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In this paper, based on the analysis of lab experimental results of 20 core samples, which were drilled from tight sandstone reservoirs of Sichuan basin, and simultaneously applied for mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) measurements, the relationships of piecewise power function between NMR transverse relaxation T 2 time and pore-throat radius R c are established. A novel classified piecewise power function (CPPF) method, which is used to transform NMR reverse cumulative curve as pseudo capillary pressure ( P c ) curve is proposed, and the corresponding model is established based on formation classification. By using this model, formation pseudo P c curves can be consecutively synthesized in the intervals with which field NMR logs were acquired. The pore throat radius distribution, and pore structure evaluation parameters, such as average pore throat radius (Rm), threshold pressure (Pd), maximum pore throat radius (Rmax) and so on, can also be precisely extracted. Several tight sandstone reservoirs are processed by using this method, and the predicted results are compared with core derived results. Good consistency between the evaluated results with core derived results illustrates the dependability of the proposed method. [ABSTRACT FROM AUTHOR]

Abstract: Permeability, irreducible water saturation and other information associated with the pore size, such as capillary pressure, are very important input parameters in tight gas reservoir evaluation. Fortunately, those parameters can be well estimated from field NMR logs, if the proper calibrations between NMR relaxation time and capillary pressure were established with measurements from core samples. In this study, a novel calibration technique, which is used for constructing capillary pressure curve from NMR logs by combining the classical SDR model with J function, is proposed and the corresponding model is established. Based on the laboratory measurements of 54 plug samples taken from the Upper Triassic sandstone formation (Xujiahe Formation) in Anlu district, Sichuan basin, Southwest China, the model parameters are calibrated. With the calibrations established and procedures proposed in this paper, the synthetic capillary pressure curves of reservoir can be constructed from field NMR spectrum. Also from those laboratory measurements, the parameter of (defined as the pore structure index) can be well related to the Swanson''s parameter derived from capillary pressure curve. This correlation could then be used for estimating reservoir permeability from the synthetic capillary pressure curve. After formation is classified into three types by using the pore structure index, the pore throat radius cutoffs can be acquired to calculate irreducible water saturation from the pore throat radius distribution. The proposed techniques and procedures are applied in field logs and the estimated information and parameters are well verified by core and drill stem testing data. By combining with the information of reservoir pore structure, permeability and irreducible water saturation, potential low resistivity contrast gas bearing reservoirs can be identified from water layers. [Copyright &y& Elsevier]

Formation pore structure prediction is of great importance in tight gas sands evaluation, and NMR logs are considered to be an advantage in this aspect. The common method that is used to evaluate formation pore structure from NMR logs is transforming the NMR T 2 spectra as capillary pressure curves and then extracting the pore throat radius distribution from the synthetized capillary pressure curves and predicting pore structure evaluation parameters. However, field applications demonstrate that the existing methods are not always practicable; additionally, many existing methods involve input parameters that are difficult to obtain, especially in tight gas sandstone reservoirs. This study is based on the analysis of the morphology of lab mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR). This experiment used 20 core samples that were drilled from tight gas sands of the central Sichuan basin. Our findings indicate that rock pore structure, the values of mercury injection saturation ( S Hg ) under every capillary pressure ( P c ), and pore component percentage compositions are interrelated. To quantitatively express this interrelationship, seven T 2cutoff s, which are used to classify NMR T 2 spectra into eight parts, are defined to calculate eight pore component percentage compositions from the NMR logs. A novel technique, which connects the S Hg s and eight pore component percentage compositions, is proposed to construct pseudo P c curves. Once this technique is extended to field applications, consecutively synthetic P c curves can be predicted in the intervals with which field NMR logs were acquired. Additionally, pore throat size distribution and pore structure evaluation parameters can also be calculated. A field example of the Sichuan basin illustrates that this technique is reliable and that the predicted results from the field NMR logs match the core experimental measurements very well. The greatest advantage of this proposed technique is that no intermediate parameters need to be acquired from other methods. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs identification and evaluation face a great challenge due to the characteristics of ultra-low porosity, ultra-low permeability, complicated pore structure and strong heterogeneity. To improve tight sandstone reservoirs evaluation, the pore structure should be first quantitatively evaluated. The NMR logs are advantageous in formation pore structure evaluation, whereas they may not be always practicable, because the NMR logs are acquired only in the limited wells. In this study, an alternative empirical technique, which is established based on the lab experimented mercury injection capillary pressure (MICP) curves for 54 core samples drilled from tight sandstone reservoirs of central Sichuan basin, Southwest China, is proposed to construct pseudo capillary pressure ( P c ) curve from conventional porosity and permeability. The reliability of this technique is verified by comparing constructed pseudo P c curves with lab measured results. To make this empirical approach can be widely practicable, formations should be first classified into three types based on the difference of J function, and the models are calibrated by using plenty of core derived results. After this technique is extended to field application, consecutive pseudo P c curves are constructed, and the parameters associated with reservoir pore structure evaluation, such as the pore throat radius distribution, the average pore throat radius, the threshold pressure, and so on, are estimated. Combining with these parameters, tight sandstone reservoirs pore structure are quantitatively evaluated, and effective formations are easily identified. Comparing with methods based on the NMR logs, the proposed technique is much universally applicable in tight sandstone reservoirs pore structure evaluation in formations with which field NMR logs were not acquired. [ABSTRACT FROM AUTHOR]

Abstract: It is difficult to obtain rock resistivity parameters by using the cross plots of porosity vs. formation factor and water saturation vs. resistivity index to calculate reservoir water saturation in low permeability sandstones. The cementation and saturation exponents (m and n separately) are divergent, and no fixed values can be obtained due to the complicated pore structure. This leads to a problem in water saturation calculation. To investigate the main factors that heavily affect the cementation and saturation exponents, 36 core samples, which were drilled from low permeability sands of Xujiahe Formation, Sichuan basin, southwest China, are chosen for laboratory resistivity and nuclear magnetic resonance (NMR) measurements, 20 of them for mercury injection capillary pressure (MICP) measurements and 10 of them for casting thin-section analysis. The results show that these two parameters are associated with rock pore structure. For rocks with good pore structure, the proportion of macropore components is dominant, high cementation exponents and low saturation exponents can be obtained, and on the contrary, rocks with poor pore structure will be dominated by the proportion of small pore components, and they will contain low cementation exponents and high saturation exponents. To quantitatively acquire reliable cementation and saturation exponents for water saturation estimation, a logarithmic function is established to calculate cementation exponent from porosity. Irreducible water saturation (S wi), which is estimated from NMR logs by using the optimal T 2cutoff, is presented to characterize the proportion of small pore components. A technique of calculating saturation exponent by combining with S wi, (1−S wi) and the logarithmic mean of NMR T 2 spectrum (T 2lm) is proposed, and the corresponding model is established. The credibility of these techniques is confirmed by comparing the predicted cementation and saturation exponents with the core analyzed results. The absolute errors between the predicted cementation exponents and the experimental results are lower than 0.08, and the absolute errors between the predicted saturation exponents and the experimental results are lower than 0.2. These techniques proposed in this study are extended to several low permeability sands for field applications; the field examples illustrate that cementation and saturation exponents can be accurately estimated in the intervals with which NMR logs were acquired. By using the variable rock resistivity parameters, precisely water saturation can be calculated for low permeability sandstones evaluation. [Copyright &y& Elsevier]

Xiao et al. (2012) proposed a method of the synthetic pseudo-capillary pressure ( P c ) curves from the NMR logs based on a J function method. This method was verified to be effectively applied in the field with similar formation properties. However, it cannot be applied if the formation properties are different, which results to the different J function from the experimented P c curves of core samples. To improve the applicability of the method of Xiao et al., 84 core samples, drilled from 4 different basins in China, are supplemented for nuclear magnetic resonance (NMR) and mercury injection capillary pressure (MICP) experimental measurements, and they are classified into three types by using a hydraulic flow unit (HFU) method based on a flow zone indicator (FZI) parameter. For each type of rocks, the properties are similar, and they can be expressed by the same J functions. In this case, Xiao's method is applicative. These 84 core samples are reused to calibrate the involved input parameters in the model of Xiao et al., separately. Several wells from different regions are processed by using the calibrated models. Field applications illustrate that the improved models are much more effective in formation evaluation. [ABSTRACT FROM AUTHOR]