Study on the Mechanism of Geostress Difference Effect on Tight Sandstone Resistivity and Its Correction Method
- Zhiying Liu (Yangtze University) | Chengguang Zhang (Yangtze University) | Gongming Zheng (Yangtze University) | Yi Xin (PetroChina Tarim Oilfield Company)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- February 2018
- Document Type
- Journal Paper
- 82 - 98
- 2018. Society of Petrophysicists & Well Log Analysts
- 4 in the last 30 days
- 92 since 2007
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Formation resistivity of tight sandstone in the Keshen area of the Tarim oilfield is greatly affected by geostress. Fluid identification and the accuracy of water saturation calculations are drastically affected by significant increases in rock resistivity resulting from geostress. In this paper, the rock constitutive relation (i.e., the relationship between stress and strain) calculation method of geotechnical structural mechanics and electric field theorem and the finite element method (FEM) numerical simulation are applied to study the effective mechanism of geostress difference on rock resistivity, and a method of geostress correction for rock resistivity is proposed. The paper explains why resistivity increases significantly with the horizontal geostress difference in the water-bearing layer subject to the high geostress of the Keshen area, and also establishes the basis for calculation of water saturation in the high-stress formation.
The Keshen area of the Tarim oilfield is one of the main blocks of Tarim Basin undergoing natural gas development. The target zones are in the Cretaceous Bashijiqike Formation, which consists of tight sandstone reservoirs with very low porosity and permeability. The actual logging data indicate that the formation resistivity in the area is significantly increased by the difference between the maximum horizontal stress and the minimum horizontal stress (hereinafter referred to as the horizontal geostress difference), resulting in resistivity logs that to fail to directly reflect the influence of the formation fluid on resistivity measurements, and lead to the inaccurate estimates of water saturation using the Archie equation. The purpose of this study is to establish a geostress-correction method for rock resistivity suitable for the area, and thereby calculate the resistivity of rock in the absence of geostress difference (hereinafter referred to as the true resistivity).
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