查看更多>>摘要：Understanding geomechanical properties is essential for an optimal completion strategy that ensures commercial productivity in shale reservoirs with ultra-low permeability. This study provides details to identify the variability of Instantaneous Shut-in Pressure (ISIP) in multidisciplinary approaches using petrophysical and geophysical data. ISIP is one of the input data required for well completion design. Operators commonly ignore the variability of ISIP and use uniform geomechanical assumptions due to cost and time constraints. In the case study, the variability of ISIP was observed up to 30 % in a single horizontal well, which could lower the efficiency of hydraulic fracturing with uniform geomechanical assumptions along a horizontal well trajectory. We propose practical approaches to identify the variability of ISIP from a case study in the Eagle Ford shale. MWD (Measurement While Drilling) Gamma Ray log, which is commonly acquired for a geosteering purpose, was utilized to prove its correlation with actual ISIP data gained at frac-stages in horizontal production wells. Correlation between ISIP and microseismic data was analyzed to identify the integrated relationship among the variability of Gamma Ray, ISIP, and hydraulically-induced fracture. In addition, practical approaches using a series of seismic inversion and attribute data were proposed to help identify geological variabilities affecting ISIP. This study was designed to provide operators hands-on approaches to capturing the variability of ISIP under limited data accessibility. We suggest multifunctional ways using the basic datasets commonly available at field sites. It can be applied to mitigate risks concerning well planning, drilling, and completion design in where operators face frequent challenges from geomechanical heterogeneity in global shale plays.
查看更多>>摘要：Unpropped fractures constitute a major part of the fracture network in shale after hydraulic fracturing, and their closure process under in situ stress has a significant effect on the well production. In this study, a 3D simulation method based on core experiments and reverse engineering technology was established to simulate the unpropped fracture surface. A contact and deformation model of rock with a rough boundary and normal stress was constructed to investigate the closure process. The plastic characteristics of shale were taken into account through the embedment of the Drucker-Prager yield criterion. An unpropped fracture closure experiment was conducted for validating the model, and the influence of stress state, mechanical properties, fracture roughness and slippage degree on the closure process was analyzed. Numerical results demonstrate that an unpropped fracture with greater roughness and slippage degree possesses more flow regions and a larger residual width under a low stress state, which decreases rapidly as the normal stress rises because of the small contact area. Young's modulus is considered to have a positive effect on resisting fracture closure and maintaining effective flow regions under a high stress state. For the shale of the deep Longmaxi formation in south Sichuan, China, the simulation results indicate that the unpropped fractures always retain a residual width greater than 0.7 mm under a closure stress of 60 MPa, which is suitable for flow passages of oil and gas in shale. Methods for moderating the closure process were also analyzed. The numerical simulations provide insights on the unpropped fracture closure process under in situ stress, which will be helpful for fracturing design and production prediction of shale wells.
查看更多>>摘要：Slickwater is commonly used during hydraulic fracturing in coalbed gas well, which is often prepared with polyacrylamide as the friction reducer. To investigate the impact of polyacrylamide on CBM ad-/desorption after a fracturing operation, slickwater with different concentrations of polyacrylamide were prepared and used to treat Illinois coal samples. Both isothermal ad-/desorption experiments and molecular simulations were conducted to investigate the impact of slickwater. Nuclear magnetic resonance measurements were performed to analyse the evolution of surface functional groups of coal during slickwater treatment. The results show that there is no significant chemical reaction between the coal component and polyacrylamide. Because of the porosity of the polyacrylamide coating adhered onto the surface of the pore, polyacrylamide has a significant sorption capacity of methane. The methane sorption amount of the 'coal-slickwater' system is higher than that of raw coal. However, with the increase of the amount of residue entering the pores, the porous structure of the polyacrylamide coating becomes denser, blocking some pores and the pore surface area is reduced;; thus, the overall sorption capacity of the 'coal-polyacrylamide' system decreases.
查看更多>>摘要：Low-permeability sandy conglomerate reservoirs are inevitably vulnerable to interaction with foreign fluids, which seriously affects oil recovery. Hydraulic fracturing, which is normally achieved by squeezing high-viscosity fracturing fluids into the reservoir, is the primary method to exploit low-permeability reservoirs. Although fracturing fluids can rupture rocks and produce fractures, they also cause formation damage. Therefore, it is critical and essential to explore the formation damage caused by fracturing fluids to protect the formation and enhance oil recovery. In this paper, two types of conventional fracturing fluids (i.e. slickwater and guanidine gum gelout) were used to study the mechanisms of pore-scale formation damage, which was assisted by the three-dimensional mineral distribution reconstruction based on in-situ micro-computed tomography. During the formation damage caused by slick water, gaps at pore edges were found to be corroded and closed. In contrast, during the guanidine gum gelout-caused formation damage, the formation of flocculent deposits led to the accumulation of particles in the pores. Porosity decreases are observed in all cores flooded with fracturing fluids and distilled water. Gray-scale values of minerals in the micro-computed tomography images were identified based on the mineral distribution map determined by scanning electron microscopy and energy-dispersive X-ray spectroscopy. These gray-scale values were compared with mineral contents obtained by X-ray diffraction. Eventually, a three-dimensional digital core was established based on mineral features, and clay minerals were demonstrated to be critical in the formation damage by blocking pores and thus separating large pores into small pores.
查看更多>>摘要：We describe a stacked model for predicting the cumulative fluid production for an oil well with a multistage-fracture completion based on a combination of Ridge Regression and CatBoost algorithms. The model is developed based on an extended digital field data base of reservoir, well and fracturing design parameters. The database now includes more than 5000 wells from 23 oilfields of Western Siberia (Russia), with 6687 fracturing operations in total. Starting with 387 parameters characterizing each well, including construction, reservoir properties, fracturing design features and production, we end up with 38 key parameters used as input features for each well in the model training process. The model demonstrates physically explainable dependencies plots of the target on the design parameters (number of stages, proppant mass, average and final proppant concentrations and fluid rate). We developed a set of methods including those based on the use of Euclidean distance and clustering techniques for offset wells selection (search for similar wells in terms of certain metrics), which is useful for a field engineer to analyze earlier fracturing treatments on similar wells. These approaches are also adapted for obtaining the margings for optimization parameters for a particular pilot well, as part of the field testing campaign of the methodology. An inverse problem (selecting an optimum set of fracturing design parameters to maximize production) is formulated as optimizing a high dimensional black box approximation function constrained by boundaries and solved with four different optimization methods;; surrogate-based optimization (pSeven), sequential least squares programming, particle swarm optimization and differential evolution. A recommendation system containing all of the above methods is designed to advise a production stimulation engineer on an optimized fracturing design.
查看更多>>摘要：Micro-indentation tests were performed adopting the grid-type of analysis on two types of shale rocks, which are porous natural materials characterized by strong geological structural and microfabric features. Emphasis was placed on understanding the influence and linkage between these microstructural-microfabric characteristics and the mechanical properties obtained from indentation, including hardness, modulus, elastic and plastic deformation fractions, and their anisotropic and heterogeneous distribution. The indentation loading-unloading curve patterns also functioned as a way to explore this linkage besides the global variation in the mechanical properties. Statistical tools and machine learning techniques were further employed in the indentation data analysis, providing a means of quantifying the intricacies involved with the mechanical properties of the shales. The creep behavior of these porous materials was quantified and linked to the anisotropic structure and mechanical properties, providing insights into one of the most complex natural materials, as many researchers have characterized. Opposite to previously published data on transversely isotropic shale rocks with vertical axis of symmetry, especially shales with high clay and organic content, the results in the present study showed higher modulus normal to the bedding planes compared to the parallel direction, which observation was attributed to the low clay (and organic) content of the shales and the possible low depth the shales were obtained.
查看更多>>摘要：Carbon dioxide miscible flooding has been proven to be one of the most effective enhanced oil recovery (EOR) technologies, particularly for light and medium oil reservoirs. However, specific effects of pore structure on CO2 miscible flooding recovery in low permeability reservoir lack in-depth understandings. In this paper, pore structures are specifically studied by means of the molecular mechanics to evaluate their effects on the CO2 EOR in the low permeability reservoir. First, a series lab experiments are performed for the pore and fluid characterization. More specifically, the pore throat size and distribution frequency are measured from the high-pressure mercury injection and nuclear magnetic resonance. The minimum miscibility pressure is determined from the slim-tube tests with known oil compositions tested from gas chromatography analysis. Second, the regularity of CO2 extraction is explored on the basis of molecular mechanics and the thickness of raffinate is calculated. Finally, the raffinate volume and recovery ratio in the pores are calculated after the CO2 miscible flooding. The results show that a raffinate-layer with thickness of 0.13 μm remains on the surface of the pore after the CO2 miscible flooding, which would cause the oil to be immobile since the throat could be blocked when the throat radius is smaller than 0.26 μm. The recoveries of cores C-l and C~(-2) are 70.1 % and 61.4 % from calculations and 68.4 % and 59.8 % from experiments, whose errors are 2.5 % and 2.7 %, respectively. This study would be beneficial to analyze the CO2 miscible flooding in reservoirs with different pore structures and provide technical support for improving CO2 utilization efficiency.
查看更多>>摘要：Nanoemulsions (NEs) are kinetically stable dispersions with droplet sizes on the order of 100 nm. Properties of NEs, such as the kinetic stability and rheology, have attracted substantial consideration in the oil and gas industry. This comprehensive review stretches from the theoretical mechanisms and fundamentals all the way to oilfield applications. Current low-energy and high-energy laboratory and industrial scale emulsification routes are included with detailed insight on synthesis challenges and opportunities. An included compendium of NE characterization techniques encompasses their applicability, advantages, and disadvantages. The oilfield applications segment covers recent uses of NEs in drilling, reservoir, and production operations. The review touches economical aspects for NEs oilfield scale implementations, and proposes solutions to mitigate foreseen challenges. This review emphasizes the scientific aspects of NE preparation and provides insight from the engineering and economic point of view of the oil and gas industry.
查看更多>>摘要：In the past, sand retention testing (SRT) research for steam-assisted gravity drainage (SAGD) injector was a parametric study, or general solution, rather than a case-specific solution for a particular problem in terms of testing parameters. Testing parameters were assigned based on hypothetical assumptions due to a lack of necessary field data to perform such a study. Field data are often confidential and nearly impossible to obtain for a hazardous scenario like a thermal injector flowback. The literature is rich with multiple proposed slotted liner (SL) design criteria for SAGD production wells. The SRT designed for SAGD producer takes into consideration some measurable factors to specify suitable aperture size. In contrast, SRT for the SAGD injector seems to rely on industry experience or rules of thumb. A systematic methodology, consisting of three steps, was developed for SL design using the SRT setup for SAGD injector wells. The first step involves estimating the laboratory testing variables or operational parameters based on case-specific data and reservoir simulations to assess the consequences of steam flowback. A 2D Computer Modelling Group (CMG) thermal simulator was coupled with gas flow correlation and used to predict the outcomes of SAGD injector flowback. The second step involves developing a new SRT setup dedicated to SAGD injector flowback laboratory testing. Intensive testing was performed to troubleshoot the problems associated with a high-velocity gas flow. The third and final step involves conducting six tests to verify the developed setup's performance. The consideration of thermodynamic equilibrium improved testing by avoiding inconsistent assumptions used in the previous studies. Neither low steam quality (SQ) nor high SQ flowback scenarios used in the past are found to fully represent SAGD injector flowback. SAGD injector depth and other operational parameters are linked to flowback differential pressure. In other words, one set of testing parameters cannot be representative for all SAGD injector, which makes developing design criteria a challenge. The preliminary results of SRT for SAGD injector indicate the possibility of using small aperture size slots of 0.010 inch that do not cause significant permeability reduction at laboratory scale, unlike the case for SAGD producer SRT. This may justify why the industry tends to prefer small aperture size slotted liner for SAGD injector completion. Eventually, this research should be considered as a first step only in SRT for SAGD injector flowback, and necessary methodology enhancements and facility upgrades should be investigated in future work.
查看更多>>摘要：For addressing the behavior of a reservoir with different fluid types, the Biot-Gassmann equation often is the base of the practical simulation. Despite the prevalence application of this equation with the isotropy conditions, the unforeseen errors always expose in simulation results because of the anisotropy state in reality. We investigated the anisotropy model with the integration of two analytical strategies using a three-component VSP data set. We obtained an initial anisotropy model in the region of acquired walkaway VSP using slowness polarization inversion, and updated the anisotropy model with the application of anisotropy ray-tracing and tomography. We applied a layer-stripping approach to the anisotropy model during raytracing to optimize the inversion. Given a computed geomechanical model and extracted rock properties of a carbonate reservoir, we developed the anisotropy Biot-Gassmann model, for finding the elastic moduli. We used the substitution strategy to generate the dynamic model of elastic moduli. We showed how the compressional modulus and rigidity change with the anisotropy model in different fluid content. We found that integration of slowness polarization and raytracing tomography increases the maneuverability to control the predicted anisotropy model and intensifies the convergence rate of the inverse problem. We observed that the isotropy assumption in modeling the elastic parameters makes around 8~(-1)0 % drift value in compressional modulus relevant to the reality, whereas rigidity showed reluctant behavior to fluid.
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