查看更多>>摘要：Abundant oil potential has been confirmed in the Eocene Shahejie Formation (Es) in the Chezhen Sag, but the genetic mechanism of its reservoir has not been fully understood. Based on core observation, thin-section identification, scanning electron microscopy (SEM), X-ray diffraction, and formation pressure measurement, the factors controlling the quality of glutenite reservoirs in the Shahejie Formation in the Chezhen Sag were revealed. The petrophysical structure, depositional facies, diagenesis, and formation overpressure jointly control the development and evolution of glutenite reservoirs. Reservoirs with high composition maturity exhibit better physical properties. Depositional facies control the development of high- and poor-quality reservoirs but have little influence on the physical properties of medium-quality reservoirs. Compaction is the main factor for reducing reservoir porosity during burial, and the cementation of carbonate and clay minerals affects the reservoir quality to some extent. Each secondary structural zone shows different dissolution models. The reservoirs in the steep slope zone are in a closed system with weak dissolution, hardly contributing to improving the reservoir porosity. The dissolution of reservoirs in the depression zone has been enhanced; however, the migration of dissolution products is restricted because of the closed reservoirs, causing the redistribution of pore space and limited contribution to improving the physical properties. The reservoir in the gentle slope zone is in an open system with a shallow buried depth, and severe dissolution can effectively improve the reservoir's physical properties. A grading assessment scheme is proposed based on the depositional facies, diagenesis, and formation overpressure, and favorable exploration zones are predicted. The western part of the gentle slope zone in the upper part of the Es4 member and the eastern part of the steep slope zone in the lower part of the Es3 member of the Chezhen Sag are the principal target areas for exploration.
查看更多>>摘要：As the need for reaching fuel reserves at greater depths increases, scientists have been exploring and developing the technology required to efficiently drill rock at highly pressured environments over the past 30 years. However, there are still gaps in the understanding of the physical phenomena involved. One of the basic problems has to deal with is the rock-bit interaction during the rock breaking process. In order to evaluate the drilling efficiency and bit wear in ultra-deep formation, parameters of ultra-deep drilling simulator were calculated based on the similarity principle, followed by the design and manufacture of ultra-deep rock-bit interaction simulator. Considering the influence of formation differential stress change on bit footage, the effects of weight on bit (WOB) and rotary speed on torque, bit wear and rate of penetration (ROP) were studied firstly under the conditions of in-situ stress of 240 MPa and temperature of 200 degrees C. The results of similar simulation test show that under the conditions of in-situ stress of 240 MPa and temperature of 200 degrees C, WOB at 8.0-8.5 kN and rotary speed at 180-240 rad are the optimal drilling parameters. Correspondingly, the on-site WOB at 128-136 kN and rotary speed at 45-60 rad are the optimal mechanical parameters on the drilling site, which can lead to high cutting efficiency and low wear rate of the bit. The horizontal differential stress has a significant impact on the ROP. With the increase of the stress difference in the horizontal direction, the footage efficiency decreases obviously, and the decrease degree is about 87.5%. The test results will help optimize rock breaking tools and drilling parameters to improve rock breaking efficiency.
查看更多>>摘要：Shale samples containing single pre-machined slots were tested under uniaxial compression directed along with bedding layers. Observed fractures were initiated by the slots, however, their further development was controlled by the shale bedding layers. Slots normal to the loading direction also cause fracturing and reduce failure load. The fracturing is initiated by tensile stress generated at the slot surface due to convergence of the initial open slot. Therefore, the presence of initial aperture in pre-existing cracks or fractures is capable of changing the mechanism of fracture generation. Three kinds of macroscopic fractures are observed: tensile fractures along bedding layers, shear fractures along the inclined direction, and buckling fractures at the final failure moment. Three types of microcracks are identified: shear microcracks produced from initial weak micro-planes unrelated to the bedding layers, tensile microcracks produced from the shear microcracks in the form of wing cracks, and shear microcracks induced by high uniaxial compression stress. The growth of wing cracks can be assisted by the bedding layers. The research results will contribute to the understanding of failure mechanisms of rocks containing prominent bedding layers.
查看更多>>摘要：Accurate determination of coalbed methane (CBM) content can not only prevent and control coal and gas outburst disasters in mines, but also facilitate the prediction of coal seam methane resource reserves, which is significant for injecting carbon dioxide to enhance CBM recovery. The freezing core sampling technique proposed in recent years for accurate determination of the CBM content has faced temperature-pressure coupling effects on gas desorption in coalin a low-variable temperature environment. In this paper, we utilized a self-developed high-low temperature alternating adsorption-desorption experimental device to study the gas desorption process in coal under temperature-pressure coupling conditions in a low-variable temperature environment. The experimental results show that the main promotion and inhibition effects of temperature on the gas adsorption and desorption in coal in a low-temperature environment were consistent with those in an ambient temperature environment, i.e., the decrease of temperature promoted coal gas adsorption, while the increase of the temperature promoted coal gas desorption. It is found that throughout the temperature change process (TCP), there existed a 'pseudo-desorption' phenomenon, and the cumulative desorption amount showed a consecutive risingdeclining-rising trend and finally stabilized. The whole process can be regarded as consisting of a main desorption process and a retrograde desorption process. In the main desorption process with the same TCP, the cumulative desorption amount and initial desorption rate increased with the increase of initial equilibrium pressure. In the retrograde desorption process, there existed a peak retrograde desorption rate, and the staged retrograde desorption amount conformed to the linear decrease law. A comprehensive analysis of the temperature-pressure coupling effect on gas desorption throughout the TCP was conducted. Thus, we proposed some considerations and precautions for the configuration and supply monitoring of the cold source in the process of the measurement of CBM content using the freeze-coring method.
查看更多>>摘要：The reservoir quality, pore spaces and pore size distributions of Bashijiqike sandstones in Kuqa depression were described by thin sections, SEM (scanning electron microscopy) analysis and nuclear magnetic resonance (NMR) tests. In order to quantitatively characterize the complexity of pore structure, NMR T-2 (transverse relaxation time) spectrum was used for fractal analysis. This study unravels the relationships between petrophysical parameters, NMR parameters and fractal dimension of the ultra-deep tight sandstones. Primary intergranular pores are the main component of pore spaces, followed by secondary dissolution pores and micropores, and there contain microfractures. The NMR T-2 spectra are either bi-modal or uni-modal distribution. The uni-modal T-2 spectrum reflects uniform pore spaces. The coexistence of intragranular pores and intergranular pores leads to the bi-modal T-2 spectrum. There is a positive correlation between fractal dimensions and T-2gm (geometric mean of the T-2 distribution). Consequently, four pore structure types are determined according to irreducible water content, T-2gm, RQI (reservoir quality index), and the characteristics of individual pore structure are summarized. Because of the uniform pore space, Type I and Type IV have the lowest fractal dimensions. The structure of Type II and Type III are most heterogeneous due to their combination of intergranular and intragranular pores. The results help clarify the internal relationships between petrophysical parameters and microstructure, and have implications for pore structure evaluation of ultra-deep sandstones worldwide.
查看更多>>摘要：Water injection into the subsurface, inherent in improved hydrocarbon recovery and extraction of geothermal energy, often suffers from injectivity decline, even when water carries only nano-sized particles at low concentrations. This study investigates the propagation of such nano-sized particles experimentally and by modelling. Water with dispersed silica nanoparticles of about 140 nm diameter was used as a proxy to ultra-filtered water. Dispersion of the nanoparticles in brine is investigated by varying their concentration, the brine composition, salinity, pH and the presence of iron ions. The measured apparent hydrodynamic size and zeta potential indicate that nanoparticles remain dispersed with the expected size only for salinity below 3000 ppm with pH ranges 6.5 to 8.5. For higher salinity or pH outside that range or presence of iron ions, agglomeration becomes strong. Core flood experiments are conducted on high permeability Bentheimer sandstone, and the transport and retention of nanoparticles in the cores was analysed using multiple pressures measured along the core and by influent/effluent analysis. Core flood results show that stable injectivity can be reached with a good propagation of the nanoparticles through the permeable core with no external filter cake formation, provided the pH and salinity of the injected fluid are kept within the dispersion range and free of iron ions. However, injectivity decline still occurs in three characteristic stages well captured by our mechanistic model used to match the data. This study will contribute to better understanding of the transport dynamics of nanoparticles in the subsurface and to better modelling prediction and assessment of technologies where transport of nanoparticles is key.
查看更多>>摘要：Directional electromagnetic (EM) logging while drilling (LWD) tools have been widely used in well navigation for its azimuthal sensitivity and deep depth of detection (DOD). In this paper, an efficient 2.5D finite difference method (FDM) associated with non-uniform meshes and parallel computing is applied to evaluate the formation structure effects on the EM tools. The algorithm is validated by comparing the numerical results with analytical/ 3D FEM solutions in 1D and 2D heterogeneous formations. The effects of complex formation structures, e.g., faults, curved boundaries and unconformities are further analyzed by using the FDM code. Two new curves GPF and GAF are defined by subtracting the geosignals in a 1D layered model from that of a 2D fault model to quantitatively evaluate the fault detection capability. Numerical results show that the EM tools are sensitive to faults, especially the low dip angle ones. The depth of detection to fault reduces from 10.6 m to 4.0 m as the dip angle increases from 15 degrees to 45 degrees. Numerical cases also indicate that the EM tool responses can be affected by the sharp slope curved boundaries, while the effects of the gentle slope (<0.2) boundaries can be neglected. Furthermore, examples in unconformity and oil/water contact formation demonstrate that a 2D structure can affect the signal amplitude significantly.
查看更多>>摘要：Two new, non-intrusive reduced order frameworks for the faster modelling of gas reservoirs with time-varying production are presented and compared. The first method is an extension of a method using proper orthogonal decomposition (POD) in conjunction with radial basis functions (RBFs) that has previously been applied to predicting the performance of oil reservoirs undergoing a constant rate waterflood. The second method uses an autoencoder rather than RBFs to estimate the flow dynamics (pressure distributions) in hyperspace for unseen cases. Both frameworks are 'trained' using sample outputs from off-line, commercial reservoir simulations of a realistic heterogeneous gas reservoir with time-varying production controls typical of gas field operation. These controls include time-varying rate and switching between bottom hole pressure and rate control as well as cases where wells get shut-in. Both POD-based models produce reasonable forecasts of the reservoir performance for new unseen/prediction cases and are between 0.22 and 300 times faster than conventional simulation, including the time spent performing training simulations with conventional simulation solutions. The POD-RBF models are more accurate and consistent with reference commercial simulation outputs than the POD-AE models. In addition, the POD-AE models required more trial and error to set up as the number of hidden layers needed, depends on the particular scenario being modelled. There is no ab initio way of predicting the best number of layers for a given type of scenario. This makes them less suitable for practical application by reservoir engineers. Overall the POD-RBF framework is the most robust and accurate of the two methods.
查看更多>>摘要：The shale of the Middle Permian Lucaogou Formation in the Jimusaer Sag has become one of the most important prospects for unconventional shale oil production in China. The Lucaogou Formation develops two sweet spots, the upper (P(2)l(2)(2)) and lower (P(2)l(1)(2)) sweet spots, which are not more than 150 m apart. Interestingly, oils produced from the lower sweet spots have higher densities and viscosities than those in the upper sweet spots. Meanwhile, these high-viscosity oils have brought technical challenges to shale oil production. In this paper, geochemical data from 78 source rock solvent extracts and 17 oil samples from various depths were employed to elucidate unconventional petroleum systems and the reason for high viscosities. The source rocks of the Lucaogou Formation are good to excellent, primarily type II and II-III kerogens, and are now in the early-peak oil window. Oils produced from the P(2)l(1)(2) interval are characterized by abundant polar compounds, which are derived from the P(2)l(1)(2) and P(2)l(1)(1) intervals source rocks with relatively high marine sources contribution. Oils in the P(2)l(2)(2) interval are generated from the P(2)l(2)(2) interval source rocks with relatively high terrigenous-marine sources contribution. Terpane biomarker ratios indicate all oil samples are derived from lacustrine source rocks. The distributions of terpanes and steranes in these samples are similar, with high quantities of C-29 steranes and low amounts of diasteranes. Migration from sources to reservoirs in the Lucaogou shale oil systems is short-distance given thermal maturity considerations. All oil samples have experienced slight biodegradation (PM level 1-2) at most. The sources and thermal maturities are the primary reasons for the high viscosities of the crude oils produced from the P(2)l(1)(2) interval. This research also indicates the Lucaogou shale petroleum system could be divided into two subsystems, and the production of different unconventional petroleum subsystems requires different strategies.
查看更多>>摘要：Significant fracture conductivity can be achieved using a much lower material cost based on the optimal partial-monolayer proppant concentration (OPPC) theory. However, experimental validation and investigation of the OPPC theory have been extremely rare in the literature. In this study, we used a laboratory fracture conductivity cell to conduct well-controlled fracture conductivity experiments to comprehensively study the role of effective stress, proppant size, rock type, and water soaking on the evolution of fracture conductivity as a function of increasing proppant concentration. With seven proppant concentrations (up to 2 lb/ft(2)) and seven effective stresses (up to 6000 psi) used in the conductivity measurements, we experimentally confirmed that the correlation between fracture conductivity and proppant concentration was non-monotonic because of a competing process between fracture permeability and fracture width. We also investigated the influence of the abovementioned experimental conditions on the OPPC and the corresponding optimal fracture conductivity (OFC). This is the first study that uses well-controlled laboratory experiments to comprehensively investigate nonmonotonic fracture conductivity evolutions. The existence of the OPPC indicates that a relatively low proppant amount can be used to form a partial-monolayer proppant pack in the fracture space, which has similar or higher fracture conductivity compared to a multilayer proppant structure. This finding has important economic implications because high-strength, ultralight-weight proppant particles can be used to form partial-monolayer proppant packs in fractures, leading to sufficiently high fracture conductivity using a much lower material cost compared to multilayer proppant structures. Our experiments illustrated that proppant embedment is the primary mechanism that causes the competing process between fracture width and fracture permeability and consequently the non-monotonic fracture conductivity evolution as a function of increasing proppant concentration. Without proppant embedment, there will not be such a competing process, and the non-monotonic fracture conductivity evolution will not be observed.
NSTL
Elsevier