A Study on the Material Composition and Traceability of the Wellhead Blockage in the Process of Oil and Gas Exploitation—A Case of the DH231 Well in the Tarim Basin, China

The formation of wellhead blockage increases operating costs and seriously affects the safe production of oil wells. It is crucial to correctly analyze the blockage composition and determine the reasons for wellhead blockage to improve the production efficiency of oil wells. In this study, the material composition and origin of the wellhead blockage in Well DH231 are systematically investigated by means of a thin-section observation, scanning electron microscope, energy spectrum analysis, and molecular geochemical characterization of organic matter. The results show that the wellhead blockage in Well DH231 consists of organic and inorganic materials. The inorganic material was mainly composed of dolomite, fine sand, and unknown black minerals. Four elements, Fe, O, Na, and Cl, could account for 95% of the total elemental content in the unknown minerals. It is speculated that they may have come from rust on the metal parts of the inner wall of the wellbore while being influenced by groundwater during the formation process. The organic matter in the blockage was an oil stain from the geological body. The coexistence of n-alkanes and 25-norhopanes in the oil stain was obvious. It indicated that the oldest crude oil in Well DH231 experienced severe biodegradation, followed by the secondary injection of hydrocarbons. The distribution characteristics of hopanes and steranes in oil stains in Well DH231 were comparable to those of the carboniferous crude oil in neighboring Wells DH4 and DH5. The oil stains in the wellhead blockage may be the product of oil and gas migration in Wells DH4 and DH5.