## 4 Results and Discussion  
### Component Identification of Wellhead Blockage  
#### 4.1.1 Identification of Inorganic Phases  
As shown in Figure 4, the wellhead blockage in Well DH 231 was a mixture of organic material and inorganic phases. From the reflected light image (Figure 4a), it can be observed that the wellhead blockage is mainly composed of three parts, namely, dolomite, fine sand, and a mixture that is unidentifiable under reflected light. The unknown mixture is purplish red in color and may be formed in a partially oxidized environment. Dolomite mixed with fine sand can be clearly discernible. Larger oil stains can be identified in polarized light micrographs (Figure 4a,b). The quantitative results show that the oil stains of the blockage could be up to 80%, and inorganic mineral accounts for less than 20% (Table 2). It is presumed that the inorganic phases may be rock fragments or cement in the drilled formation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Type** & **Components** & **Weight, mg** \\ \hline \multirow{4}{*}{DH 231H (before accident treatment)} & Mineral & Iron oxide, dolomite & _2.2_ \\ \cline{2-3}  & & Sat & 11.2 \\ \cline{2-3}  & & Aro & 5.1 \\ \cline{2-3}  & & Res & 1.2 \\ \cline{2-3}  & & Asp & 17.6 \\ \hline \multirow{4}{*}{DH 231H (after acid treatment)} & Mineral & Iron oxide, dolomite & 1.27 \\ \cline{2-3}  & & Sat & 12.6 \\ \cline{1-1} \cline{2-3}  & & Aro & 6.9 \\ \cline{1-1} \cline{2-3}  & & Res & 2.8 \\ \cline{1-1} \cline{2-3}  & & Asp & 16.3 \\ \hline \end{tabular} Note: Sat: saturated hydrocarbon; Aro: aromatic hydrocarbon; Res: resin; Asp: asphaltene.  
\end{table} 
Table 2: Absolute mass statistics of components before and after acidification of wellhead blockage in Well DH231. Figure 4: Micrograph of the inorganic and organic portions of the wellhead blockage (**(a)**: reflected light images; (**b,c**): polarized light images). To determine the mineral composition of the unknown mixture in the reflected light images, we performed scanning electron microscopy observations [16; 17] and mineral energy spectroscopy identification [18] of the unknown mixture, respectively (Figure 5). High-resolution scanning electron microscopy (SEM) can visualize the surface morphology and mineral composition of blockages, combined with auxiliary X-ray energy spectroscopy (EDS) for quantitative or semi-quantitative compositional analyses to determine the mineral composition. The unknown mixture mainly contained two inorganic phases. One of the inorganic phases was a crystal with a smooth surface and a cubic structure. The other inorganic phase was an unknown mineral with a distinct metallic luster. Under the 180-4700\(\times\) magnification SEM, numerous densely distributed cube crystals in the mixture can be observed, with unknown minerals attached to their surfaces (Figure 5a-c). From the SEM image, it can be observed that the structure of this crystal was very similar to that of the NaCl crystal, but its exact composition was still unknown. Under the SEM field of view, the crystals can be up to 300 um and less than 20 um. Unknown minerals were distributed in blocks or cemented together with crystals. The energy spectrometer performs component analysis by detecting the energy of the characteristic X-rays of the sample. The energy of the characteristic X-rays is a function of the atomic number of the sample. Therefore, by detecting the energy of the characteristic X-rays, the corresponding elements can be identified. As shown in Figure 5c, three points were taken on the cubic crystal as well as on the unknown mineral for elemental determination. The characteristic X-ray yield and energy of light elements with an atomic number lower than that of sodium were low; coupled with factors, such as spectral peak shift and interference, accurate detection results cannot be obtained. Mineral types can be inferred from the morphology, grayscale, point analysis of energy spectrum, and atomic ratio of minerals in SEM images.