Fracture Flow Characterization with Low‐Noise Spontaneous Potential Logging

Abstract

Geophysical well logging has been applied for fracture characterization in crystalline terrains by physical properties measurements and borehole wall imaging. Some of these methods can be applied to monitor pumping tests to identify fractures contributing to groundwater flow and, with this, determine hydraulic conductivity and transmissivity along the well. We present a procedure to identify fractures contributing to groundwater flow using spontaneous potential measurements generated by electrokinetic processes when the borehole water head is lowered and then monitored while recovering. The electrokinetic model for flow through a tabular gap is used to interpret the measured data and determine the water head difference that drives the flow through the fracture. We present preliminary results at a test site in crystalline rocks on the campus of the University of São Paulo.

Laboratory simulations of self-potential signals to assist groundwater studies

“…Despite modern technology to visualize fractures and respective apertures from opti-cal/acoustic imaging of the borehole wall, the identification of fractures effectively contributing to groundwater flow is still a challenging task of importance in hard rock tunnelling, subsurface mining or contaminant repository development. Kowalski et al (2020) conceptualized a procedure to identify hydraulically active fractures by tracking their electrokinetic response after a well is pumped and left to recover to its previous condition. In this test, a transmissible fracture shows a characteristic sigmoidal variation in the electrokinetic potential, its polarity indicating flow direction (if entering or exiting the borehole), and its amplitude linearly varying as the static level is recovered.…”

“…The proposed recovering method was tested in field conditions (Kowalski et al, 2020) but some particularities of the test site (Kowalski et al, 2021), namely a single hydraulically active fracture at a very impermeable environment, left as open questions how the proposed test would respond if applied to boreholes with at least two fractured systems in more permeable environments. A laboratory-scale experiment was then outlined to simulate a situation in which a borehole intercepts two fractures, each one connected to aquifer systems with distinct hydraulic heads.…”

Fracture Critical Length Estimative Using Percolation Theory and Well Logging Data

“…As shown in Table 1, a set of 32 fractures were identified from OPTV imaging of the 20 m open section of borehole P2 with apparent apertures varying from undistin-guishable up to a few centimeters. A full image for P2 borehole is presented in Kowalski et al (2021).…”

“…One commonly used logging technique is the direct imaging of the borehole wall with optical or acoustical probes allowing a quantitative fracture analysis as it is done in structural geology with outcrops. Borehole wall imaging can distinguish active and sealed fractures (Shapiro, 2001), evaluate the fracture transmissivity when integrated with adequate pumping tests (Hamm et al, 2007) or hydraulic head estimates from monitoring electrokinetic signals after pumping a well (Kowalski et al, 2021). Image logs also allow predetermination of depths of interest for selective measurements with flowmeter probes (Williams and Paillet, 2002) to detect potential fractures as a nearby well is pumped (Vitale et al, 2019).…”