3.2.1. RMIP

RMIP analyses were carried out using an ASPE-730 Automatic Mercury Intrusion Porosimeter (Coretest Systems, Inc., Closter, NJ, USA) at the Exploration and Development Research Institute of Daqing Oilfield Company Ltd. Tight rock samples were first cut into cubes, with a volume of <2.0 mL or a weight of <5.0 g, to avoid excessive experimental time, and then were dried to a constant weight at 105 ◦C before the RMIP experiment. To keep a quasistatic mercury intrusion, the intrusion rate of mercury was set as an extremely low fixed value of ≈5 × 10−<sup>5</sup> mL/min. The mercury intrusion pressure ranged from ≈0.025 MPa to ≈6.2 MPa, matching a throat radius range of ≈0.12–29.4 μm based on the Washburn equation that is shown as follows [36]:

$$\tau = \frac{2\sigma\cos\theta}{P\_c} \tag{1}$$

where *r* is the throat radius; *Pc* is the mercury intrusion pressure; σ is the mercury interfacial tension, with a value of 485 mN/m [5]; and θ is the mercury wetting angle, with a value of 140◦ [5].

As shown in Figure 2A,B, the most important process that occurs repeatedly in the RMIP experiment is the drop and increase of mercury intrusion pressure. The sudden drop of mercury intrusion pressure, marked as rheon by Yuan and Swanson [37], represents the passage of nonwetting mercury from the narrower throats into the wider pores, and the pore-throat radius can be obtained from the initial pressure at the beginning of rheon process using Equation (1). The process of increasing mercury intrusion pressure can be further divided into two subprocesses, namely subison and rison, according to Yuan and Swanson [37]. The subison process mainly refers to the region of rising mercury intrusion pressure immediately after its sudden drop, and the increased pressure needs to be below the pressure at which the rheon process occurred (Figure 2A,B). Thus, a subison process represents a connected void space with lower capillary pressure, i.e., pore systems. Rison primarily refers to a region of continuously increasing mercury intrusion pressure, when the subison process finished (Figure 2A,B), until the next rheon occurs, and the increasing capillary pressure has never been reached previously. Therefore, rison usually corresponds to the filling of a connected void space with greater capillary pressure, i.e., throat systems. Based on the above process, we can obtain mercury intrusion curves within pores, throats, and total space (pores + throats), respectively, according to the fluctuations of mercury intrusion pressure. Several commonly used parameters, such as the average ratio of pore to throat radius (*RPTa*) and mercury intrusion saturation in pores or throats (*Spore* or *Sthroat*) can be calculated using the following equation:

$$\begin{cases} RPT\_d = \sum\_{i=1}^{n} RPT\_if\_i\\ S\_{pore} = \frac{V\_{pore}}{qV\_{sample}}, \ S\_{tlraat} = \frac{V\_{throat}}{qV\_{sample}}\\ S\_{total} = S\_{pore} + S\_{tlraat} \end{cases} \tag{2}$$

where *fi* is the normalized distribution frequency of *RPTi*; *Vpore* and *Vthroat* are the mercury volumes intruded into pores and throats, respectively; ϕ is the porosity of tight rocks; and *Vsample* is the volume of tight rock sample used in the RMIP experiment.

**Figure 2.** A small piece of raw RMIP data for sample #2 (**A**), mercury intrusion process analysis of RMIP (**B**), and the pore-throat structure models assumed in RMIP (**C**) and PMIP (pressure-controlled mercury intrusion porosimetry) (**D**) methods, respectively.

Compared with the RMIP method, pressure-controlled mercury intrusion porosimetry (PMIP) can only obtain one mercury intrusion curve [5], with a maximum mercury intrusion pressure of up to ≈400 MPa corresponding to a pore size of ≈3.7 nm [36]. The testing time used in the PMIP method is usually 2–3 h, evidently shorter than that spent on RMIP analyses (i.e., 1–2 d). In addition, in the RMIP method, porous media are usually assumed to be composed of pores and throats with different diameters (Figure 2C), which is more consistent with the fine pore structure characteristics of tight reservoirs [38]. However, the porous media primarily consist of capillary bundles with various diameters in the PMIP method (Figure 2D), and thus, this method cannot recognize the difference between pore and throat diameters.
