Study On Characteristics Of Microscopic Pore Structures Of Sandstone Reservoirs Of Xingnan Oil Field In Daqing

In this study, the sandstone reservoir of Xingnan Sa and Pu oil groups in Daqing Oilfield is the main research object, Through the collection of a large amount of materials, such as casting body chip, fluorescence chip, electron microscope and mercury penetration, systematic analysis and discussions were conducted on characteristics, classification and influence factors of reservoir pore structure and its relation with remaining oil using laboratory displacement experiment, fractal mathematics and computer simulation method.Apart from the conventional pore structures for description of parameters, mathematical fractal theory was also introduced in this paper, and distribution of microscopic pore structure was studied using parameters including fractal dimensionality, fractal porosity, and fractal porosity/total porosity. It was found that the pore throat distribution of most samples in the studied area was binary fractal, i.e., pore throat distribution could be divided into two main groups, namely, big pore throat and small pore throat. The big pore throat group mainly reflected the characteristics of sedimentary process, while the small pore throat group reflected the characteristics of diagenesis. Binary fractal porosity formula was deduced on the basis of previous researches in this paper, and the ratio of the fractal pores of each sample to the total fractal pores was calculated. It was concluded in this paper that the fractal dimension of large pore path did not differ greatly in general, and it was the distribution of small pore path which decided the fractal dimension:the higher the distribution frequency of small pore path, the larger the fractal dimension, fractal porosity and the ratio of fractal porosity to total porosity was.With the consideration of correlative degree of porosity and permeability, all the samples were divided into five categories of pore structures by integrating conventional pore structure description parameters and parameters of fractal characteristic. From the first category to the fifth category, the radius of pore throat decreased gradually, and pore-to-throat ratio gradually increased; porosity and permeability became increasingly worse; the content of interstitial matter, fractal pore ratio and heterogeneity all increased.The study on different sedimentary microphases found that the first, second and third categories of pore structures mainly developed in microphase sand body of distributary channels, while the first and second categories of pore structures mainly developed in the mid-lower river channels or predominant pathways of the river channels; and the third and fourth categories of pore structures developed mainly at the upper or the edge of river channels. Types of pore structures of primary shelf blanket sands and secondary shelf blanket sands mainly belonged to the third and fourth categories of pore structures. The third category of pore structure developed mainly at the upper or kernel part of sand body, while the fourth category of reservoir developed mainly at the lower or the edge of sand body. The pore structures of external thin sand body primarily belonged to the fourth and fifth categories.A detailed study of the influence of diagenesis on pore structure was conducted. Mechanical compaction could decrease the fractal dimension of pore throat distribution, rendering pore structure more uniform different cementing agents increased the number of pores under development to different extent due to the difference in occurrence and shapes. Among them, flake cementing agent produced the largest number of fine pores. Corrosion also made pore throat surface rougher, and increased the ratio of small pore throat, but with less influence on fractal pores than that of cementing agent.Different water washing period had different influence on pore structure. As the water washing intensified, radius of pore throat increased, and relative fractal became better, with enhanced connectivity. Data of the actual core was used to establish the network model of pore throat, and the numerical simulation of two-dimensional and three-dimensional displacement of oil by water was conducted, respectively. The microscopic distribution pattern of remaining oil in pore structures during different water washing periods was summarized using microscopic chips. The first and second categories of pore structures mainly occurred in the form of drusy or pore surface thin film during the strong water washing period;the third, fourth and fifth categories of pore structures were primarily of types of remaining oil as intragranular pore, intergranular pore or film.Discussion of the relationship between pore structure and oil displacement efficiency revealed that the correlation between all microscopic parameters of different sedimentary microphases and oil displacement efficiency was higher than that between data and oil displacement efficiency. This founding indicated the control of sedimentary process on microscopin pore structure. Values of average radius of pore throat, porosity, permeability, and permeability/porosity were positively correlated with oil displacement efficiency, while fractal porosity/total porosity was negatively correlated with oil displacement efficiency. The oil displacement efficiency of meandering type of distributary channel was the lowest, while that of straight distributary channel was the highest, with that of the three categories of blanket sands at the middle. The oil displacement efficiency of different sedimentary microphases decreased with the increase in fractal porosity/total porosity. Thus, pore structure was formed under the coaction of sedimentation and diagenesis, with the heterogeneity of pore structure being the main factor affecting oil displacement efficiency.Extraction scheme for remaining oil of different microphases was proposed based on the above research. Oil extraction of distributary channels should be targeted at the upper part of sand bodies or external part of the predominant pathways, and profile control agents, polymer injection and cyclic waterflooding could be used to extract remaining oil;for primary shelf blanket sands and secondary shelf blanket sands, medium water washing was mainly adopted. The larger amount of remaining oil in sand body could be extracted by doubling the amount of water injection or changing fluid flow direction of cyclic waterflooding; blanket sands of the external thin layer could be extracted using unwashed or weak washing methods. A small amount of remaing oil might be extracted by changing the injection direction of cyclic waterflooding, but the majority of oil remaining in thin pore throat should be extracted by increasing seepage channel through fracturing measures.