Applying bridging agents to prevent seepage losses is a common practice during drilling reservoir sections which limits the invaded zone and reduces stuck pipe possibility. Unfortunately, the initial particle size distribution (PSD) design of bridging agents based on static models does not prevent actual seepage losses due to the induced fractures which have different sizes comparing to the initial reservoir pore sizes.

This paper reviews an actual case study with provided solutions in an offshore field located in the Middle East which had a seepage loss circulation problem through induced fractures. It also presents analyzing natural and induced fractures size of the reservoir layer to choose optimized possible bridging agents’ PSD to cure/prevent loss circulation problems. The maximum/average pore size of formation can be measured from routine core analyses. A geological method to estimate the induced fracture widths with geo-mechanical data were used. Finally, optimum blends of bridging agents for loss circulation pills or background treatment to prevent mentioned problems were designed.

Based on the laboratory testing on cores taken from previously-drilled wells in the mentioned field, the maximum size of pore throats was measured as 20 microns. Therefore, using the Ideal Packing Theory (IPT) method, the result for selecting bridging agents through pore throats (for seepage loss) indicates that optimum treatment is using of bridging agents with D50 and D90 6.5 and 16 microns, respectively.

Also, for improving the treatment selection through parameters such as PSD of bridging agents, investigation on behavior of fracture growth were done. As a result, induced fracture width in studied well, with provided geo-mechanical (such as Poisson's Ratio & Young Modulus) and drilling fluid data was calculated approximately to be 230 microns through the porous medium in the near-wellbore region. Therefore, optimization for bridging these new fractures while drilling was performed again and it was concluded that optimum bridging agent size distribution at the tip of these newly-created induced fractures is applying bridging agents with D50 and D90 of 64 and 170 microns respectively, which are approximately 10 times higher than normal treatment in size.

This paper describes the historical seepage circulation and related problems in the mentioned field and presents a methodology to prevent these issues by predicting induced fractures and optimizing bridging agent PSD to block them. Considering this methodology, the gap between the design and actual drilling is reduced and both rig downtime and related drilling and drilling fluids costs can be saved.

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