Xiaoqi Wang

Application of charging effects in evaluating storage space of tight reservoirs: A case study from Permian Lucaogou Formation in Jimusar sag, Junggar Basin, NW China

Abstract According to the study of the electron beam charging effects of oil-bearing samples from tight reservoirs, a “Two-step Cross-section Back Scattered Electron (BSE) Imaging Method” is put forward, revealing the distribution of residual oil (the oil without loss in the vacuum conditions) in the tight reservoir. With the injection of charging agent, the connectivity of the pore system can also be characterized by this method, and a novel quantitative method of evaluating the effectiveness of reservoir space is established. For the tight reservoir samples from the Permian Lucaogou Formation in the Jimusar sag in the Junggar Basin, which was taken as an example, the strong charging effects mainly result from the residual oil in pores. The oil from pyrolysis of organic matter was widely stored in the pores both inside and nearby organic matter, or existing in poor organic matter area. Due to the super-large analyzing area, the Two-step Cross-section BSE Imaging Method can solve the weak representation of analysis area caused by the heterogeneity. The method of injection of charging agent works well in revealing the connectivity of the pore system. The total plane porosity of oil-bearing samples from the tight reservoir of the Lucaogou Formation is 12.56%, and the connectivity of the whole pore system in the sample reaches as high as 90%. The effectiveness of the reservoir space is very high.

Three dimensional characterization and quantitative connectivity analysis of micro/nano pore space

Abstract Evaluation is performed on pore connectivity of unconventional reservoirs using 3-D FIB-SEM imaging characterization and digital rock techniques. 3-D images are first obtained by FIB-SEM device and then transformed into digital pore structure model through shape correction, brightness correction, depth-of-field correction and phase distinguishing. Based on this model, a new connectivity evaluation method for micro/nano pores in unconventional reservoirs is proposed. This method differentiates dead and live pore space, grades live connected domains (1-grade is the worst and 3-grade is the best) and calculates the connectivity rates of all grades. Selected connected domains can be quantitatively characterized through statistical analysis of connected domain distribution, volume and shape. The applications on nano material, shale and carbonate get distinctive connectivity rates (96%, 22% and 82%) and characteristic differences on connected domain distribution, volume and shape. Through these statistical parameters (connectivity rate, connected domain distribution, volume and shape), the three demonstrated materials and reservoirs are quantitatively characterized and differentiated. Thus, validity of the proposed connectivity method in this paper is proved.

Do Shale Pore Throats Have a Threshold Diameter for Oil Storage

In this work, a nanoporous template with a controllable channel diameter was used to simulate the oil storage ability of shale pore throats. On the basis of the wetting behaviours at the nanoscale solid-liquid interfaces, the seepage of oil in nano-channels of different diameters was examined to accurately and systematically determine the effect of the pore diameter on the oil storage capacity. The results indicated that the lower threshold for oil storage was a pore throat of 20 nm, under certain conditions. This proposed pore size threshold provides novel, evidence-based criteria for estimating the geological reserves, recoverable reserves and economically recoverable reserves of shale oil. This new understanding of shale oil processes could revolutionize the related industries.

Significance of source rock heterogeneities: A case study of Mesoproterozoic Xiamaling Formation shale in North China

Abstract Taking Mesoproterozoic Xiamaling Formation, Northern China as an example, the heterogeneities of source rock in different scales and hydrocarbon microscopic occurrence are studied based on observation of outcrops and observation with microscopy, and geochemical analysis. The large scale heterogeneities of source rocks are considered to be controlled by the plate movement and paleo-latitude location, while the micro-scale might be controlled by climate changes driven by the astronomical orbit. The constant existence of heterogeneities includes the differences of organic matter, debris sources and porosities. The heterogeneities of source rock should be seriously treated during the evaluation of oil and gas resources, especially the unconventional oil and gas. This kind of heterogeneous source rocks provides excellent source-reservoir assemblage of oil and gas generation, expulsion and accumulation, and new reference indexes for the economic evaluation of unconventional oil and gas. Therefore, quantitative study of the heterogeneity of source rock is of great significance for investigating formation mechanism and resource estimation of unconventional oil and gas.

Physical simulation and quantitative calculation of increased feldspar dissolution pores in deep reservoirs

Abstract The physical simulation of diagenesis was conducted for the Cretaceous Bashijiqike Formation deep sandstone reservoir in Kelasu structural belt of Kuqa Depression, Tarim Basin, and the dissolution rate and increased dissolution pores of feldspar matrix grains in such reservoirs were quantitatively calculated in the process from long-term shallow burial in early stage to quick deep burial in late stage. Through the field emission large-area SEM analysis, the dissolution rate and increased dissolution pores of feldspar matrix grains in core samples taken from Dabei and Keshen areas were quantitatively calculated. After the experimental data and the actual core data were cross-checked, the evolution model was established for increased feldspar dissolution pores in deep continental reservoirs with high content of feldspar matrix grains. According to the calculation results, the maximum increased feldspar dissolution pores in Keshen area during the process from long-term shallow burial in early stage to quick deep burial in late stage is by 0.86%−2.05%. The simulated sandstone reservoir with burial depth of more than 7 000 m reveals a larger quantity of increased feldspar dissolution pores, with the absolute error value of 0.23% after calibration. In Dabei area, calcite is the primary contributor to secondary dissolution pores, followed by feldspar. Quantitative calculation shows the maximum increased feldspar dissolution pores in Dabei area to be by 0.62%−1.48%. Similarly, the simulated sandstone reservoir with burial depth of more than 7 000 m reveals a larger quantity of increased feldspar dissolution pores, with the absolute error value of 0.15% after calibration. There are two causes of the experiment errors: One cause is that the simulation experiment uses ideal conditions and the simulation reservoirs are homogeneous; Another one is that deep reservoirs have strong heterogeneity and there are big differences in the dissolution within different areas.