2D Optimized Electrode Arrays for Borehole Resistivity Tomography and CO2 Sequestration Modelling

Abstract

Modern optimization approaches for electrode configurations can significantly improve the resolution of 2.5D resistivity imaging surveys. This study presents a brief review of the 2.5D optimization approach, particularly for borehole-borehole surveys with applications for mapping virtual CO 2 plumes sequestrated in deep saline reservoir formations. The applied algorithm searches for arrays that maximize the spatial resolution of the survey among the comprehensive dataset of best possible spatial resolution (i.e. least temporal resolution). A main goal of this study is to increase the temporal resolution of ERT boreholeborehole surveys by selecting optimized electrode configurations in order to minimise the required data acquisition time while sustaining a high spatial resolution. The optimized dataset starts with a base set and is iteratively increased based on the model resolution matrix (R) until the required number of data points is achieved. Among four different optimization methods, the compare R (CR) method of the best resolution is applied to directly calculate R for each new array added to the optimized dataset. Small optimized datasets generated by this technique are only \5% of their comprehensive sets but of an average resolution ratio (R r ) of[0.95 (i.e. almost the same resolution). With increasing the size of the optimized dataset (during its generation), the algorithm progressively enhances R r values in the central interwell region (of low sensitivities and low resolution) far higher than in the near borehole region (of high sensitivities). Also the inverted tomogram reliability increases by increasing the optimized data size. Briefly, the optimized arrays improve the resolution in the interwell region which is commonly low in borehole-borehole ERT studies. The inverted output model is evaluated quantitatively using the model difference relative to the input model. The results reflect the common smearing effects and artefacts of varying degrees that overpredict volumes, underpredict magnitudes and blur boundaries of the target anomalies. This input model is a synthetic resistivity model that was used to generate synthetic (forward solution) data used during the inversion. Applications on synthetic CO 2 models show that the mapping resolution for optimized datasets is better than that for other highly resolving arrays of the same number of data points. Problems of smeared boundaries and thin layers are less visible in the optimized array than in the other highly resolving arrays.

Optimized arrays for 2D cross‐borehole electrical tomography surveys

“…There have been many significant developments in algorithms to automatically select arrays to maximize the resolution of the inversion model for linear surface arrays and cross-borehole surveys (Stummer et al 2004;Wilkinson et al 2006aWilkinson et al , 2006bLoke et al 2010a;Nenna et al 2011;Hagrey 2012). A non-linear method that calculates the model resolution (the 'Compare R' method) by Wilkinson et al (2006b) proved to be the best method (Loke et al 2010a).…”

2D Model Study of CO2Plumes in Saline Reservoirs by Borehole Resistivity Tomography

“…This comprehensive set includes all possible viable electrode configurations conducted within this array and possesses the maximum possible resolution, see next sections. The application of this 2D array optimization was recently extended into borehole-borehole and surface-borehole surveys [13][14][15]. The last algorithm is applied here.…”

“…Optimization algorithms, noises, depth effect, and modeling constraints are not shown here. They are contained in Hagrey [2,14,15] and Hagrey and Petersen [21].…”

“…The effective comprehensive set in boreholes contains more than 80,000 data points and is even about 0.65 of its whole set but more than 450 times each of the standard dipole-dipole and Wenner sets. This justifies applying the new approach of electrode optimization to generate optimized data sets of far lower size and almost the same resolution as the comprehensive ones, that is, of highly spatiotemporal resolution (for more details, see [14,15]). Briefly this opt array improves the ERT resolution particularly in the interwell region which is commonly low compared to that of the region close the boreholes.…”

“…Tomogram W 1 shows strong smearing effects for the lowermost apex of plume triangle and the cap rock with their surroundings. Both targets have the thin layer problem with electrode coverage of ≤1 which causes this smearing governed by the equivalence principle (e.g., [15]). For a thin resistive CO 2 plume target, the inversion code is not able to resolve thickness and resistivity parameters of certain resistive model features individually but only their product (resistance).…”