Evaluation of fluid rheology and pore space characteristics in the inner and external cake formation in overbalanced drilling operations

Abstract

The increase in energy demand has been improving the extraction techniques of energy resources. In the oil industry, understanding the behavior of drilling fluids in the inner and external cake formation and the inner cake displacement during flow back (i.e., oil extraction) is important because these situations may impact formation damage and, subsequently, cause financial and environmental risks. To address this issue, this Study investigated the effect of fluid rheology and pore space characteristics in the formation of filtered cake in overbalanced drilling operations. Study 1A aimed to evaluate the change in the media permeability by polymer retention and the effect of viscous and inertial forces during polymer flow. The flow of xanthan gum (XG) 0.2 and 0.4% (m/m) through 50 and 120 μm ceramic filters was analyzed as a function of pressure drop by high-temperature, high-pressure (HTHP) filter press. Study 1B aimed to evaluate the properties of external cakes and the change in the medium permeability by polymer retention and the clogging of pores by small solid particles. Suspensions of calcium carbonate (CaCO3) 5% (v/v) in XG solutions were filtered, similar to Study 1A, and a model to estimate the medium and the external cake permeabilities was proposed. Study 2A aimed to analyze inner cake formation and its displacement by mineral oil at pore levels. XG 0.2 and 0.4% (m/m) was injected through miniature carbonate core plugs, followed by oil injections at different flow rates and pore fluid occupancy imaging using a Heliscan micro-CT system. Study 2A also proposed a novel methodology to eliminate the partial volume effect and to account for the porosity from micropores. Study 2B aimed to investigate the impact of the initial saturation of the core in the formation of inner filter cakes. XG 0.2 % (m/m) was injected through similar carbonate cores as in Study 2A with different wetting and non-wetting saturations. Then the pore fluid occupancy and phase saturation were imaged using the same experimental apparatus of Study 2A. The main findings show that Darcy’s law best characterized the XG flow in Study 1A. Polymer retention was noticed by the decrease in the medium permeability and the filtrate consistency index. As the pressure drop increased, the difference in the permeability of the media shortened as a function of pressure. In Study 1B, the medium and cake permeabilities decreased as the pressured drop increased. The cakes presented compressibility, and the ones formed by XG 0.4% (m/m) showed higher permeability and porosity values. The suspension filtrate presented lower values of consistency index and higher values of behavior index than the suspension before filtration. In Study 2A, the number of pores occupied by XG in the centers decreased, and the oil resided in the large and intermediate regions while cycles of oil injections were performed. Polymer retention was observed in the inlet face of cores 1 and 2. The well-connected pore space influenced the fluid phase mobility in both cores. While the fluid flow was favored in core 1, it was hindered in core 2. In Study 2B, higher initial wetting saturation favored an increase in XG saturation after its injection. The invaded XG disconnected most of the previous oil clusters that became trapped in the pore space. So, the studies presented in this thesis intend to expand further the knowledge of rock-fluid interaction and the formation of filter cakes during overbalanced drilling operations to minimize formation damage by fluid loss.