Study On The Relationship Between Permeability And Electrical Parameters Of Tight Sandstone

Along with the progress of the exploitation and mining technology, the tight sandstone gas has become the main force to achieve steady growth of gas production in our country. Generally, tight sandstone formation is characteristical of lithology density, low porosity and low permeability, high capillary pressure, diverse clay minerals content etc.. The invasion of exotic fluid and the stress change might cause severe sensitivity damage and water phase trapping damage. Tight sandstone's permeability could be measured difficultly, while its electrical parameters'acquisition is relatively easy. By establishing the link between permeability and rock-electrical parameters, evaluating formation damage through rock electrical parameters is significantly favorable for quickly obtaining the tight formation damage evaluation results and improving the formation protection theory and technology.This article mainly focuses on the typical tight sandstones which were choosen in a gas reservoir from Permian, upper Palaeozoic formation in the Ordos basin. Through core analysis, rock electrical experiment and sensitivity damage experiment, the dominant influence of pore structure on the rocks'absolute or relative permeability is testified. The tight sandstone rock-electrical parameters'characteristics are summarized and the mechanism how the saturation index response to the rock wettability and pore structure heterogeneity is explained in detail. The relation between absolute or relative permeability and rock-electrical parameters is confirmed, and the feasibility of evaluating sensitivity damage and water phase trapping damage through rock electrical parameters is discussed.The dominant effect of the tight sandstones'pore structure on permeability is classified. The tight sandstone pore space mainly contains nano-micro scale pores, also little amount of original residual porosity which has major contribution to the rock permeability. Tight sandstone pore throat is extremely narrow, resulting to the high capillary pressure and the "permeability jail" in the gas and water seepage. The lower the rock permeability is, the wider the water saturation of "permeability jail" is.The relationship between the tight sandstone's electrical parameters and pore structure parameters is analyzed. Under the in-situ stress, the cementation index m change between 1.8-2.0, and the lithology coefficient b is less than 1. The saturation exponent n fluctuate mainly betweenl.0?1.4, which reflects the tight sandstone's characteristics of sheet-like pore throat. The formation factor decreases with the increase of the average radius of pore throat. During low water saturation stage (less than 20%), water-wet tight sandstone's resistivity index and water saturation I-Sw curve may bend downward with the saturation index less than 2, while the I-Sw curve of water wet-rock with poor pore connection may bend upward with saturation index larger than 2. Most of the tight sandstones have more than two kinds of different sized pore systems. During the high water saturation (70%-90%), the sandstone's I-Sw curve may become more horizontal with saturation index less than 2.The relationship between the absolute permeability and formation factor is firmly demonstrated, which bases on the average pore throat radius. The absolute permeability and formation factor of one specific well-cemented tight sandsone sample under different effective stress are in accordance with the former relation. On this basis, a new method of evaluating the tight sandstone stress sensitivity damage is put forward. For the tight sandstones with permeability between (0.05?0.5)×10-3 ?m2, the stress sensitivity damage value by this new method is nearly the same with that through experiments.A new method is formed to evaluate the water phase trapping damage by using the resistivity index. For the tight sandstones with permeability between (0.05?0.5)×10-3?m2, when the rock permeability becomes lower, the gas phase relative permeability with new method gets closer to the measured values, and the water phase trapping damage evaluation result with new method becomes more consistent with that based on the experimentally measured permeability.