Impacts of clay on pore structure, storage and percolation of tight sandstones from the Songliao Basin, China: Implications for genetic classification of tight sandstone reservoirs

This paper explores the applicability of nuclear magnetic resonance (NMR) technology on pore size distribution (PSD) and movable fluid distribution (MFD) in tight sandstones. Centrifugation experiments and NMR tests are performed on saturated samples. The fluid changes in pores corresponding with three different types of NMR T2 distribution after each centrifugation is then analyzed. In addition, a new method to determine the conversion factor from NMR T2 distribution to PSD is developed. In comparison with the PSD obtained by mercury intrusion porosimetry, the new method is more suitable for PSD calculation in tight sandstones. Afterward, the optimum centrifugal force to determine the threshold radius for fluid flow is obtained. On the basis of this, we analyze MFD in tight formation. Through study, the following results are arrived at: patterns of NMR T2 distributions of outcrop and subsurface cores at water saturation condition can be classified into three types (I, II, and III). Among which, type I an...

Application of NMR T2 to Pore Size Distribution and Movable Fluid Distribution in Tight Sandstones

A multiscale comprehensive study on pore structure of tight sandstone reservoir realized by nuclear magnetic resonance, high pressure mercury injection and constant-rate mercury injection penetration test

Pore structure characteristics of tight sandstones in the northern Songliao Basin, China

Pore structure is the most important factor affecting reservoir quality and petrophysical property of tight reservoir. The effective characterization of pore structures, including pore radius distribution (PRD), throat radius distribution (TRD), pore-throat radius distribution (PTRD), relevant pore structure parameters, etc., is of great importance for the oil exploration and exploitation. Taking the tight sandy conglomerate reservoir as research target of tight reservoir, this paper characterizes the pore structures by a combination of experiments on parallel core samples. These experiments include high-pressure mercury injection (HPMI), constant-rate mercury injection (CRMI), nuclear magnetic resonance (NMR), as well as microscopic analysis of casting thin sections and scanning electron microscopy (SEM). This paper systematically analyzes the advantages and shortcomings of these commonly used experimental techniques. And then, novel methods are proposed to characterize the pore structure (especially the...

Pore Structure Characterization of the Tight Reservoir: Systematic Integration of Mercury Injection and Nuclear Magnetic Resonance

Combining nuclear magnetic resonance and rate-controlled porosimetry to probe the pore-throat structure of tight sandstones

pore structure is the key to evaluating tight reservoirs. Since tight reservoirs mainly develop nano-pores which are difficult to fully characterize pore throats’ distribution by single conventional experimental method, there’s an urgent need to establish a characterization method by jointing several conventional experimental methods. The tight gas of Shahezi Formation in Xujiaweizi Fault Depression had made breakthroughs, the main pore types observed by the microscope were dissolution pores, intergranular pores and some micro fractures. The dissolution pores were combined by interparticle dissolution pores which dissolved cements and intragranular dissolved pores which dissolved the unstable components like feldspars and rock fragments. The intergranular pores were mainly the pores between crystals of illite/smectite and illite. The micro fractures improved the permeability of reservoirs greatly which were often developed in conglomerates. The remnant primary pores only local visible which were seldom developed. * There is a tremendous amount of research shows that NMR T2 spectra have close links with the pore size distribution curves which obtained by mercury injection. Since both of them can reflect the pore structure of rock, it’s able to use mercury injection curves to calibrate the NMR T2 spectra. However, the mercury injection curves were only able to reflect part of the larger pore space because the tight reservoirs have narrow pore throats and low pore connectivity. When the pore radius less than 50 nm, there was a big distribution discrepancy between T2 spectra and capillary pressure curves. So the previous methods couldn’t characterize micropores and mesopores of tight reservoirs well. The accuracy of nitrogen adsorption was restricted by samples because the macropores were destroyed, so this method was just effective for the micropores and mesopores. It turns out that the development trend of pore structures characterization techniques of tight reservoirs is the effective combination of various experimental methods. The advantage of NMR is that the measurement of pores are comprehensive, it can identify the pores larger than 10 nm. The disadvantage of NMR is that the measurement unit is relaxation time which needs to transformation. Thus, it’s able to characterize the NMR T2 spectra by combine the nitrogen adsorption and mercury injection. The compound cumulative porosity curves which obtained by integrating the data of nitrogen adsorption and mercury injection were used to calibrate the value of C. Then we transformed the NMR relaxation time into pore radius by C. Specific experimental steps are as followes: (1) Transform the experimental data of nitrogen adsorption and mercury injection into porosity component (Fig.1a), (2) Obtain the cumulative porosity distribution curves (Fig.1b), (3) Determine the best calibration coefficient C (Fig.1c), (4) According to the optimized C, the NMR T2 spectra were converted into pore radius, then the pore volume distributions of the full scale pore size in tight reservoirs were obtained (Fig.1d). In this paper, the NMR T2 spectra were calibrated by the method mentioned before and the pore size distribution characters of tight glutenite reservoirs of Shahezi Formation were analyzed. The pore radius of tight glutenite reservoirs was ranged from 2 nm to 100 μm (Fig.2a) and the distribution curves had two peaks: the larger pores were mainly distributed from 2 μm to 10 μm and the smaller pores were mainly between 10 nm to 100 nm. Totally, the geometric average aperture mainly distributed from 50 nm to 150 nm. The amount of the pores which their radius larger than 1μm were less than 20% , representing Shahezi tight reservoirs developed narrow pore throats and nano-grade pores (Fig.2b).

Application of NMR on Quantificational Characterization of Tight Glutenite Reservoirs Pore Size

organic carbon(TOC)content in mud shale:geochemical analysis and ΔlgR model. Geochemical analysis can obtain accurate TOC content, but it has the problem of higher cost and less samples. The ΔlgR model can obtain TOC content variable values in well profile conveniently and quickly, but it can’t predict TOC content in mud shale in cross-hole area. Both the methods can’t meet the needs of resources evaluation of shale oil in the back ground of strong organic heterogeneous in mud shale. We propose a new method for predicting TOC content based on the improved ΔlgR model and geostatistical inversion. The method combines good lateral resolution of seismic data and high vertical resolution of logging data, and can predict the spatial distribution of TOC in mud shale accurately. It has great practical significance for the exploration of shale oil. The new method requires to obtain TOC variable values in well profile at first. In the traditional ΔlgR model, acoustic travel time and resistivity curves are overlapped together artificially with a fix overlapping coefficient, then defined the baseline value and background content of TOC to calculate ΔlgR and TOC content. The traditional ΔlgR model has several deficiencies on applicability, objectivity, convenience from the aspects of baseline value, background content of TOC and overlapping coefficient. Therefore, we improve the ΔlgR model based on the theory of the traditional ΔlgR model. The improved ΔlgR model can select overlapping coefficient automatically, obtain TOC content accurately and rapidly by computer in absence of mature degree parameters, and manual definition of TOC background value. Haas (1994)proposed geostatistical inversion firstly, and Dubrule(1998)and Rothman(1998)further developed this theory. Its fundamental process is: starting from well point, the inversion is faithful to logging data at well point and conforms to seismic data in cross-hole area. Geostatistical inversion is generally used to predict the spatial distribution of sandstone and obtains good application effects. Using the geostatistical inversion to calculate TOC content requires to analysis the relaxationship between p-wave impedance and TOC content in mud shale. Since organic-rich mud shale poses to less density, higher acoustic interval travel time and lower velocity, obviously, p-wave impedance decreases with the increasing of TOC content in mud shale(Fig.1).

Prediction of Total Organic Carbon Content Based on Geostatistical Inversion: A Case of Qingshankou Formation in Wangfu Depression, Songliao Basin

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Papers

Combining nuclear magnetic resonance and rate-controlled porosimetry to probe the pore-throat structure of tight sandstones

Application of NMR on Quantificational Characterization of Tight Glutenite Reservoirs Pore Size

Prediction of Total Organic Carbon Content Based on Geostatistical Inversion: A Case of Qingshankou Formation in Wangfu Depression, Songliao Basin