Dynamic Filtration: Seepage Losses on Tyrihans

摘要

Drilling of the Tyrihans reservoir sections was affected by significant seepage losses. These added up to ~1000 m3 after drilling five wells with 13 reservoir sections, including pilots and laterals, and a total exposed reservoir length of ~15000 m. In this study, we estimate whether the reported losses can be explained by dynamic filtration mechanisms and related seepage losses. Daily drilling reports were evaluated to reconstruct the downhole drilling environment. Accumulated dynamic and static filter loss periods were calculated. A dynamic filtration HPHT filter press was used to measure dynamic filter losses of laboratory and field samples of the drilling fluid used on Tyrihans. Measurement results were evaluated and extrapolated to field scale. Finally, losses experienced in the field and calculated values were compared. Results strongly indicate that the reported losses on Tyrihans were dynamic seepage losses and that these losses dominate the total loss volumes clearly. The filtration volumes measured in the laboratory were significantly influenced by the shear rate applied. High shear rates caused a larger dynamic filtration component. There are indications that a reduction of coarse bridging particles during drilling and an increase of finer particles relative to the optimum particle size distribution increases the dynamic filtration component. Dynamic fluid losses may in magnitude be misinterpreted as lost circulation into microfractures or in developing fracture systems. A correct assessment of the nature of the losses is essential to select an efficient treatment. Elevated downhole and circulation temperatures as well as long reservoir sections creating large filtration areas cause increased dynamic seepage losses. To reduce these losses higher viscosity base oil could be considered in OBM or a less turbulent flow regime should be engineered. The latter can be achieved for example by increasing drilling fluid viscosity, reducing pump rates or choosing a smaller drill pipe diameter. Efforts should be put into maintaining the optimum particle size distribution of filtercake-building bridging particles. Such changes in fluid design should be carefully evaluated, as they can have a negative impact on other fluid parameters such as equivalent circulation density, swab and surge or reduced hole cleaning efficiency. Research should be initiated to develop low-dynamic-loss fluids. This requires a better understanding of dynamic loss mechanisms and the identification and verification of additives that protect filtercakes against shear and erosion.