the influence of NaCl crystals on permeability, the relationship between permeability and true effective stress can be closely fitted with an exponential function. This study is helpful to understand the effects of water and saline solution on the permeability of shale gas reservoirs in the process of hydraulic fracturing.
Ning, YanruiKazemi, HosseinTura, AliDavis, T. L....
11页
查看更多>>摘要：A large amount of unproduced oil remains in unconventional shale reservoirs. In this paper we present the use of water and oil tracers as a key factor in flow channel characterization and in constructing a viable numerical model to forecast oil and gas recovery by depletion drive and enhancing oil recovery (EOR) by hydrocarbon gas or CO2 injection in Niobrara and Codell formations in the Denver-Julesburg (DJ) Basin. First, a dual-porosity compositional model was built based on seismic interpretation results, well logs, and core analysis. Two hydraulic fracture scenarios, (1) uniform dimensions and (2) variable dimensions, were included in the static rock frame of the numerical model. Production performance matching demonstrated that the use of variable length and height for hydraulic fractures led to a more realistic representation of the reservoir performance. On another front, injection of water and oil tracers and flowback analysis provided the means to better quantify the fracture network distribution, flow communication between wells, and hydraulic fracture performance in each well. Finally, two 'huff-and-puff' (injection, shut-in, production) cycles of lean gas and CO2 injection into the reservoir were modeled to assess EOR potential of cyclic gas injection. With identical gas injection rates, lean gas produced more oil than CO2; however, CO2-EOR modeling results indicated that a substantial amount of CO2 was stored in the reservoir. The net carbon stored after CO2-EOR was approximately 13% of the injected CO2 and CO2 uti-lization was 39,000 scf per incremental oil barrel produced which is much larger than the average CO2 utilization of about 12,000 scf per incremental oil barrel produced in conventional reservoirs. However, the produced CO2 from unconventional reservoir EOR operation can be recycled to achieve complete storage of CO2 - rendering CO2-EOR an effective means of decarbonization. Finally, transmissibility analysis of an existing major fault zone in the study area indicated that CO2 did not leak via a major fault in the study area nor via the associated nearby natural fractures in the study area.
查看更多>>摘要：Fracture stress is the key parameter relating the laboratory evaluations of bulk gels to their plugging efficiency. By the in-situ gelling method, the complete deformation curves of polyacrylamide (PAM) gels are successfully obtained. We observe the strain hardening behavior from all the interested gels, which exhibits an exponential relation between the apparent modulus and the strain. Based on the exponential equation, the fracture stress is factorized into three rheological parameters, i.e., the plateau modulus, degree of stiffening, and fracture strain. In this article, we focus on the concentration dependence on these mechanical properties of PAM gels. It is observed that the plateau modulus of unentangled gels is dominated by the crosslinking density, and the entangled gels exhibit a scaling law with exponent of 2.2-2.3 between the plateau modulus and polymer concentration. In general, the degree of stiffening would be advanced by increasing the crosslinker concentration or decreasing the polymer concentration, while the fracture strain is reduced with both of them. In addition, these variations would be significantly exacerbated if the polymer concentration exceeds the entanglement concentration. As a result, the contribution of gellant concentration to the fracture stress of gels is weakened due to entanglement. Around the entanglement concentration, the fracture stress-polymer concentration curves of the organic gels are continuous but the increasing rates are decreased, while the inorganic gels experience a sudden reduction of fracture stress. Finally, we briefly discuss the fingering-rupture process of the bulk gels, and find the rupture pressure gradient is in linear relation with the fracture stress.
查看更多>>摘要：To characterize matrix pores and microfractures which are widely developed in unconventional reservoirs and also to analyze flow law in multiscale flow region, some related research has been carried out in this study. Based on the statistical characteristics of microscale and nanoscale scanning images of cores, digital cores of micro fractures are reconstructed by an improved Voronoi method and the digital cores of matrix pores are reconstructed by the quartet structure generation set (QSGS). By means of regional superposition algorithm, the digital cores of microfractures are firstly magnified in each divided region and then integrated with the corresponding digital cores of matrix pores. After the superposition is completed in whole regions, numerical model after superposition is reduced to the original size of the digital core of microfractures. Finally, the multiscale digital core is reconstructed. Based on reconstructed multiscale digital core, flow simulation considering the permeability of matrix is carried out by the gray lattice Boltzmann method (GLBM). The results show that by constraining the generation areas of random points, the improved Voronoi method can better reflect the geometric characteristics of specific microfractures and the average relative errors of the two-point function and variation function are 13.36% and 2.52% respectively compared with the original microfractures. The multiscale digital core reconstructed by the regional superposition algorithm integrates the anisotropy of matrix pores while retaining the original characteristics of microfractures. With the increase of permeability of matrix, pressure drop range in the flow simulation region expands, and the average velocity at the outlet of microfractures increases. Compared with the case not considering the permeability of the matrix, the maximum increase of average velocity at the outlet of microfractures can reach 48.37% when the permeability of the matrix is set as 0.75 mD. When the flow direction in the microfractures is transverse, compared with the multiscale digital core with transverse connectivity in the matrix, the multiscale digital core with longitudinal connectivity in the matrix has higher flow velocity at the outlet of microfractures, and the average increase of velocity is up to 1.73%.
查看更多>>摘要：Lost circulation has been a serious problem to be solved in many drilling practices during oil, gas and geothermal well drillings. Many materials have been developed and evaluated for the purpose. However, their performance to plug severe leakoff is very limited. Herein, an injectable self-healing hydrogel based on polyampholyte with sulfonated and quaternary ammonium functionalities (P(MPTC-co-NaSS)) was developed and comprehensively evaluated to prevent the severe loss of fluids to formation. By incorporating cation-7t (7t is for aromatic residues) interaction, the hydrogel shown self-healing property and robustness in severe environment (temperature, salt) by comparison with other hydrogels merely consisting of cation-anion and H-bonding interactions. Aromatic residues enhanced thermal stability above 310 degrees C. The plugging measurement shown that an addition of 2 wt% dried gel particles can plug high-permeability formation and endure a high pressure of 6 MPa, produce much lower circulation loss and result in a dramatically increased loss volume reduction rate (63.5%) compared with a commercial polymer gel product and an inert material (9.4%) after a self-healing process. Markedly, P(MPTC-co- NaSS) can be used in a wide range of formation temperature (as high as 150 degrees C) and salt concentrations (NaCl, CaCl2, as high as 15 wt %). In addition to suitable particle size and mechanically robustness, it was also attributed to the soft, swelling, deformable, toughness and self-healable features of P(MPTC-co-NaSS) gel par-ticles as well as the strong adhesion to negatively charged formations in water, even under high thermal and saline condition. These characteristics also contributed to a long-term plugging performance, beneficial to avoid repeated lost circulation in drilling operation. Besides, this self-healing polyampholyte gel particles dispersed well in saline fluid and maintained stable rheological properties after hot rolling, which was favorable to drilling fluid circulation. This study shown the application potential of self-healing materials as plugging material candidate in petroleum drilling industry.
NSTL
Elsevier