Filter cake formed by barite-based drilling fluid is often soaked with chelating agent removal treatment 24 hours to be dissolved after the drilling operation. During the removal process, the chelating agent absorbs barite from the filter cake to form a barite chelate. Due to the heterogeneity of the filter cake thickness where the thinner parts may dissolve faster and due to long soaking time, the removal fluid will have more time to invade the formation. This paper investigates the interaction between the barite chelate and the calcite formation after filter cake dissolution.
The barite filter cake was formed over the face of calcite rock samples using a High-Pressure High Temperature (HPHT) fluid loss apparatus. Chelating agent solution was then used to dissolve the filter cake. NMR spectroscopy and X-ray CT images were applied to study the changes in the pore size distributions of the calcite rock samples at pre and post-invasion states.
The result of this work showed that barite chelate released barite precipitates into the pores of the cores and then absorbed Calcium cations from the rock minerals. The barite chelates released barites into the macropores and absorbed cations from the micropores. CT scan and spatial NMR analysis also showed that the chelating agent was not able to create wormholes after releasing barite. A detailed explanation of the observed results is presented with supporting results.
Based on the obtained results in this work, several recommendations were drawn for removing the barite filter cake in the filed application without causing any severe secondary formation damage.