An alternative method for estimating the porosity and the permeability of (potential) geothermal reservoirs using magnetotelluric data; A case study of the Sherwood Sandstone Group in the Lough Neagh Basin, Northern Ireland

Abstract

Since traditional methods for estimating the hydraulic properties of reservoir rocks, such as laboratory analysis of core samples and pumping tests, are time-consuming, invasive and expensive, a method for the reliable estimation of reservoir rock properties from surface measurements would be beneficial. In this thesis an alternative method is introduced for the estimation of the porosity and the permeability of a (potential) geothermal reservoir. In this method, the electrical resistivity of the reservoir rock is determined based on magnetotelluric (MT) data. Then, petrophysical relations (i.e. Archie’s first law and the RGPZ model) are used in to derive values for the porosity and the permeability from the resistivity values predicted by the MT models. A case study is carried out of the Sherwood Sandstone Group in the Lough Neagh Basin, Northern Ireland, utilizing this alternative approach. For this purpose, wireline logging data and core sample data from three boreholes are utilized in conjunction with MT data from five sites located close to the boreholes. The results are used to advance the assessment of the geothermal energy potential of the Sherwood Sandstone Group.

The wireline logging data are evaluated and modeled using Interactive Petrophysics software and a porosity-permeability relationship is derived from core sample measurements. The results are used to determine average values for the resistivity, porosity and permeability of the upper part of the Sherwood Sandstone Group. The MT data are analyzed and processed, before being used for extensive 1D resistivity modeling. For this, three different inversion codes are used: Occam, the WinGLink sharp boundary code and Minim. The Minim models are used to determine the resistivity of the upper part of the Sherwood Sandstone Group. From the resistivity values the porosity and permeability are derived using Archie’s first law and the RGPZ model respectively. The values of the parameters of Archie’s first law and the RGPZ model are defined based on wireline logging data and core sample data from the boreholes.

Comparing the results of the MT and petrophysical modeling, it is concluded that accurate estimates of the porosity and the permeability can be obtained using the method introduced here, provided that the resistivity determined based on the MT models does not deviate from the actual average resistivity of the target formation by more than 1 m. Additional geological data are essential for calibration of Archie’s first law and the RGPZ model. Therefore, this method is best utilized in combination with additional geological data from a reference site, allowing calibration of the petrophysical models. After calibration the petrophysical models can be applied to the entire MT survey area.

From the borehole data analysis porosity and permeability values for the upper part of the Sherwood Sandstone Group are obtained, with the porosity varying between 18 and 21 % and the permeability varyingbetween 83 and 723 mD. These values indicate a good to excellent quality of the Sherwood Sandstone Group as a geothermal reservoir. Future work on the geothermal energy exploration of the Sherwood Sandstone Group in the Lough Neagh Basin should focus on identifying the areas in the basin with the highest geothermal potential.