查看更多>>摘要： Old strata and deep petroleum systems are becoming increasingly important. The Cambrian-Lower Ordovician petroleum reservoirs in the eastern Tazhong Uplift, central Tarim Basin, western China were studied comprehensively using various methods, including biomarkers, carbon isotopes of oil and gas, TOC and Rock-Eval pyrolysis of potential source rocks, as well as fluid inclusions, thin sections of dolomite rocks and Raman spectra of reservoir bitumen. The results show that the content of C_(28) steranes among the C_(27-29) steranes in most of the oil samples is greater than 0.25. The original hydrocarbon generation and expulsion potential of Cambrian and Lower Ordovician shale samples from the nearby Manjiaer Depression is considerable, and the oils are considered mainly derived from the Paleozoic shales in the Manjiaer Depression and the surrounding slops. Three notable types of oil and gas reservoirs were recognized in the Cambrian and Ordovician dolomite rocks: 1) the relatively well preserved primary reservoirs; 2) the residual reservoirs after deep oil-gas cracking with participation of thermochemical sulfate reduction (TSR); 3) the reformed reservoirs by the late charged high mature gas. In the vicinity of the No.l fault zone, the effect of late gas charging and invasion is remarkable. They are mosdy the mixture of gas cracked from both crude oil and heavy hydrocarbon gas in high mature stage. Oil and gas-liquid inclusions detected in the Cambrian reservoirs together with the Raman spectral characteristics of reservoir bitumen confirm the existed Cambrian paleo-oil reservoirs and oil cracking process. The gypsum-salt rock developed in Cambrian system is considered as a crucial factor for the primary oil and gas reservoirs, and the nearby areas within or below the strata where these gypsum-salt rock developed are suggested as favorable targets for deep to ultra-deep petroleum exploration.
查看更多>>摘要： Tight sandstone gas (TSG) reservoirs are developed in large buried depths and complex geo-stress field environments. During exploitation, the surrounding rock of a drill well is often damaged or can collapse under artificial engineering disturbances. Understanding the failure mechanism of tight sandstone under high and complex three-dimensional (3-D) stress states is essential for the safe and efficient exploitation of TSG. In this study, using the stress Lode angle (θ_σ) as a variable, failure experiments of low porosity sandstone specimens under various 3-D stress paths are performed, and the mechanical responses (e.g., stress-strain behavior, strength, fracture pattern, and acoustic emission characteristics) are analyzed. The results show that as θ_σ increases, the strength of the specimen as well as the deviatoric stress required its failure decrease linearly, whereas its brittleness increases. The failure of the specimens is primarily due to numerous micro tensile cracks and a few macro shear cracks. θ_σ significantly affects the cracking mode during failure. Acoustic emission (AE) parameters show that the failure process can be categorized into three stages within the time-to-failure window, among which Stage 2 (acceleration stage) can be regarded as the precursor stage of the ultimate failure of the specimen. The descriptive statistical results of AE energy show that uniaxial stress, hydrostatic stress, and true triaxial stress compression impose different effects on the damage mode of the specimens. Under the high 3-D stresses, the multiple fracture surfaces formed inside the specimen are intertwined to present several 'X'-shaped fracture pairs. These findings facilitate the understanding of the failure mechanism of rock mass surrounding the well-bore, and of significance in stability designing in well trajectory of TSG reservoir actual development.
查看更多>>摘要： Building empirical models to estimate formation properties from logging curves is a key task in logging formation evaluation, which can be implemented with numerous statistical or machine learning regression algorithms. The majority of existing works are limited in the paradigm of point-to-point mapping, which implicitly assumes the relationships between logging responses and formation properties can be represented by simple pointwise mapping models. However, since logging responses of a specified measuring point are comprehensive reflections of formations near it, the above assumption is not true in fact. In this study, through the introduction and combination of deep learning techniques such as end-to-end deep network, mask loss, total variation regularization, and multi-task learning, we develop an algorithm framework for more accurate core calibrated formation property prediction. Algorithms in the proposed framework take sequences of logging responses as input, and simultaneously predict sequences of multiple formation properties with the same length. This not only makes it possible to utilize the sequential dependence and morphological features of input logging response curves, but also highlights the morphological consistency and the inner correlation among predicted property sequences. The proposed framework is realized into three algorithms, namely, the multi-task fully convolutional neural network (MtFCNN), the multi-task long short term memory network (MtLSTM), and the multi-task gated recurrent unit network (MtGRUN). Advantages of these algorithms are fully confirmed by comparative experiments and real-world applications, while their abilities to utilize morphological features of logging curves, reduce the risk of overfitting, and emphasize the inner correlation among multiple formation properties are also substantiated. We believe the proposed algorithms have provided improvements for more accurate borehole formation property prediction, while the introduced techniques will also inspire future studies on machine learning assisted logging formation evaluation.
查看更多>>摘要： The influence of karstification on gas reservoir formation is an area of interest in petroleum geology, and karst paleogeomorphology restoration is a topic of reservoir research. However, most previous studies of karst pale-ogeomorphology have focused on karst reservoirs, ignoring the influence of the karst paleogeomorphology on the overlying strata and gas accumulations. Therefore, the value of the petroleum geology information associated with karst paleogeomorphology is not maximized. The carbonate strata of the Leikoupo Formation were uplifted and exposed by the fndosinian movement, leading to the development of a weathered karst stratum (Lei,,3 sub-member). Many commercial gas wells have been developed in the Lei43 sub-member. Based on the latest drilling, logging, and two- and three-dimensional seismic data for the northwestern Sichuan Basin, we selected the residual thickness and the impression methods to restore the karst paleogeomorphology of the Lei_4~3 sub-member. The karst paleogeomorphology was generally high in the southeast and low in the northwest. The area can be successively divided into three secondary karst geomorphological units, namely, karst highlands, karst slopes, and karst depressions, and several tertiary geomorphological units. The karst geomorphology had the function of controlling reservoir, hydrocarbon source, and gas accumulation distribution. The karst processes were different in the various geomorphological units, further controlling the distribution and development of the karst reservoirs. The inherited geomorphology also controlled the sedimentation of the clastic rocks in the overlying formation. Due to the paleogeomorphological setting of higher in the east and lower in the west, the hydrocarbon generation intensity of the early Xujiahe Formation increases from the karst highland to the karst depression. Due to the cutting and lateral sealing of the formation by faults and karst valleys, there are no unified gas-water boundaries in the Lei_4~3 sub-member. As a result of the overall tectonic setting of high in the east and low in the west, the natural gas is enriched in the karst monadnocks.
查看更多>>摘要： Sustained casing pressure (SCP) was an urgent problem to be solved during shale gas exploration and development. To cope with the issue of SCP, a method of establishing gas channeling barrier was proposed, and the location of the barrier was optimized, considering engineering and geological factors. A series of tests, containing uniaxial/triaxial compression tests and cyclic loading-unloading tests, were carried out to evaluate the accumulated plastic strain. Also, a full-scale cement sheath sealing integrity evaluation device was employed to verify the appearance of the micro-annulus. Computed tomography scan and nuclear magnetic resonance tests were performed to measure and analyze the distribution of microcracks and pores in the cement sheath after cyclic loading-unloadings. Numerical models of the wellbore assembly were established, which considered wellbore structures of different intervals of an actual deep shale gas well. Research findings indicated that after loading and unloading a certain number of times, the accumulation of plastic strain showed an increasing trend with the increase of the measured depth in the vertical section, but decreased with the rise of the measured depth in the horizontal section. The method of setting gas channeling barrier was proposed and could be used to avoid SCP by using the cement slurry with low elastic modulus, which could significantly reduce the cost of cementing operation. Many factors, including the non-uniform in-situ stress, wellbore structure, fracturing stage number, casing internal pressure, and formation mechanical properties were considered to establish the optimization method of barrier locations, and this method was verified by using the logging data of an actual well as well as the engineering and geological data.
查看更多>>摘要： A novel photolithography-based technique was developed to fabricate a quasi-2D heterogeneous calcite micro-model of representative elementary volume size. The effect of brine-chemistry on the mobilization of capillarity and heterogeneity trapped oil after high salinity water injection was evaluated by using diluted seawater, and seawater modified with calcium, sulphate, and silica nanoparticles. Preliminary brine screening was performed based on modified contact angle experiments under dynamic salinity alteration. The main findings are that the chemical composition of brine impacts both the ultimate oil recovery and its speed. The highest and fastest oil recovery was obtained with diluted seawater and seawater augmented with nanoparticles. We also found that the ex-situ contact angle results, indicative of wettability alteration, can be predictive of each brine performance at the pore network-scale. A slow recovery process, from 7 days up to 12 days, without any oil banking, was observed with all the brines. Due to the time-dependent nature of the wettability alteration process, mere injection of brines, even several pore-volumes and the viscous force exerted by flooding, were not sufficient to result in any additional oil production. Oil production was obtained only during the shut-in period via enhanced spontaneous imbibition of brine into the pores and throats. This highlights that a sufficient soaking time (at least for laboratory scale experiments) would be necessary to assess suitability of brines and determine accurately the incremental oil recovery by low salinity waterflooding (LSWF).
查看更多>>摘要： Economic and efficient energy production from enhanced geothermal systems in naturally fractured rocks depends on the stimulation of natural fractures by hydraulic fractures to generate complex connected fracture networks. This fracture network provides pathways for maintaining high fluid injection and production rates which lead to high heat extraction rates. However, little work has been conducted to describe the formation of these complex fracture networks formed by the interaction of multiple hydraulic fractures with natural fractures. This paper presents a dynamic (time dependent), fully coupled hydraulic fracturing model that is capable of capturing fracture propagation, fracture deflection, the interaction between propagating hydraulic fractures and pre-existing natural fractures and fluid flow inside the fractures. The proposed model is validated against an analytical model for a penny-shaped hydraulic fracture. This model is then applied to conduct a detailed parametric study to investigate the effect of pre-existing natural fracture networks on the connectivity and propagation of hydraulic fractures. The orientation, length distribution, width distribution and connectivity of the created fracture network is studied. Simulation results indicate that: (1) the hydraulic fracture network is oriented in the direction of the dominant natural fracture network when the length of the natural fractures is large, and this leads to a large connected fracture area; (2) If the hydraulic fracture propagates at a large angle to the natural fractures, it results in a smaller connected fracture area and a complicated hydraulic fracture network that is dominated by shear failure events; (3) Natural fracture density plays a dominant role in the propagation direction of hydraulic fractures and the resulting connected fracture area; (4) Increasing the injection rate increases the connected fracture area and width while creating more complex hydraulic fracture networks and leads to more fracture dilation and less shear opening of cracks. The detailed parametric study helps us better understand the creation of fracture networks and can serve to guide us in hydraulic fracture design and optimization in naturally fractured geothermal reservoirs.
查看更多>>摘要： Mudstone stringers in an oil sand reservoir can significantly affect the propagation of the steam chamber in the process of steam-assisted gravity drainage (SAGD). The micro-fracturing by water injection in strongly heterogeneous oil sands can result in these problems such as hard stringer breakthrough and aggravating heterogeneity. This paper proposed a comprehensive numerical model for conventional SAGD wells and vertical well-assisted SAGD (VWA-SAGD) wells to predict the coupled thermo-hydro-mechanical responses under hot water injection in a typical Karamay oil sand reservoir with two stringers, considering skeleton shear dilation, the 'phase change' of bitumen, formation heterogeneity, the permeability evolution induced by elastoplastic deformations, and the theory of heat and mass transfer. Major conclusions were drawn that there are three relatively narrow separate transition zones effectively heated around the SAGD wells and vertical well. The vertical well makes about 0.5 MPa increase to the pore pressure of the reservoir embedded with mudstone stringers in vertical directions and rises by about 0.6 MPa for the reservoir pore pressure above the upper stringer to reduce the pore pressure differential along the wellbore. Water injection-induced ultimate stress states can't reach the shear failure line under field operations, so the shear dilation can't be induced. The thermoporoelastic deformation determines whole reservoir deformation. The reservoir between the two stringers contributes to most of the caprock uplift. The VWA-SAGD technique can improve the porosity by about 1% in the whole vertical direction and reduce the anisotropy of porosity along the wellbore, which is beneficial to the uniform and fast propagation of the steam chamber in subsequent preheating and production stages. These findings can be employed to accurately predict the temperature, pore pressure, stress/displacement, and porosity evolutions for the field engineers to properly evaluate the uplift of reservoir and caprock, oil output changes, and heat utilization efficiency.
查看更多>>摘要： Proppants play an important role to increase oil and gas output in shale oil and gas development process. Tracers are used to study the degree of heterogeneity and dynamic equivalence between injection and production wells to judge the state of oil reservoir. However, it is difficult to integrate both slow-release tracing and shale supporting functions in one material. The development of functional coated proppants with tracer slow-release function is of great significance. Here a simple method is developed to introduce a carbon quantum dots (CQDs) into polyvinyl alcohol (PVA) coated proppants and fabricate novel self-suspension proppants with tracer slow-release function. The PVA coating layer on ceramic proppants achieves the self-suspension ability. In the well, the PVA film melted in fracturing fluid and the CQDs were released as a tracer for downhole monitoring. The surface adhesion force of PVA-CQDs coated proppants is 2 times larger than that of ordinary ceramic proppants, and the liquid conductivity of the self-suspension slow-release tracer proppants (SSTP) can reach 51 D cm, which is 22.45% higher than original ceramic proppants at the closure pressure of 40 MPa. This work combined the coated proppants with the quantum dot tracer and obtained a new type of coated proppants with tracer slow-release function, which has a promising significance to improve oil and gas production and conducting cross-well monitoring.
查看更多>>摘要： The two-phase viscosity modifier (T-PVM) is a new chemical agent for enhancing sweep and displacement efficiency in heavy oil reservoirs due to multiple-effects of reducing oil viscosity by emulsifying and increasing water viscosity. In the work, the dynamic interfacial tension (IFT) between simulated oil and chemical solution, viscosity, rheological characteristics, emulsification and stability of emulsion are measured to evaluate performance difference among the agents. Then, microfluidic visualization experiments were performed for subsequent chemical flooding after initial water flooding. Based on the advanced image processing technology, morphology and migration characteristics of remaining oil were described quantitatively and the mechanism of enhanced oil recovery was explored. Results show that large amount of residual oil are remaining in the non-main streamline area and dead-end pores after water flooding, which is observed in the morphology of spot, membrane, column, cluster and network. There would be migration path separation between polymer and surfactant during surfactant-polymer (SP) combination flooding. However, T-PVM, as a single chemical agent, penetrates in remaining oil and emulsifies columnar and membrane-shaped oil, which erodes the cluster and network-shaped oil simultaneously. Compared to the traditional viscosity reducer (VR) and SP flooding, multi-effective T-PVM enhances microcosmic sweep efficiency by approximately 35.93% and 21.83%, respectively. It is thereby expected that T-PVM flooding is effective for enhancing heavy oil recovery using the chemical cold flooding.
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