Borehole Induced Polarization

Basic Concept

Induced polarization (IP) borehole logging consists of electrical wireline methods that investigate the capacitive properties of subsurface materials intersected by a wellbore. The IP phenomenon occurs when materials become electrically charged (i.e., polarized) after being subjected to an external electrical field. When the external field is removed, the materials discharge over time and produce a measurable voltage signal.

The ability of earth materials to hold electrical charge (i.e., its chargeability) represents the magnitude of electrical charge polarization within the material. Polarization magnitude can depend on minerology, water content, pore-fluid chemistry, and fluid-grain boundary interactions. The IP logging methods can identify variations of capacitive properties in borehole-adjacent subsurface materials and, thus, aid material interpretation.

IP measurements can be collected in the time domain or the frequency domain. Time-domain IP measures the decay of the polarization over time, and frequency-domain IP (see surface induced polarization for further information) makes use of the dispersive nature of the IP measurement. Because signal amplitudes and phases are related to frequency, measurements collected using more than one frequency can be used to more deeply investigate subsurface chargeability.

Theory

Subsurface electrical charge is transported by electrons through metallic materials and ions in the pore fluid and along mineral-grain surfaces. When an external electric field is applied via current injection through two current electrodes, positive and negative ionic charges are transported in a direction opposite the field. Charges are often blocked by and accumulate on fluid-mineral interfaces, which results in interfacial-charge polarization and is the primary source of the induced-polarization effect.

Polarization generates internal electric fields, the strengths of which depend upon the chemical properties of the subsurface materials. Once the external electrical field is eliminated, the electric fields within the IP-affected materials drive the charges to resume their equilibrium positions. During re-equilibration, a current is generated as polarized materials discharge the energy that was stored under the influence of the external electric field. The generated current produces a measurable voltage signal that decays over a measurable period of time.

The chargeability of a material is a measure of how well that material can hold electrical charge and can be investigated in the time- and frequency domains. The more common and simplistic measure of chargeability is collected by a time-domain IP tool, which can produce quantitative values of intrinsic chargeability. However, chargeability is commonly expressed qualitatively in milliseconds (ms) and can also be described in milliradians and percent frequency effect (PFE).

Chargeability primarily depends upon the fluid-grain boundary interactions and is significant in areas with disseminated metals and clay mineralogy. Porous materials containing bound water within tight, clay-rich pores have high chargeability that increases with the increase of dissolved ions. However, the presence of some matrix clays may affect the intended IP response. For example, phyllosilicates, which are clay minerals containing oppositely charged layers, typically have negatively charged surfaces that attract pore-water cations and imbalance the polarization.

Applications

IP-logging tools are similar to those employed in borehole resistivity logging, and they can be integrated together as one tool. Like the resistivity tool, the time-domain IP tool injects current into the subsurface using current electrodes and measures the voltage response with potential electrodes. The tool transmits pulses of direct current (DC) square waves. The signal polarity is reversed with each measurement to average out constant potentials (see self-potential) and increase signal to noise.

The transmitted current (I) and measured voltage (V) are monitored until they stabilize at primary values (I_p and V_p), which indicate that charge polarization has occurred. The current is turned off, and a relaxation time near that of the transmission time is allotted for discharging. After current is shut off, the voltage response (i.e., residual voltage (V_R) curve) is measured over time as it decays from the value maximum-residual voltage to zero.

Theoretically, intrinsic chargeability (M), which is dimensionless, can be determined by with the ratio of max V_R/V_P. The maximum-residual voltage, which occurs immediately after current-shutoff, is difficult to measure because, as it dissipates, the transmitted current still influences the measurement. By integrating and normalizing the voltage-decay curve by the primary voltage, apparent chargeability (M_a) is calculated. Additionally, relative changes in chargeability are related to changes in current-dissipation time, and chargeability is most often represented in units of milliseconds.

The frequency-domain IP methods make use of the dispersive nature of the IP response (i.e., signal amplitude is inversely proportional to transmission frequency). As such, frequency-domain IP measurements inject current at different frequencies and consider the change in magnitude of voltage response to estimate subsurface-capacitive properties. Spectral-IP measurements are frequency-domain measurements use multiple frequencies and can estimate the tortuosity, which characterizes the pore spaces and connectivity of pores.

IP-logging methods have certain requirements and potential complications. Electrical coupling is required for successful data collection, and, thus, data can only be collected in fluid-filled boreholes. Additionally, all electrodes must be submerged and out of casing. Furthermore, the electrodes must also be nonpolarizing (i.e., have a fixed potential) as to not affect the received signal. Lastly, IP data bear a certain degree of inherent non-uniqueness, which is referred to as the uncertainty response and requires careful analysis.

The processes involved in induced polarization are not completely understood, as the IP-effect is a very complex phenomena that results from several physio-chemical conditions. However, surface induced polarization methods have received much attention and been the subject of more research. As such, knowledge of the mechanisms contributing to IP and applications of IP data are advancing. Though not as expansive in scope as its surface counterpart, IP-borehole logging methods have successfully aided the following:

Mapping of lithology and orebodies
Estimation of water content and soil moisture
Determination of solute concentration/salinity
Mapping of clays, contaminates, hydrogeological boundaries, and fracture zones

Examples/Case studies

Bacon, L.O., 1965, INDUCED‐POLARIZATION LOGGING IN THE SEARCH FOR NATIVE COPPER: Geophysics, v. 30, no. 2, p. 191-337, doi:10.1190/1.1439564.

Abstract: Induced‐polarization logging of underground boreholes, in the footwall zones of the Osceola Mine, has provided an increased sample volume to detect the presence of copper mineralization as well as to provide information on geological contacts which aid in geologic interpretation and in the planning of mining operations.

Briggs, V., Sogade, J., Minsley, B.J., Lambert, M., Reppert, P., Coles, D., Rossabi, J., Riha, B., Shi, W., and Morgan, F.D., 2004, Mapping Of Tce And Pce Contaminant Plumes Using A 3-D Induced Polarization Borehole Data, in proceedings, EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems, 17^th, European Association of Geoscientists & Engineers, doi:10.4133/1.2923360

Abstract: In-situ complex resistivity (CR) or Induced Polarization (IP) data are collected using a 3D array of surface and borehole electrodes, over an area known to be contaminated with DNAPLs (Dense Non- Aqueous Phase Liquids). The contaminants include Tetrachloroethylene (TCE) and Trichloroethylene (PCE), which, until recent years have been disposed of directly into the environment. The design of the surface and cross-borehole array allows for a 3D IP inversion. Data are measured at two frequencies (1/4 and 1/16 Hz), and are inverted for resistivity magnitude and phase. The inversion results are compared with PCE and TCE contaminant concentrations measured from core samples taken from three ground truthing wells drilled within the region of interest. The phase and imaginary resistivity are shown to be well correlated with the concentration data from two of the three ground truthing boreholes where the TCE and PCE concentrations are above 1mg/kg.

Spitzer, K. and Chouteau, M., 2003, A dc resistivity and IP borehole survey at the Casa Berardi gold mine in northwestern Quebec: Geophysics, v. 68, no. 2, p. 430-760, doi:10.1190/1.1567221.

Abstract: In the spring of 1996, a direct current (dc) resistivity and induced polarization (IP) borehole survey was carried out at the Casa Berardi gold mine in northwestern Quebec to study the spatial extent of the economic disseminated zone of an auriferous quartz vein type orebody. Crosshole pole‐pole and single‐hole pole‐dipole configurations were used to delineate the geometry of the body associated with the Casa Berardi fault system. Since the spatial data sampling was insufficient for 3D inversion, the interpretation has been done using 3D dc and IP forward modeling. Model changes were applied iteratively to match synthetic with field data. Sensitivities provide information on how to alter the models efficiently. Furthermore, they indicate the significant regions of the model, giving evidence on where the model is meaningful. A model study using a simple prismatic block structure is shown to enhance understanding of the physical response associated with the two types of borehole survey. The algorithms used for the interpretation offer a so‐called grid‐independent electrode positioning technique, which is a helpful modus operandi to significantly facilitate the simulation process. The result is a resistivity and chargeability model that produces the observed physical response and incorporates all known geological a priori information. Particularly, the IP response carries detailed information on the appearance of the orebody, whereas the potential response reflects the resistivity contrast between metavolcanic and metasedimentary rocks at the Casa Berardi fault.

Tong, M., Li, L., Wang, W., and Jiang, Y., 2006, A time‐domain induced‐polarization method for estimating permeability in a shaly sand reservoir: Geophysical Prospecting, v. 54, no. 5, p. 623-631, doi:10.1111/j.1365-2478.2006.00568.x.

Abstract: It is known that the time‐domain induced‐polarization decay curve for a shaly sand reservoir depends on the pore structure of the reservoir, and this curve can be used to estimate permeability, which is a determining factor in making production decisions in the petroleum industry. Compared with NMR logging tools, induced polarization has several advantages, such as a deep depth of investigation and a high signal‐to‐noise ratio. The purpose of this paper is to establish an appropriate model using induced polarization to estimate the permeability. The curve can be modelled as a weighted superposition of exponential relaxations. The plot of weight versus the relaxation time constant is defined as the relaxation time spectrum. Induced‐polarization decay‐curve measurements were performed on 123 samples from the Daqing oilfield using a four‐electrode technique. A singular‐value decomposition method was used to transform the induced‐polarization decay data into a spectrum. Different models to estimate the permeability were discussed. The results of the research indicate that the induced‐polarization measurements greatly improve the statistical significance of permeability correlations. Compared with the traditional forms, AφC and AFC, the forms, ATBφC and ATBFC, have lower error factors, where T, Φ and F are the geometric mean time constant of the induced‐polarization relaxation time spectrum, the porosity and the resistivity formation factor, respectively, and A, B and C are constants. The mean time constant is the decisive parameter in the permeability estimation and it is not completely independent of the resistivity formation factor. The additional use of the porosity and the resistivity formation factor leads to an appreciable improvement. It is concluded that this new model will make it possible to estimate the permeability of a shaly sand reservoir downhole.

Worthington, P.F. and Collar, F.A., 1984, Relevance of induced polarization to quantitative formation evaluation: Marine and Petroleum Geology, v. 1, no. 1, p. 14-26, doi:10.1016/0264-8172(84)90117-X.

Abstract: The role of electrical induced polarization (IP) in the petrophysical estimation of shaliness and permeability remains uncertain in the wake of widely varying degrees of optimism within the literature of the past 30 years. Therefore, there is no general agreement on the potential usefulness of IP methods within the framework of formation evaluation. A refinement of the broad scatter of data previously indicated by IP-shaliness-permeability correlations has been accomplished through new empirical relationships involving time-domain IP parameters. It is demonstrated that chargeability can be related smoothly and definitively to excess conductivity or intergranular permeability. These relationships are based upon measurements of water-saturated sandstones over a wide range of electrolyte conductivity. Their field application is viable provided that three formation constants have been determined from laboratory measurement. In the light of these developments the possible contribution of induced polarization measurements to petrophysical evaluation is briefly re-examined with more confidence than has been justifiable up to now. In so doing particular attention is paid to those precautionary procedures of core preparation and data acquisition which must be followed if the IP approach is to be beneficial from a quantitative standpoint.

References