*2.3. XRD*

X-ray di ffraction tests were carried out on reservoir samples in accordance with the SY/T5163–2010, and quantitative analysis of mineral composition was conducted using D8 Advance x-ray di ffractometer of Bruker Company in Bremen, Germany. The contents of hard and brittle minerals and clay minerals were determined to study the microscopic mechanisms a ffecting the adsorption properties and wettability of samples.

#### *2.4. Rheological Properties Test*

The rheological properties test was conducted according to API standards. The time required to flow up to 500 mL of drilling fluid was measured by the funnel viscometer produced by Qingdao Tongchun Petroleum Instrument Co., Ltd. (QTPI) (Qingdao, China), which is the viscosity of drilling fluid (FV).

The ZNN-SD6 rotary viscometer produced by QTPI was used to obtain the drilling fluid viscosity and shear stress. The drilling fluid was placed in a circular space between two concentric cylinders. The outer cylinder rotates at a constant speed through variable transmission, and the outer cylinder produces a torque through the action of the measured drilling fluid on the inner cylinder, which makes the inner cylinder connected with the torsion spring rotate at a corresponding angle. According to Newton's law, the size of the angle is proportional to the viscosity of the drilling fluid, so the measurement of the viscosity turns to the measurement of the inner cylinder angle. By the sensor display value, readings at the di fferent rpm values, i.e., 600, 300, 200, 100, 6 and 3 were measured, including gel strengths. The apparent viscosity (*AV*), yield point (*YP*) and plastic viscosity (*PV*) can be obtained by Equations (1)–(3).

$$AV = 0.5\varphi\epsilon\alpha\alpha\tag{1}$$

$$PV = \varphi\_{600} - \varphi\_{300} \tag{2}$$

$$YP = 0.511(2\varphi\_{300} - \varphi\_{600})\tag{3}$$

where ϕ600 is a 600-rpm dial reading of the viscometer; ϕ300 is a 300-rpm dial reading of the viscometer.

#### *2.5. Filtration Test*

According to the API standard, the filtration test was conducted using the ZNS-5A medium pressure filter press assembly produced by QTPI. First, 500 mL drilling fluid was injected into the cylindrical drilling fluid cup and the lid was tightened; then, the air source was connected to adjust the pressure to 0.69 MPa; and finally, the air valve was opened to let the air source enter the drilling fluid cup. The filtration area of the instrument was 45.60 ± 0.60 cm<sup>2</sup> (1 ± 0.1 in2). The time of filtration and the amount of filtration (FAPI) after filtration were measured. Meanwhile, the quality of filter cake was observed to measure the effect of borehole wall protection.

#### *2.6. Laser Particle Size Analysis Test*

Added 2 g of rock powder over 200 mesh sieve into 200 g solid-free drilling fluid, then stirred for 5 min and stood for 30 min, and extracted 20 mL of upper liquid. Distilled water was selected as the dispersion medium, and the particle size distribution of suspended rock powder in the liquid was analyzed by Rise-2002 laser particle size analyzer produced by Jinan Rise Science & Technology Co., Ltd. (Jinan, China), so as to further evaluate the suspension performance of the solid-free drilling fluid.

## *2.7. Wettability Test*

Different solid-free drilling fluids were configured by varying the surfactant concentration and drilling fluid pH. The samples were soaked in different drilling fluids for 48 h and dried naturally for 24 h. JCY series contact angle instrument produced by Shanghai Fangrui Instrument Co., Ltd. (Shanghai, China) was used to measure the dynamic contact angle of distilled water on the samples within 0~12 s and obtain the contact angle photos. Then, the wetting modification effect of the solid-free drilling fluid on the samples was analyzed by comparing with the samples that were not soaked or soaked in water.
