Evaluation of secondary formation damage caused by the interaction of chelated barite with formation rocks during filter cake removal

Filter cake formed by barite-based drilling fluid are often removed using chelating agents such as Diethylenetriaminepentaaceticacid (DTPA) in combination with other catalysts. During the removal process, the filter cake is soaked with the chelating agent for approximately 24 h, after which they absorb barite from the filter cake to form a barite chelate (BaDTPA). The filter cake thickness may not be homogenous along the wellbore surface, especially in directional wells. As a result, the chelating agent will dissolve the thinner filter cake faster and earlier than the thicker filter cake. Thus, for a given treatment period, the thin filter case is quickly dissolved and the resulting barite chelate will have more time to invade the formation. This paper investigates the interaction between the barite chelate and the rock formation after filter cake dissolution. A suite of lab scale experiments was conducted to simulate filter cake formation on the face of four rock samples (limestone and sandstone) using a High-Pressure High Temperature (HPHT) filter loss apparatus. DTPA (20 wt %) was then used to dissolve the filter cake. A core flooding apparatus was used to inject the effluent barite chelate into the rock samples to simulate its invasion and interaction with the rock. NMR spectroscopy and x-ray micro CT images of the samples revealed interesting changes in the pore size distributions of the samples. Barite chelate released barite precipitates into the pores of the rock and then absorbs cations from the rock minerals. In sandstones, the barite chelates released barites into the small - intermediate pores and absorbed cations ions from the large pores. In carbonates, the barite chelates released barites into the big pores and absorbed cations from small - intermediate pores. Micro-CT scan and spatial T-2 NMR analysis also showed that no wormholes were generated during filter cake dissolution. A detailed explanation of the observed results is presented with supporting results.