Finite-Element Modeling of Spontaneous Potential in an Axisymmetric Reservoir Model with Account of Its Shale Content

(1) This article is devoted to the development of a theoretical and algorithmic basis for numerical modeling of the spontaneous potential method (SP) as applied to the study of sandy-argillaceous reservoirs. (2) In terms of coupled flows, we consider a physical–mathematical model of SP signals from an electrochemical source, with regards to the case of fluid-saturated shaly sandstone. (3) An algorithm for 2D finite-element modeling of SP signals was developed and implemented in software, along with its internal and external testing with analytical solutions. The numerical SP modeling was carried out, determining the dependences on the reservoir thickness and porosity, the amount of argillaceous material and the type of minerals. We performed a comparative analysis of the simulated and field SP data, using the results of laboratory core examinations taken from wells in a number of fields in the Latitudinal Ob Region of Western Siberia. (4) The results of the study may be used either for the development of the existing SP techniques, by providing them with a consistent computational model, or for the design of new experimental approaches.

Inversion-based Interpretation of Borehole Spontaneous Potential Measurements for Accurate Estimation of Water Resistivity in the Presence of Shoulder-Bed and Mud-Filtrate Invasion Effects

Electrical resistivity of formation water is a fundamental property used to quantify in situ water quality for human consumption or for assessment of hydrocarbon pore volume. Resistivity interpretation methods commonly used to quantify the electrical resistivity of formation water invoke rock porosity and fitting parameters that require additional and independent core measurements. Alternatively, the spontaneous potential (SP) log can be used to calculate water resistivity without knowledge of rock porosity in wells drilled with water-based mud. In combination with resistivity and gamma-ray logs, SP logs can be used to estimate water quality, apparent volumetric concentration of shale, and for qualitative assessments of permeability. However, SP logs often exhibit both shoulder-bed and mud-filtration effects; these effects need to be mitigated before using SP logs for calculation of water resistivity. We develop a new inversion-based method to simultaneously mitigate shoulder-bed and mud-filtrate invasion effects present in SP logs via fast numerical simulations based on Green functions. The interpretation method is implemented on SP logs acquired across aquifers with various degrees of complexity using noisy synthetic and field measurements to estimate equivalent NaCl concentration, radius of mud-filtrate invasion, and sodium macroscopic transport number. Interpretation results compare well to those obtained from resistivity and nuclear logs, provide estimates of uncertainty, and can incorporate a priori knowledge of aquifer petrophysical properties in the estimation.

Finite-Element Modeling of Spontaneous Potential in an Axisymmetric Reservoir Model with Account of Its Shale Content

(1) The article is devoted to the development of a theoretical and algorithmic basis for numerical modeling of the spontaneous potential method (SP) as applied to the study of sandy-argillaceous reservoirs. (2) In terms of coupled flows, we consider a physical-mathematical model of SP signals from an electrochemical source, with regard to the case of fluid-saturated shaly sandstone. (3) An algorithm for 2D finite-element modeling of SP signals was developed and implemented in software, along with its internal and external testing with analytical solutions. The numerical SP modeling was carried out, with determining the dependences on the reservoir thickness and porosity, the amount of argillaceous material and the type of minerals. We performed a comparative analysis of the simulated and field SP data, using the results of laboratory core examinations taken from wells in a number of fields in the Latitudinal Ob Region of Western Siberia. (4) The results of the study may be used either for the development of the existing SP techniques, by providing them with a consistent computational model, or for the design of new experimental approaches.

Codell continuous oil accumulation in the northern Denver and SP logs Basin as defined by resistivity, density, and SP logs

A continuous Codell Sandstone oil accumulation is present in the northern Denver Basin downdip from water-wet Codell. The Codell oil accumulation can be defined by resistivity, spontaneous potential (SP), and density logs. Updip from the oil accumulation, average deep resistivity of the Codell decreases to below 4 ohm-m, SP response increases, and density porosity increases. Codell sandstones are continuous across the transition from downdip oil to updip water, so the updip seal does not seem to be caused by a stratigraphic trap. The transition corresponds to a change in thermal history; the area of the oil accumulation was subject to much higher heat flow than the updip wet area. This thermal maturity may have had an impact on clay diagenesis resulting in reduced porosity in the more thermally mature part of the Codell. This paper presents a wireline log-based workflow that can be used to identify and map regional changes in thermal maturity that control hydrocarbon accumulations and sweet-spots in low-permeability rocks such as the argillaceous Codell Sandstone.

Mapping Contaminant Plume at a Landfill in a Crystalline Basement Terrain in Ouagadougou, Burkina Faso, Using Self-Potential Geophysical Technique

The delineation of pollution plumes generated by household waste landfills is not easy, particularly in the case of discontinuous or intricately extending water tables, such as those developed in a fractured crystalline bedrock context. In Ouagadougou (Burkina Faso), there are many uncontrolled landfills throughout the urban area. The water table, generally located between 3 and 10 m deep, is likely to be contaminated by the leachate from these landfills. More than 1000 measurements of spontaneous potential (self-potential), referenced by GPS, have been carried out on a landfill and its immediate surroundings to the south of the urban area. The geostatistical processing by analysis of variograms and correlograms highlights an adapted prospecting technique and reliable cartography. The response seems to be mainly due to the electrochemical component with hot spots within the landfill and a plume heading towards the North-East. The distribution of the spontaneous potential seems to be controlled, not by the topography of the site, but by the fracturing of the mother rock of dominant direction 15° N, and by the mother rock/saprolite contact. Thus, the plume does not flow to the market gardening just below the landfill but rather to a residential area where monitoring of the quality of the borehole water is required.

Landfill Pollution Plume Survey in the Moroccan Tadla Using Spontaneous Potential

In many parts of the world, the impact of open landfills on soils, biosphere, and groundwater has become a major concern. These landfills frequently generate pollution plumes, the contours of which can be delineated by non-intrusive geophysical measurements, but in arid environments, the high soils resistivity is usually an obstacle, which results in the low number of studies that have been carried out there. In addition, such prospecting using geophysical techniques do not provide information on the intensity of the processes occurring in the water table. This study was carried out on an uncontrolled landfill in the arid Tadla plain, Morocco’s main agricultural region. A survey based on geo-referenced spontaneous potential measurements was combined with measurements of anoxic conditions (Eh-pH and O2 equilibrating partial pressure) in the groundwater and leachates, in order to highlight a pollution plume and its geometry. The range of spontaneous potential measurement is wide, reaching 155 mV. Ponds of leachate with high electrical conductivity (20 to 40 mS cm−1) form within the landfill, and present very reducing conditions down to sulphate reduction and methanisation. The plume is slowly but continuously supplied with these highly reducing and organic carbon-rich leachates from the landfill. Its direction is towards N-NW, stable throughout the season, and consistent with local knowledge of groundwater flow. The fast flow of the water table suggests pollution over long distances that should be monitored in the future. The results obtained are spatially contrasting and stable, and show that such techniques can be used on a resistive medium of arid environments.

Instrumentation of soil columns for time-lapse monitoring of the phenomenon of capillary rise through spontaneous potential, soil moisture sensor, electrical resistivity, and GPR measurements

The continuous monitoring of capillary rise via indirect measures aims to predict and generate alerts regarding the soil mass deformations, transport leachate from landfills to the soil surface, and carry salts that can damage buildings. Through time-lapse monitoring of the electromagnetic wave's electrical potential and speed, it is possible to correlate via petrophysical relations the measures of electrical potential, electrical resistivity, and dielectric permittivity to the volumetric water content and capillary height. For this, four acrylic columns filled with civil construction material were instrumented. Column 1 - silver electrodes to measure the potential difference with a bench multimeter that measures the spontaneous potential generated by water flow. Column 2 - low-cost soil moisture sensors that measured the electrical potential and converted to bits. Column 3 - resistivimeter that measured the voltage and that was later converted to electrical resistivity and, Column 4 - 2.6 GHz antenna that measured the speed of the electromagnetic wave that was later converted into dielectric permittivity. The instrumentation assembled proved to be satisfactory to monitor the phenomenon. The monitoring lasted 187 h, and it was found that the maximum capillary height remained constant for a long time.

Estimation of in situ hydrocarbon saturation of porous rocks from borehole measurements of spontaneous potential

It has been previously shown that Nernst’s equation is not reliable for the interpretation of spontaneous potential (SP) measurements acquired in hydrocarbon-bearing rocks. We have examined whether the difference between borehole SP measurements and Nernst-equation predictions could be used to estimate in situ hydrocarbon saturation of porous rocks. For this purpose, a new petrophysical model and a mechanistic finite-difference algorithm for simulating SP borehole measurements in the presence of mud-filtrate invasion are used to establish the limits of detectability of hydrocarbon saturation using only SP logs. We find that the optimal conditions for the detection of hydrocarbon saturation from SP borehole measurements are when (1) capillary forces dominate the process of mud-filtrate invasion, (2) the matrix-pore interface region, known as the electrical double layer, has a relevant impact in the diffusion of counterions, and (3) the electrolyte concentration of drilling mud is greater than that of formation water. We also determine why high values of the endpoint of the water relative permeability favor the detection of hydrocarbon-saturated rocks with the SP log. Using measurements acquired in three key wells within a mature and active hydrocarbon field, three blind tests find that our petrophysical model together with the mechanistic SP simulation algorithm enable the estimation of hydrocarbon saturation from SP borehole measurements without the need for resistivity logs or porosity calculations. The estimation is reliable when the volumetric concentration of shale is negligible, the pore network structure is constant throughout the reservoir, and radial invasion profiles are similar to those observed in the calibration key wells.