An enhanced accuracy method to determine oil saturation by carbon/oxygen logging in tight reservoirs

The low porosity and permeability characteristics of tight oil reservoirs have brought challenges to monitoring oil saturation recently. Although carbon/oxygen logging is effective for oil saturation evaluation, the statistical fluctuations of the measured energy spectrum in tight reservoirs make it impossible to distinguish the different signals between oil and water. Thus, Noise Adjusted Singular Value Decomposition (NASVD) is applied to denoise the raw energy spectrum and evaluate the oil saturation quantitatively. The energy spectrum matrix, which is composed of the energy spectrum of the measurement point and its adjacent depth points, is decomposed and reconstructed to remove non-informative signals and improve the signal-to-noise ratio (SNR) of the raw energy spectrum. The parameter K evaluates the smoothness of the logging curves, reflecting the influence of the number of energy spectra and singular values on NASVD. Meanwhile, the NASVD, Savitzky-Golay (S-G) filtering and depth averaging methods are compared for calculating the accuracy of C/O, Si/Ca and oil saturation with the Monte Carlo method, indicating that NASVD is better than the other two methods for eliminating the statistical fluctuations of the raw energy spectrum. A simulation example indicates that NASVD can control the calculation errors of tight reservoir oil saturation to within 15%, which significantly improves the accuracy of the estimated oil saturation. An oil field example shows that the oil saturation interpretation result for tight reservoirs is in good agreement with the oil saturation from open hole log analysis, signifying that the NASVD energy spectrum denoising method can provide a quantitative estimate of oil saturation in tight oil reservoirs.

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A Hybrid Model for Selecting Horizontal Candidate Wells for Re-Fracturing of Tight Oil Reservoirs—A Case Study in the Baikouquan Formation, North Mahu Oil Field, Western China

10.2118/196975-ms ◽

2019 ◽

Author(s):

Zhaozhong Yang ◽

Rui He ◽

Jianlin Chen ◽

Xiaogang Li ◽

Bin Xie ◽

...

Keyword(s):

Hybrid Model ◽

Oil Field ◽

Oil Reservoirs ◽

Western China ◽

Tight Oil ◽

Tight Oil Reservoirs

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A Semianalytical Approach for Production of Oil from Bottom Water Drive Tight Oil Reservoirs with Complex Hydraulic Fractures

Journal of Chemistry ◽

10.1155/2019/5186831 ◽

2019 ◽

Vol 2019 ◽

pp. 1-8 ◽

Cited By ~ 1

Author(s):

Shuhui Dai ◽

Chunqiang Chen ◽

Hao Wang ◽

Zhaohui Fu ◽

Dameng Liu

Keyword(s):

Bottom Water ◽

Oil Production ◽

Oil Reservoirs ◽

Hydraulic Fractures ◽

Tight Oil ◽

Matrix Permeability ◽

Cumulative Production ◽

Tight Reservoirs ◽

Tight Oil Reservoirs ◽

Half Length

In order to improve the recovery rate, fractured horizontal wells are widely used in tight reservoirs. In general, a complex fracture system is generated in tight reservoirs to improve oil production. Based on the fractal theory, a semianalytical model is presented to simulate a complex fracture system by using the volumetric source method and superposition principle, and the solutions are determined iteratively. The reliability of this model is validated through numerical simulation (Eclipse 2011); it shows that the result from this method is identical with that from the numerical simulation. The study on influence factors of this model was focused on matrix permeability, fractal dimension, and half-length of main fracture. The results show that (1) the production increases with the increase of half-length of main fracture and matrix permeability during the initial production stage, but the production difference becomes smaller in different half-length of main fractures and matrix permeability in middle and later stages and (2) the cumulative production increases with the increase of fractal dimension, and the increments of cumulative production in different fractal dimensions gradually increase during the initial production stage, but the increments tend to be stable in middle and later stages. This study can be used for forecasting the oil production from bottom water tight oil reservoirs with complex hydraulic fractures. It can also guide in optimization of horizontal well fracturing design to improve oil production and oil recovery in bottom water tight oil reservoirs.

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A Hybrid Model for Selecting Horizontal Candidate Wells for Re-Fracturing of Tight Oil Reservoirs—A Case Study in the Baikouquan Formation, North Mahu Oil Field, Western China (Russian)

10.2118/196975-ru ◽

2019 ◽

Author(s):

Zhaozhong Yang ◽

Rui He ◽

Jianlin Chen ◽

Xiaogang Li ◽

Bin Xie ◽

...

Keyword(s):

Hybrid Model ◽

Oil Field ◽

Oil Reservoirs ◽

Western China ◽

Tight Oil ◽

Tight Oil Reservoirs

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Nanopore Confinement Effect on the Phase Behavior of CO2/Hydrocarbons in Tight Oil Reservoirs considering Capillary Pressure, Fluid-Wall Interaction, and Molecule Adsorption

Geofluids ◽

10.1155/2021/2435930 ◽

2021 ◽

Vol 2021 ◽

pp. 1-18

Author(s):

Zhixue Zheng ◽

Yuan Di ◽

Yu-Shu Wu

Keyword(s):

Phase Behavior ◽

Pore Size ◽

Capillary Pressure ◽

Confinement Effect ◽

Oil Reservoirs ◽

Co2 Injection ◽

Tight Oil ◽

Tight Reservoirs ◽

Tight Oil Reservoirs ◽

Nanopore Confinement

The pore sizes in tight reservoirs are nanopores, where the phase behavior deviates significantly from that of bulk fluids in conventional reservoirs. The phase behavior for fluids in tight reservoirs is essential for a better understanding of the mechanics of fluid flow. A novel methodology is proposed to investigate the phase behavior of carbon dioxide (CO2)/hydrocarbons systems considering nanopore confinement. The phase equilibrium calculation is modified by coupling the Peng-Robinson equation of state (PR-EOS) with capillary pressure, fluid-wall interaction, and molecule adsorption. The proposed model has been validated with CMG-Winprop and experimental results with bulk and confined fluids. Subsequently, one case study for the Bakken tight oil reservoir was performed, and the results show that the reduction in the nanopore size causes noticeable difference in the phase envelope and the bubble point pressure is depressed due to nanopore confinement, which is conductive to enhance oil recovery with a higher possibility of achieving miscibility in miscible gas injection. As the pore size decreases, the interfacial tension (IFT) decreases whereas the capillary pressure increases obviously. Finally, the recovery mechanisms for CO2 injection are investigated in terms of minimum miscibility pressure (MMP), solution gas-oil ratio, oil volume expansion, viscosity reduction, extraction of lighter hydrocarbons, and molecular diffusion. Results indicate that nanopore confinement effect contributes to decrease MMP, which suppresses to 650 psi (65.9% smaller) as the pore size decreases to 2 nm, resulting in the suppression of the resistance of fluid transport. With the nanopore confinement effect, the CO2 solution gas-oil ratio and the oil formation volume factor of the oil increase with the decrease of pore size. In turn, the oil viscosity reduces as the pore size decreases. It indicates that considering the nanopore confinement effect, the amount of gas dissolved into crude oil increases, which will lead to the increase of the oil volume expansion and the decrease of the viscosity of crude oil. Besides, considering nanopore confinement effect seems to have a slightly reduced effect on extraction of lighter hydrocarbons. On the contrary, it causes an increase in the CO2 diffusion coefficient for liquid phase. Generally, the nanopore confinement appears to have a positive effect on the recovery mechanisms for CO2 injection in tight oil reservoirs. The developed novel model could provide a better understanding of confinement effect on the phase behavior of nanoscale porous media in tight reservoirs. The findings of this study can also help for better understanding of a flow mechanism of tight oil reservoirs especially in the case of CO2 injection for enhancing oil recovery.

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