--> Experiments and Characteristic Analysis of Mud Filtrate Invasion in Low Porosity and Low Permeability Sandstone Reservoir

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Experiments and Characteristic Analysis of Mud Filtrate Invasion in Low Porosity and Low Permeability Sandstone Reservoir

Abstract

Abstract

The mud filtrate invasion during drilling has caused lots of problems in oil field exploration for a long time. The drilling mud and small particles invade the original reservoirs and change the initial status of formation around wellbore, which not only cause the difficulty in logging interpretation, but also affects the oil and gas productivity. In order to figure out the invasion mechanism, experiments and characteristic analysis of mud filtrate invasion in low porosity and low permeability sandstone formation module are showed in this paper.

A multifunctional physical simulation system of mud filtrate invasion in formation module is designed and built to realize the experimental analysis of mud invasion under complex drilling conditions. The main results are summarized as followed:

The internal influence factors of mud filtrate invasion include reservoir porosity, permeability, heterogeneity, etc., and the external influence factors include mud pressure differential, mud properties (viscosity, filtration performance, solid-liquid ratio, etc.), mud hydrodynamic and invasion time, etc.

At the beginning of the mud filtrate invasion, the mud filtrate invasion velocity in low porous and low permeable sandstone reservoir is slower than that in medium-high porosity and permeability sandstone reservoir, while in the middle and later periods of mud filtrate invasion, the former invasion velocity is faster than the latter. The difference in the mud cake permeability caused by the different mud cake formation rate is the direct cause of this phenomenon.

In the same sandstone reservoir, the bigger mud pressure differential results in faster mud filtrate invasion velocity. At the beginning of the mud filtrate invasion, with the invasion of the polymer and fine clay particles, the longer invasion time results in the larger damage rate of sandstone reservoir permeability. The damage rate will keep increasing until the mud cake is steadily formed.

Although with smaller pore radius, shallower invasion depth of polymer and fine clay particle and smaller damage rate of formation permeability, the original permeability of the low porosity and low permeability sandstone reservoir is 2-3 orders of magnitude smaller than that of medium-high porosity and permeability one, thereby the productivity of low permeability sandstone reservoir is more seriously affected by mud filtrate invasion.