Experimental investigation of formation damage during drilling of ultradeep fractured carbonate reservoir

Abstract

Severe formation damage easily occurs in ultradeep fractured carbonate reservoirs during drilling, yet the formation damage mechanisms have not been studied extensively. This study focused on typical fractured carbonate reservoirs in the Shunbei field, China. The multiscale reservoir space characteristics and core wettability of ultradeep fractured carbonate reservoirs were investigated. The results showed that the reservoirs mainly comprise a fourth-order fracture network. The high-temperature and high-pore-pressure steady-state method also identified fluid sensitivity and phase trapping damage. The fluid sensitivity damage at the formation temperature (170 °C) indicated that this reservoir is affected by water sensitivity, salt sensitivity (critical value: 3/4 formation water salinity (FWS)), alkali sensitivity (critical value: pH 7.5), and liquid phase trapping damage. Particle movement and deposition and liquid phase trapping damage are thought to be the primary damage mechanisms. Furthermore, abnormally low water saturation, mixed wettability, abundant clay minerals, and the fourth-order fracture network contribute to fluid sensitivity and liquid phase trapping damage. Finally, a formation damage control strategy was proposed.