Characterizing Dual-wettability Pore Network of Unconventional Rocks by Analyzing Water and Oil Imbibition Data

• Author / Creator

  Shi, Yue

• Recent studies show that the pore network of unconventional rocks generally consists of inorganic and organic parts. The organic part has high wetting affinity towards the oleic phase but low wetting affinity towards the aqueous phase. In contrast, the inorganic part has high wetting affinity towards both oleic and aqueous phases. Separate characterization of the organic and inorganic pore networks is of great importance to understand the fluid storage and transport mechanisms, and to develop more accurate models for fluid flow in porous media. However, the traditional methods such as mercury injection capillary pressure (MICP) and CO2/N2 isotherm adsorption tests fail to characterize the dual-wettability pore network. As mercury is the non-wetting phase towards all the pores and CO2 (or N2) is the wetting phase towards all the pores, these techniques can only evaluate the total PSD (PSDtot). The objective of this study is to compute the oil-wet and water-wet pore size distributions (PSDs) of the unconventional rocks by analyzing the comparative oil and brine imbibition data. To achieve this objective, a modified analytical fractal model for co-current spontaneous imbibition is developed. In previous studies, spontaneous imbibition of wetting phase is considered as a piston-like displacement phenomenon which leads to similar rates of imbibition in pores with different diameters. However, imbibition rate of wetting phase is positively proportional to the square root of pore diameter based on Lucas-Washburn equation. Therefore, we extend the previous fractal models by assuming non-piston-like imbibition in pores with different diameters. In the proposed model, first the larger pores are filled by wetting phase, followed by smaller pores. Considering non-piston-like liquid imbibition, we use available imbibition data and propose a novel Imbibition Transient Analysis (ITA) to calculate PSD. We use the history matching technique to match the experimental imbibition data with the proposed fractal model, and obtain the unknown parameters, such as fractal dimension (Df) and minimum pore diameter (Dmin), controlling imbibition profile. The determined parameters are then used to calculate PSD. The PSD of oil-wet pores (PSDoil) is calculated by oil imbibition data and PSD of water-wet pores (PSDwater) is calculated by brine imbibition data. The PSD of water-repellant pores (PSDorg) is calculated by decoupling of PSDoil and PSDwater. On the basis of proposed non-piston-like fractal model, the minimum pore diameter (Dmin) controls the equilibrated time in the spontaneous imbibition process. The calculated Dmin from oil imbibition data is smaller than that from brine imbibition data, proving the assumptions about imbibition of oil in small organic pores that are not accessible for brine imbibition. Comparing PSDwater and PSDwater-repellant shows that water-repellant pores are generally smaller than water-wet pores. This is in agreement with the results of scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) analyses that demonstrate the abundant number of nanopores within the organic matter with low wetting affinity towards water. Moreover, compared with the results of MICP test, PSD calculated by oil imbibition data can detect very small pores (< 3 nm) which are not accessible by mercury, especially for low-permeability rock samples. Thus, the proposed method in this study can complement the conventional MICP technique for a more comprehensive characterization of the pore network of unconventional tight rocks.

• Subjects / Keywords

  • Pore size distribution
  • Spontaneous imbibition
  • Fractal theory
  • Dual-wettability

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3F47H873

• License

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