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After over four decades of exploration, a major breakthrough has been made in the Hetao Basin recently, that is, commercial oil flow of 62.6 m 3/d is tested from the Paleogene Linhe Formation in Well Song 5. A comprehensive study of petroleum geological features of the Linhe Depression reveals that the Langshan fault, Hangwu fault and Huanghe fault controlled the deposition and evolution of the depression and hydrocarbon generation center, and the basin experienced Early Cretaceous depression and Cenozoic faulting. There developed two sets of saline lake hydrocarbon source rocks, Cretaceous Guyang Formation and Paleogene Linhe Formation. The source rocks, dominantly type II1 and I, have high abundance of organic matter and large potential of hydrocarbon generation, and their maturity ranges from low mature to over mature owing to wide variation of burial depth. The Guyang Formation and Linhe Formation are the clastic reservoirs, which have good physical properties with burial depth less than 5 000 m. In the Jixi uplift, the weathering fractures in matrix also have storage capacity. Faulted block, faulted anticline and matrix fracture reservoirs are found through exploration. The low mature to mature oil, is generated from Guyang Formation and Linhe Formation. The study shows that the Linhe Depression has rich resources and huge exploration potential, where the main exploration targets are the Guyang and Linhe formations, the favorable exploration areas are the Hangwu fault belt, Jixi uplift belt and the deep sag in the north.
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The dominant factors controlling development of microfractures in the black shale and the origin of microfractures in the sweet spot intervals were discussed of the Ordovician Wufeng Formation-Silurian Longmaxi Formation in Shuanghe outcrop profile, Changning, Sichuan Basin. For the target interval, holographic photograph statistics of microscopic composition of 203 big thin sections and 203 small thin sections, TOC content of 110 samples, 110 whole rock X-ray composition, and main trace elements of 103 samples were tested and analyzed. The results show that the microfractures include bedding microfractures and non-bedding microfractures. The bedding microfractures are mostly plane slip microfractures, lamellation microfractures and echelon microfractures. The non-bedding microfractures are largely shear microfractures and tension microfractures. Vertically, the density of microfractures is the highest in SLM1 Member of Longmaxi Formation, decreases from SLM2 Member to SLM5 Member gradually, and drops to the lowest in Wufeng Formation. The microfracture density is positively correlated with siliceous content and negatively correlated with the carbonate content. The finer the grain size of the black shale, the higher the density of the microfractures is. The microfracture density is controlled by biogenic silicon: the higher the content of biogenic silicon, the higher the microfracture density is. Under the effect of ground stress, microfractures appear first in the lamellar interfaces. Regional tectonic movements are the key factor causing the formation of microfractures in the sweet spot interval, diagenetic contraction is the main driving force for lamellation fractures, and the pressurization due to hydrocarbon generation is the major reason for the large-scale development of microcracks.
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This study applied seismic-sedimentological workflow to deeply buried marine carbonate sequences in western China. The workflow aimed at integrating core, wire line log and 3D seismic data to investigate the paleogeomorphology qualitatively and reservoir thickness quantitatively in the carbonate systems. In Gaoshiti-Moxi area of Sichuan Basin, the Longwangmiao Formation was formed in sedimentary environment of carbonate platform facies. Lithofacies vary from dolomitized grainstone to mudstone, which formed shoal, lagoon, and deep shelf sub-facies. The high-porosity (porosity > 4%) reservoir rocks are related to shoal sub-facies. Seismic geomorphology was employed to restore paleostructure of Longwangmiao Formation in the study area by methods of cast and isopach, which were calibrated by well data. It was found that the paleogeomorphology and depositional facies were largely controlled by a syndepositional, en echelon fault system. Following the strategy of seismic lithology, quantitative distribution of reservoir rocks and sedimentary facies was assessed by applying principle component analysis on amplitude attributes generated from multiple frequency panels. Seismic prediction of diagenetic facies (selective dissolution) was achieved by interpreting similarity variance, a seismic geometric attribute. In summary, the procedure, combining seismic geomorphology and seismic lithology, is effective and can be used as a reference for seismic-sedimentological study in other carbonate basins in China and elsewhere.
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Based on fine geological modeling and taking the evolution of key reservoir-forming elements as the main line, the mechanism of orderly distribution and differential enrichment of hydrocarbon was revealed by studying the joint evolution and coupling effect of pressure, fluid and reservoir properties. Orderly development of sedimentary systems in a rift basin is the base of orderly distribution of reservoirs, and the continuity of pressure structure is the key to controlling orderly distribution of reservoir. From the sag center to the margin, in the sag, second-order sequence and large-scale sedimentary system, the reservoirs appear in an orderly distribution from lithologic reservoir to structural reservoir to stratigraphic reservoir. Alternative acid and alkaline actions controlled the development of high quality reservoirs in the mid-fan sandy conglomerate bodies in the steep slope, resulting in the oil and gas accumulation pattern of sealing at the root fan and enrichment at the mid-fan, in which the breakthrough pressure difference between root fan and mid-fan determines the reservoir enrichment level. The action of acidic fluid controlled the development of high quality reservoirs in beach bar sand and turbidite. The pressure difference between high-pressure source rock caused by pressurization of hydrocarbon generation and low-pressure reservoir caused by reservoir improvement provided driving force for oil and gas charging, giving rise to the pressing-absorbing oil and gas charging mechanism controlled by source rock overpressure. The research results have guidance and reference significance for fine exploration in mature exploration areas.
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Based on the three dimensional seismic data and drilling data in Gaoshiti-Moxi area, the structural characteristics and evolution process of strike-slip faults in the study area and their significance for gas accumulation were examined using the method of fault structure analysis. The results show that: (1) Transtensional strike-slip faults are developed in the Paleozoic in study area. On the profile, there are three types of structural styles: steep and erect, flower structures, "Y" and reversed "Y" type faults. On the plane, strike-slip faults developed in the Cambrian extending linearly and along the nearly EW, NW and NE direction. The faults are composed of multiple secondary faults in en-echelon arrangement; faults in the Permian are mainly along nearly EW and NW direction, and the Permian faults are distributed in the middle east part of the study area and they are more in the north than in the south. (2) The nearly EW and NW trending faults are dextral strike-slip faults, and the NE trending faults are sinistral strike-slip faults. The strike-slip strength of the nearly EW trending faults are the strongest and the maximum horizontal displacement is 550 m, while the strike-slip strength of the NE trending faults is the weakest. The Cambrian faults had stronger activities than the Permian faults. (3) The transtensional strike-slip faults were active in two periods, the early Caledonian period and the late Hercynian period, and were the products of oblique extension of pre-existing weak zones in Xingkai and Emei taphrogenesis, with certain inheritance in the main faults. (4) The strike-slip faults and surrounding fractures enhance the porosity and permeability of the reservoir in Cambrian Longwangmiao Formation and control the distribution of karst reservoirs in the Permian Qixia Formation-Maokou Formation, bringing about the situation of multiple gases bearing series in this area.
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Based on the corresponding relationship between the paleoproductivity, redox conditions and volcanism within a chronostratigraphic framework, the effects of volcanic events in the Wufeng-Longmaxi period on organic abundance of shale were examined. Bentonite layers were mostly developed in the transgressive systems tract 1 (TST1, Wufeng Formation) and transgressive systems tract 2 (TST2, Longmaxi Formation), and the two systems tracts corresponded to favorite shale lithofacies with high silica and total organic carbon (TOC) contents. According to the stratigraphic characteristics of bentonite rich interval, TST1 is classified as the interval with dense bentonite layers with the frequency of bentonite layer (bentonite layers/time) of more than 1.5 layers/Ma and the cumulative thickness ratio of bentonite layers (thickness of bentonite layers/thickness of shale) of more than 1%; TST2 is classified as the interval with sparse bentonite layers (frequency < 1.5 layers/Ma; cumulative thickness ratio < 1%). TST1 (dense interval) witnessed more intense and high-frequency volcanic activities than TST2 (sparse interval), so the TST1 has generally higher TOC than TST2. The intense and frequent volcanic activities had dual effects on organic-rich shale: on one hand, volcanic ash provided a sufficient supply of nutrients, which triggered high marine productivity; on the other hand, the extremely anoxic environment caused by volcanic activity enhanced the burial amount and preservation rate of organic matter.
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Based on cores, well logging and karstic paleogeomorphology, the identification method of multi-period paleokarstic water tables and their distribution and control on reservoirs were researched of Yingshan Formation in Tazhong Area, Tarim Basin. Three periods’ water tables numbered 1 to 3 have been identified from bottom to top of Yingshan Formation in Tazhong Area. The corresponding typical horizontal undercurrent zones are characterized by single karst fabrics such as sack-like dissolved vugs or subhorizontal channels, combination of various karst fabrics like pre-existing high-angle fractures intersected by small caves and subhorizontal channels, and combination of multi-layered caves and different karst fabrics of large pores. On the plane, periodical rising sea levels caused the development of No.1, No.1 and 2, No.1, 2 and 3 water tables in karst depressions, slopes and highlands respectively. As the result of the nearly horizontal occurrence of Yingshan Formation, submembers of Yingshan Formation had tiny angles with fracture-cave bodies of each period’s water table, so the karst reservoirs are characterized by “crossed strata regionally and developed along the stratum locally”. The distance between water table and unconformity, thickness of horizontal undercurrent zone and average thickness of reservoir all tended to increase with the rise of karst landforms. The karst highland and slope had 2-3 periods’ water tables and thicker horizontal undercurrent zones, so they had thicker and better quality reservoirs, making them favorable exploration areas.
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Taking organic-rich shale in the first member of Cretaceous Qingshankou Formation in the Gulong Sag, northern Songliao Basin as an example, this study examined the lithofacies classification, petrological characteristics, pore size distribution and their implications on the enrichment of shale oil of lacustrine detrital fine-grained shale. The spatial variation of lithofacies, controlled by Milankovitch cycle and influenced by sediment provenance, has an obvious sequence. The fine-grained sedimentary rocks of studied section could be classified into seven lithofacies according to a three-step classification criterion that consists of total organic carbon (TOC), sedimentary structure and mineral composition. Among them, the laminated siliceous mudstone lithofacies with moderate TOC has high hydrocarbon generation potential and abundant reservoir space, making it the most favorable lithofacies for the enrichment of matrix shale oil. Under the background of abnormally high pressure, the laminated siliceous mudstone lithofacies with moderate TOC deposited between the top of SSC2 and the bottom of SSC3 is stable in lateral distribution in the delta-lacustrine transition zone, with continuous thickness greater than 30 m. The massive siliceous mudstone lithofacies with high and moderate TOC developed in the middle of the these two cycles can act as the roof and floor seal for shale oil, therefore, the study area has good conditions for forming matrix type shale oil reservoirs.
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Through Laplace transform and Stehfest numerical inversion, this research established a mathematical model for constant temperature electric heating of dual-horizontal-well steam-assisted gravity drainage (SAGD) start-up. To verify the model, a finite element simulation based on a typical SAGD block was completed, which proved the excellent agreement between the model solution and simulation results. Moreover, by means of superposition principle, underground temperature distribution and energy-related parameters during the multiwell operation were obtained, as well as a computational method targeted at quantifying the conserved energy, water and fuel of electric heating against steam heating. A parametric sensitivity analysis of electric heating was undertaken, which proved that the start-up effect is most sensitive to heating temperature, featuring a nonlinear negative correlation. Additionally, start-up effect is positively linear-correlative to porosity, and negatively linear-correlative to wellbore radius and oil saturation. The proposed model can be employed to predict key indexes such as underground temperature distribution, energy-consumption parameters, and accumulated conserved amounts of energy, water and fuel versus steam heating. Under a specified terminal temperature, temporal and energy quantities required by the start-up can also be determined.
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Five full diameter cores were obtained from a conglomerate reservoir of Xinjiang oilfield. They were assembled into a full diameter long-core group, with a total length of 52.3 cm, 10 cm in diameter. With the full diameter long-core flooding system designed and produced in China, experiments were conducted to simulate depletion, water flooding, flue gas flooding, flue gas-water alternative flooding, CO2 flooding, and flue gas storage in the conglomerate reservoir, and the characteristics and effect of the flue gas flooding were evaluated. The study shows that full diameter long-core flooding experiment is more effective in simulating the gas displacement and percolation characteristics in the conglomerate reservoir. According to the experimental results, the flue gas produced from fire flooding can be used to enhance oil recovery of the water-drived conglomerate reservoir. The key to reaching a favorable development effect is to select the right timing, injection pressure and mode of flooding. In the middle of the water flooding stage, when the dominant percolation channel has not been formed, injecting flue gas and water alternatively after increasing reservoir pressure could avoid premature gas channeling and enhance oil recovery. During the flooding, the poisonous gas, H2S, was absorbed efficiently in cores and fluids, and flue gas was stored safely.
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A visualization experimental method of foam flooding was developed by combining nuclear magnetic resonance (NMR) and traditional core flooding method. On this basis, a new method to evaluate the dynamic stability of foam in the core during displacement process was established. Using this method, the displacement characteristics and dynamic stability of foam of S-2 (the main component is sodium lauryl sulfate) and S-NP-2 (the main components are sodium lauryl sulfate and silica nanoparticles) in two different displacement modes, i.e. direct foam flooding and foam flooding after water flooding, were studied. The results show that the NMR images and the T2 (transverse relaxation time) spectrum reflected the displacement characteristics. The flooding efficiency of S-2 and S-NP-2 after water flooding was increased by 18.05% and 25.68% and reached 63.72% and 67.50% respectively at last, higher than direct foam flooding. The same foam system had better stability in foam flooding after water flooding than in direct foam flooding, and foam S-NP-2 is more stable than foam S-2 under the same displacement mode.
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The mass transfer between heavy oil and liquid carbon dioxide and the changes of the heavy phase (mixture of heavy oil and CO2) and light phase (pure CO2) in the mixture were tested in lab with heavy oil samples from Russia. The experimental results showed that the heavy oil hardly expanded when the concentration of carbon dioxide in the mixture was 10%. When the concentration of carbon dioxide was higher than 26%, the volume of the heavy phase decreased, and the viscosity of the heavy phase increased exponentially as the light components extracted from the heavy oil exceeded the carbon dioxide saturated in the heavy oil. When the concentration of carbon dioxide in the mixture was 26%, the effect of viscosity reducing to the heavy phase was the strongest. The density of the light and heavy phases, volume factor, and solubility of gas and flash viscosity of heavy phase all increased with the rise of carbon dioxide concentration in the mixture. The best concentration of carbon dioxide in the mixture was 26%, when the heavy oil expanded the most and the viscosity of the heavy phase was the lowest. When the concentration of carbon dioxide in the mixture was between 10% and 26%, the volume of the light phase was the smallest and the oil displacement effect was the best.
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Sandstone core and free-clay sandstone core were prepared in chromatography columns, and core flooding experiments were performed to determine the EOR mechanisms of low salinity (LS) water flooding and analyze the effect of clay on recovery factor. First, brine with a high salinity (HS) of 100 000 mg/L was injected to simulate formation water. Then, LS water of 1 000 mg/L was then injected at different temperatures. Concentrations of Ca 2+ and CH3COO - and pH of the fluid flowing out of the chromatography column were recorded. The results showed that during flooding the free-clay core with LS water, the carboxylic acid detached from the sand, as great as that of the clay-containing cores. It showed a noteworthy desorption of Ca 2+ from the free-clay core surface, which indicated that LS water flooding resulted in a high recovery in sandstone without clay. Compared with HS water flooding, in the LS water flooding, the pH of produced fluid increased significantly; also, as the temperature increased, the pH and Ca 2+ concentration of the produced fluid decreased; and the hydrophilic property of the sandstone was stronger. The results of oil recovery tests using the same columns were in line with the chemical analyses. The oil recovery from the free-clay columns was as great as from the column clay-containing column. The results of wettability measurements supported our theory.
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Based on the theoretical study and field application of volume stimulation in horizontal wells over the past 10 years, the core connotation of volume stimulation was further interpreted. The implementation methods, design models and key issues were analyzed, and the future development direction was put forward. The research shows that the multi-cluster limited entry technique can achieve homogenous growth of multiple fractures. The hybrid stimulation of “breaking by rock gel stimulation + carrying proppant by slick water” plus small-particle proppant can reduce the fracture complexity near the well bore and increase stimulated reservoir volume (SRV) in the far-field. The requirement of fracture conductivity in unconventional formations can be met by shear-sustained fractures and proppant-transporting slick water. The optimum well spacing between a child well and a parent well should be determined by the stimulation modes, injection volume and pressure drawdown. Reconstructing seepage field, stress field and stimulation targets is crucial for improving the stimulation results in a horizontal well. Reducing cluster spacing and well spacing is the basis for establishing development modes of fracture-controlled reserves. Fracturing-design decision system based on “spatial-mode stimulation” and geology-engineering integration is an important research direction for volume stimulation techniques.
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To tackle the challenge of great depth, high pressure and high temperature (HPHT), low permeability and porosity, serious heterogeneity in deep oil/gas reservoirs, this paper proposed high efficiency reservoir stimulation based on temporary plugging & diverting, which can realize multi-stage stimulation to improve stimulated rock volume (SRV) without mechanical isolating tools. A series of degradable nanometer ester temporary plugging material with high temperature resistance and high strength has been developed. The feasibility of the method was tested by modeling experiment, and the performance of the plugging material was evaluated. The high efficiency stimulating technology based on chemical plugging & diverting can avoid the problems of high cost and high risk operations by mechanical isolating tools in HPHT deep wells. This technology has been applied in 211 wells from 10 deep oil/gas reservoirs in China, with average production increment per well 3.5 times higher than conventional technologies. This technology enhanced reservoirs production and stimulation effect, lowered the operation difficulty and risk, and decreased multi-stage stimulation cost for deep reservoirs.
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This paper conducted the shale hydration experiments by using four different types of shale outcrop samples. The microscopic pore structure variations before and after hydration were recorded, compared and analyzed through Field Emission Scanned Electronic Microscope (FESEM) with fixed-point observation technique. The results showed that higher content of montmorillonite and carbonate minerals would contribute to the form of dissolution pores and looseness of mineral grains; some critical factors also include original alignment and cementation of mineral grains, and distribution of natural microfractures. Hydration doesn’t change the organic pore structure. Almost all dissolution pores originated from mineral intergranular and intragranular pores in matrix, and the dissolution of matrix pores also lead to mineral particles to loose and fall off. When the mineral grains are aligned and compacted along with the bedding-parallel planes, the density of dissolution pores and the number of dissolution pores of small size in bedding-vertical specimens are usually larger than that in bedding-parallel specimens. For the shale samples with few natural microfractures, carbonate minerals may contribute to the generation and propagation of microfractures during hydration.
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To examine the distribution pattern and controlling factors of oil and gas accumulation in Baiyun Sag systematically, this study analyzes the key factors controlling hydrocarbon accumulation using a large amount of seismic, drilling and geologic data. It is concluded that the oil and gas accumulation is controlled by the following factors: (1) Massive source rock is the physical basis of the formation of medium and large oil and gas fields. (2) Orderly distribution and accumulation of hydrocarbon is regulated by the differential generation of hydrocarbon and differential hydrocarbon accumulation: spacial differential leads to “inner gas, outer oil” pattern, differential accumulation of hydrocarbon controlled the pattern of “inner gas, outer oil” in the east subsag of Baiyun; the timing differential leads to the “upper gas, lower oil” pattern. (3) Late fault activity, diapirs and inherited structural ridges controlled the favorable migration in the Neogene: the activity of faults and diapirs regulated the trap formation and vertical migration of hydrocarbon; inherited structural ridges controlled the Neogene hydrocarbon accumulation. The result effectively guides the exploration in the Baiyun Sag and surrounding area, and provides experience and evidence for the exploration in deepwater area of Pearl River Mouth Basin and other similar basins.
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Azimuthal electromagnetic logging while drilling (LWD), which is capable of providing accurate position information approaching bed boundary, has been widely applied in real-time geosteering. For the inversion speed and precision of azimuthal electromagnetic LWD data, the key lies in the selection of proper inversion model and corresponding optimization algorithm. In this study, we first simplified the complex three-dimensional (3D) inversion of data into a series of one-dimensional (1D) inversion problems by using the dimensionality reduction scheme. Then, the feasibility and inversion performance of various 1D inversion models and different optimization methods were investigated, and the best combination between the inversion model and inversion algorithm was also obtained. Numerical simulation results show that the selection of 1D inversion model is dominated by the thickness of targeted beds, whereas the determination of inversion algorithm relies on the total layers amount of the inversion model. For the formation with thickness larger than 4.0 m, the single boundary inversion model and gradient optimization method are recommended. When the bed thickness is between 1.0 m and 4.0 m, the two-boundary inversion model instead of the single-boundary one is needed to estimate upper and lower boundaries around the borehole. For the inversion of azimuthal electromagnetic LWD data of thin layers, the multiple- boundary inversion model and the Bayesian algorithm should be employed.
