Study on Methods of Evaluating Plugging Capacity of Nanometer and Micrometer Sized Plugging Agents for Shale Formations

Methods presently in use for evaluating the performance of many kinds of plugging agents in nanometer and micrometer levels still lack the required accuracy and effectiveness, and no commonly followed standards are available. To deal with these problems, a mesoporous membrane and a dense sand-bed are selected as the media to simulate the microfractures in shale formations, and the filtration rate and the wetted depth of the sand-bed are used as the indices for evaluating the performance of a nanometer or micrometer plugging agents. The mesoporous membrane method uses a filter membrane with pore sizes between 100 nm and 450 nm, the fitting lines of the parallel experimental data have minimal fluctuations, and this method is thus suitable for evaluating the performance of the plugging agents with particle sizes distributed between 35 nm and 450 nm. The dense sand-bed method uses 200-mesh quartz sands as the packing material, and the variance of several experimental data is 0.2131, meaning that the parallelism of the experiments is good. This method is suitable for evaluating plugging agents with particle sizes distributed between 500 nm and 24.6 μm. Using this method, three ultra-fine plugging agents with big differences in particle sizes, which are ultra-fine calcium carbonate, emulsified rubber MORLF and ULIA, were evaluated for their performance, and the MORLF with deformability, was finally selected as the most suitable nanometer plugging agent. The evaluation methods presented and the nanometer-sized plugging agents have been applied on 7 wells drilled in the Changning block. Compared with other wells drilled with conventional oil based drilling fluids, the average percent hole washout of the 7 wells is reduced by 12.74%, and average drilling time reduced by 12 d, further proving that the methods presented have good parallelism and high accuracy.