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Pore systems of tight-gas reservoirs control important parameters such as flow capacity, producible pore volumes, hydrocarbon flow rates and storage which are complex to characterise and to quantify. Detailed understanding of 3D pore structure is vital in predicting tight-gas reservoir productivity. This study investigates the pore architecture of a sandstone sample from Collyhurst Sandstone Formation, northern England, a tight gas reservoir. X-ray Computed Tomography (XCT) was employed to quantify the pore system (pore size, shape, geometry, and connectivity). Three types of pores were identified: inter-grain pores which are at the boundary of detrital grains and/or authigenic minerals, intra-grains pores which are pores within the detrital mineral grains, and intra-cement pores which are pores within the cement phase (authigenic minerals). The first pore type is primary in origin, whereas the latter two are secondary pores. A centerline tree and a pore network model (PNM) algorithm were applied for pore network analysis and a porosity value was estimated. The image-based porosity value was validated with laboratory experimental measurement. 3D image estimates 3.1% volume porosity, while 4.6% was recorded from the laboratory experiment. Spatial quantification of pore types and analysis of pore connectivity of the studied sandstone improves understanding on gas flow behaviour and gas storage. Furthermore, the results are useful in predicting permeability and multi-flow phase properties in a tight gas reservoir. • 3D quantification of pore types in tight-gas sandstone constructs a spatial distribution of the pore system at micro-scale. • Pore model network module is more suitable than the centerline tree module when quantifying pore system of a reservoir. • Quantification of 3D pore structure is significant in building pore models and predicting multi-phase flow of a reservoir. [ABSTRACT FROM AUTHOR]

A clear understanding of pore structure of tight oil reservoirs is essential for reservoir evaluation and enhanced oil recovery. This paper presents a multiscale characterization method using a combination of pressure-controlled porosimetry (PCP), micro-computed tomography (micro-CT), and scanning electron microscopy (SEM). Four tight sandstone samples from Chang 7 Formation in the Ordos Basin were collected for petrophysical characterization. Pore-throat size distributions (PTSDs) for these samples were measured via PCP. A high-resolution micro-CT scan (1 μm/pixel) was used to acquire 3D volumetric images of small core plugs to evaluate pore connectivity of these samples. Additionally, high-resolution digital images were obtained through SEM to identify different pore types. SEM analysis shows that pores in tight sandstones could be classified into four types, i.e., residual interparticle pores, grain dissolution pores, clay pores, and micro-fractures. Residual interparticle pores are often coated by fibrous illite and chlorite. Grain dissolution pores are mainly deduced from the dissolution of grain minerals, among them the feldspar dissolution pore is the primary type. According to the PCP experiments, these samples exhibit multiscale pore structures with a wide range of PTSD from 9.2 nm to 500 μm dominated by nanopores. Average mercury intrusion saturation and permeability contribution value of the dominating nanopores are 63.61% and 80%, respectively. Given the unresolved nanopores, CT images were segmented into three phases, including pore space, grain phase, and clay minerals. The results of connectivity analysis demonstrate that macroscopic pores are mostly connected by clay phases, implying that nanopores provide the critical flow paths. This novel multiscale characterization approach provides us a better understanding of complex pore structures of tight sandstones. [ABSTRACT FROM AUTHOR]

The pore size distribution (PSD) and fractal dimension of tight sandstone are studied with mercury intrusion porosimetry (MIP). Four different fractal models are used to calculate fractal dimensions from mercury intrusion capillary pressure (MICP) and mercury extrusion capillary pressure (MECP) respectively, and the physical meanings of the fractal dimensions calculated from different fractal models are discussed. The fractal characteristics of tight sandstone pore spaces with unimodal and bimodal PSD are compared. The correlations between fractal dimensions and tight sandstone petrophysical properties are researched, and the optimal fractal model for formation evaluation is recommended. Finally, the key parameters influencing tight sandstone permeability are investigated. [ABSTRACT FROM AUTHOR]

Characterization of pore throat size distribution (PTSD) in tight sandstones is of substantial significance for tight sandstone reservoirs evaluation. High-pressure mercury intrusion (HPMI) and nuclear magnetic resonance (NMR) are the effective methods for characterizing PTSD of reservoirs. NMR T2 spectra is usually converted to mercury intrusion capillary pressure for PTSD characterization. However, the conversion is challenging in tight sandstones due to tiny pore throat sizes. In this paper, the linear conversion method and the nonlinear conversion method are investigated, and the error minimization method and the least square method are proposed to calculate the conversion coefficients of the linear conversion method and the nonlinear conversion method, respectively. Finally, the advantages and disadvantages of these two different conversion methods are discussed and compared with field case study. The research results show that the average linear conversion coefficients of the 20 tight sandstone core plugs collected from Yanchang Formation, Ordos Basin of China is 0.0133 μm/ms; the average nonlinear conversion coefficient is 0.0093 μm/ms and the average nonlinear conversion exponent is 0.725. Although PTSD converted from NMR spectra by the nonlinear conversion method is wider than that obtained from linear conversion method, the nonlinear conversion method can retain the characteristic of bi-modal distribution in PTSD. [ABSTRACT FROM AUTHOR]

The characteristics of the nanopore structure in shale, tight sandstone and mudstone from the Ordos Basin of China were investigated by X-ray diffraction (XRD) analysis, porosity and permeability tests and low-pressure nitrogen adsorption experiments. Fractal dimensions D1 and D2 were determined from the low relative pressure range (0 < P/P0 < 0.4) and the high relative pressure range (0.4 < P/P0 < 1) of nitrogen adsorption data, respectively, using the Frenkel–Halsey–Hill (FHH) model. Relationships between pore structure parameters, mineral compositions and fractal dimensions were investigated. According to the International Union of Pure and Applied Chemistry (IUPAC) isotherm classification standard, the morphologies of the nitrogen adsorption curves of these 14 samples belong to the H2 and H3 types. Relationships among average pore diameter, Brunner-Emmet-Teller (BET) specific surface area, pore volume, porosity and permeability have been discussed. The heterogeneities of shale nanopore structures were verified, and nanopore size mainly concentrates under 30 nm. The average fractal dimension D1 of all the samples is 2.1187, varying from 1.1755 to 2.6122, and the average fractal dimension D2 is 2.4645, with the range from 2.2144 to 2.7362. Compared with D1, D2 has stronger relationships with pore structure parameters, and can be used for analyzing pore structure characteristics. [ABSTRACT FROM AUTHOR]

Carbonate cement is the most important cement type of the Xujiahe Formation in the western Sichuan foreland basin. In particular, the Xu-2 Member has a much higher content of dolomite cement, and the Xu-4 Member predominately contains calcite cement. It is concluded that the formation water contained in the Xujiahe Formation in the western Sichuan foreland basin was originally sourced from meteoric water, the formation water in both members was heavily influenced by the release of water from the compaction of mud shale and coal beds, and the present-day characteristics of the water are the result of concentration and water-rock interaction during the burial process. The Xu-4 Member has a relatively open water-rock system that results in the formation water exhibiting heavier oxygen isotope values and the carbonate cements exhibiting lighter carbon isotope values, dominated by calcite. The Xu-2 Member has a more closed water-rock system, and the significantly heavier carbon isotope values of the carbonate cement in it are related to the source of cementitious material, the lithology of its underlying source rock, and the tectonic uplift erosion. [ABSTRACT FROM AUTHOR]

To investigate the effect of siliceous cementation on the densification of sandstone and the forming process of tight sandstone, based on cathodoluminescence, scanning electron microscopy and thin section analysis, the growth mechanism and characteristics of quartz particles in tight sandstone formations are explored. Meanwhile, combined with conventional core analysis and X-ray diffraction experiments, the factors affecting the crystallization of quartz particles, including the chlorite content, grain size and clay mineral, are analyzed, respectively. The entire siliceous cementation is divided into two processes. The first part is the process in which the weathered and rounded particles in the formation are restored to the hexagonal dipyramid crystal by siliceous cementation. The second part is the process of coaxial growth that the hexagonal dipyramid crystal continues to increase with the form of micro-quartz film. As siliceous cements continue to increase, the petrological characteristics of sandstones are constantly changing. The tight sandstone developed in the study area is composed of lithic sandstone and quartz lithic sandstone. Based on the analysis results, 2D and 3D evolution models are established for densification of two different lithic sandstones. When the content of siliceous cement in the study area is less than 17%, the porosity of tight sandstone increases with the increase of cement. When the content of cement is more than 17%, the porosity of tight sandstone is negatively correlated with the content of cement. When the cement content is greater than 10%, the reservoir permeability is negatively correlated with it. Furthermore, the particle size mainly affects the permeability of reservoir, and the particle size is negatively correlated with the permeability of tight sandstone. The most high-quality tight sandstone reservoir in the study area is in the first cementation stage when siliceous cements are distributed in porphyritic texture with the content of 10–15% and a grain size of 0.2–0.3 mm. In addition, the relatively high-quality reservoir is the one developing clay mineral film with a content of cementation about 5–12%. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs are often produced by shutting in the well and inducing imbibition. However, by adopting current reservoir classifications, the heterogeneity of reservoirs cannot be properly treated. Based upon the analysis of the imbibition curves and mercury intrusion porosimetry tests, Chang-7 tight sandstone reservoirs were classified into three categories according to the newly proposed standards. Imbibition tests demonstrated that for the first category, imbibition and drainage occurred continuously and never reached the plateau within the experiment duration. It was suggested that a longer shut-in time favors the production of oil. For the second category, a steady state for imbibition was reached and a shut-in time as short as three days resulted in a high imbibition rate. For the third category, a plateau was reached for the first time and imbibition restarted until a steady state was reached. The average shut-in time for the third category was eight days. Compatibility between reservoir characteristics and a soaking development regime based upon the proposed classification methods effectively enhances the oil recovery efficiency of formations with distinct petrophysical properties. This provides insight into the classification methods of tight sandstone reservoirs. [ABSTRACT FROM AUTHOR]

Pore structure determines the ability of fluid storage and migration in rocks, expressed as porosity and permeability in the macroscopic aspects, and the pore throat radius in the microcosmic aspects. However, complex pore structure and strong heterogeneity make the accurate description of the tight sandstone reservoir of the Triassic Yanchang Formation, Ordos Basin, China still a problem. In this paper, mercury injection capillary pressure (MICP) parameters were applied to characterize the heterogeneity of pore structure, and three types of pore structure were divided, from high to low quality and defined as Type I, Type II and Type III, separately. Then, the multifractal analysis based on the MICP data was conducted to investigate the heterogeneity of the tight sandstone reservoir. The relationships among physical properties, MICP parameters and a series of multifractal parameters have been detailed analyzed. The results showed that four multifractal parameters, singularity exponent parameter (αmin), generalized dimension parameter (Dmax), information dimension (D1), and correlation dimension (D2) were in good correlations with the porosity and permeability, which can well characterize the pore structure and reservoir heterogeneity of the study area, while the others didn't respond well. Meanwhile, there also were good relationships between these multifractal and MICP parameters. [ABSTRACT FROM AUTHOR]

Systematically illuminating the controlling factors on fracture development in the tight sandstone reservoirs is the key basis to accurately predict the fracture distribution. Fractures in the tight sandstone reservoirs of Kuqa foreland basin are quantitatively characterized, and a systematic research on the fracture distribution and controlling factors is carried out. Tectonic fractures are dominant in the tight sandstone reservoirs, and their development was controlled by inherent (e.g.,lithology,mechanical stratigraphy, sedimentary micro-facies and diagenesis facies) and external (structural position and overpressure) factors. Rocks with high content of brittle minerals, fine grain size, low porosity and well sorting are conducive to the development of fractures. Fractures often develop in the competent rocks and their height is equivalent to the thickness of mechanical layer. Sedimentary micro-facies with thick cumulative deposition thickness, thin single layer and fine granularity are the favorable area for fracture development. Diagenesis facies with dense compaction and cementation are also areas favorable for fracture development. Vicinity of fault and hinge of fold are area of local stress concentration,thus the fracture density increased significantly. The existence of overpressure promotes the failure of rocks. [ABSTRACT FROM AUTHOR]

Natural fracture in reservoir has important influence on oil and gas production. It is difficult to determine the distribution of fracture network's heterogeneity only rely on limited downhole data. In addition, it is also difficult to obtain a large number of the static parameters for fracture numerical simulation, so the quantitative evaluation of fracture faces greater challenge. Based on typical tectonic styles of Kuqa foreland thrust belt outcrop area (which include thrust monoclinic, high angle thrust anticline and slip anticline), this article described the fracture unique development and distribution characteristics of different tectonic styles, then established the fractures development models of different tectonic style, finally analyzed the controlling factors on fracture growth. The results show that the thrust monocline fold develops two phases fractures, the strike direction are NNW mainly. Late vertical open fractures have absolute priority in this tectonic style, and early high angle oblique fracture is filled by calcite. The density of fracture from core of anticline to wing has decreasing tendency. High angle thrust anticline develops three phases fracture, and the strike direction is NNW and NWW. The early fracture was full filled by calcite, middle-late fracture are opened. The fracture cut off the single sand body and converges to rock bed, which have obvious mechanical stratigraphic characteristics. The steep wing of anticline’s developed more tectonic fractures than that of slower wing, which shows asymmetric distribution. The compression wing of slip anticline develops two stage high angle oblique open fracture,and the fracture strike direction is NNW and NEE. The sandstone deformation of barrier wing is weak in which it doesn't have obvious fracture development. The fracture growth characteristics under different structural styles are of great significance to the numerical simulation of fracture and the prediction of reservoir distribution. [ABSTRACT FROM AUTHOR]

With deep development of the gas reservoirs of Penglaizhen layer in western Sichuan, formation energy is continually falling. Some problems have cropped up during fracturing process, including flowback rate dropping and fracturing fluid filtrate invading seriously, which restrict efficient exploitation of shallow layer Penglaizhen gas reservoirs. Based on the research of performance requirements of the fracturing fluids, technical countermeasures was proposed to reduce fracturing fluid damage to reservoir, and improve the development benefits of gas reservoirs. Finally, the two low damage fracturing fluid systems: ultra - low concentration thickener fracturing fluid system and linear fracturing fluid system are formed, which are suitable for Sichuan Penglaizhen gas reservoir and widely applied in western Sichuan Penglaizhen. Flowback rate are 65.1% and 61.9%respectively, average production are 0. 81 X 104m3/d and 1. 45 X 104m3/d respectively after fracturing. The significant effect for increasing yield is achieved. [ABSTRACT FROM AUTHOR]