Koerperich, E.A., SPE-AIME, Shell Oil Co. In this method of logging, a recording of the potential difference between two SP electrodes yields a curve that may facilitate and improve sand counts, and that might be amenable to automatic computerized interpretation. Spurious SP electrode noise signals can be substantially reduced or eliminated from the potential-difference curve. Introduction Wellbore SP currents and their applicability in the location of subsurface sand bodies were discovered in 1931 and the usefulness of this measurement has persisted to the present day. Through the steadfast persisted to the present day. Through the steadfast endeavors of many researchers, knowledge and understanding of the SP has slowly evolved from the original mere recognition of its existence to its present-day quantitative use. Some common applications present-day quantitative use. Some common applications of the SP curve include bed correlation, sand counting and formation-water resistivity determination. This report relates to the determination of net sand counts from SP data. Many influences combine to complicate SP interpretation and to restrain its use as a unique, infallible indicator of permeable bed boundaries. The distribution and intensity of SP, currents are largely influenced by properties of the drilling and formation fluids. wellbore diameter and invasion depth, and by various characteristics of the subsurface strata. Quantitative use of the SP curve is severely restricted in salt, carbonate, and fresh water provinces, Shaly sand formations create difficulties with SP interpretation, and this topic has been the subject of many publications. An even greater problem is SP noise, which has long been a source of low-quality and erroneous SP curves, particularly since the advent of offshore drilling operations. A principal source of noise is associated with the surface "ground" potential to which the wellbore spontaneous potentials are referenced. Being located at the surface, the reference potential is vulnerable to voltage disturbances created by surface objects. Thus, all these factors that mold SP character should be accounted for, as adequately outlined in the literature, when the curve is being analyzed quantitatively. Many times the exact influence of some or all of these factors affecting SP response cannot be predicted. For this reason, auxiliary tools, whenever predicted. For this reason, auxiliary tools, whenever available, are used in conjunction with the SP to determine net sand. Short spaced, highly focused resistivity devices or pad-type resistivity devices are commonly used for this purpose. Despite the limitations of the SP curve, there are a number of occasions and areas where it satisfactorily reflects permeable bed boundaries. Many times auxiliary logs are not available to refine the SP sand count, leaving the SP as the primary indicator of bed boundaries. In these cases, the method of recording and presenting SP data as outlined in this report can presenting SP data as outlined in this report can usefully supplement the SP curve. This method of presentation is directly derived from a single conventional presentation is directly derived from a single conventional SP curve or from two conventional SP curves recorded simultaneously at different depths in the wellbore. When the data presentation is derived from a single down-hole electrode the resultant curve (Delta V curve) is subject to all the limitations and required corrections associated with the SP recording, including the presence of noise signals when they exist on the SP. When the, Delta V curve is derived from two downhole electrodes, it is subject to the same limitations except the SP noise. JPT p. 1437

The apparent formation water resistivity (Rwa), the Ro overlay, and the Resistivity (Rt) Porosity (0) crossplot are among several methods used in picking out hydrocarbon-bearing zones. One condition essential to the use of these techniques is that the formation water resistivity (Rw) remains constant or varies in a narrow range over the interval analyzed. If this condition is not met, a zone with water resistivity abnormally higher than that of other zones within the interval of interest can be mistaken for a hydrocarbon-bearing zone. Also, a hydrocarbon-bearing zone with abnormally low water resistivity can be over looked. The use of Resistivity (Rt) Spontaneous Potential (SP) crossplot together with the Rt 0 crossplot can prevent such gross misinterpretations. This paper reviews the concepts of the Rt SP crossplot, its advantages and limitations. Field examples illustrating the proposed interpretation techniques are discussed.

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 135146, ’Real-Time Measurements of Spontaneous Potential for Inflow Monitoring in Intelligent Wells,’ by M.D. Jackson, SPE, M.Y. Gulamali, E. Leinov, J.H. Saunders, and J. Vinogradov, Imperial College London, prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed. Spontaneous-potential (SP) signals are generated during hydrocarbon production because of gradients in the water-phase pressure (relative to hydrostatic), chemical composition, and temperature. Measurements of SP during production, taken with permanently installed downhole electrodes, could be used to detect water that is encroaching on a well while the water is several tens to hundreds of meters away. Introduction SP measurements, logged before production or injection using wireline tools, are primarily electrochemical (EC) in origin. Contrasts in chemical composition between formation and drilling fluids give rise to “junction” or “diffusion” potentials in permeable beds, while the exclusion of (typically) negative ions from the pore space of fine-grained rocks (e.g., mudstones and shales) results in “membrane” potentials. Together, these EC potentials typically dominate the SP log, although in some cases, electrokinetic (EK), or streaming, potentials, which arise from gradients in fluid pressure (relative to hydrostatic), also may contribute. However, gradients in fluid pressure, chemical composition, and temperature also will be present during hydrocarbon production, particularly during waterflooding or steamflooding where colder or hotter water of a chemical composition different from that of the formation brine is injected into the reservoir. Consequently, if the casing is either nonmetallic or is insulated electrically from the formation, SP signals will be observed during production. This study characterized the magnitude of these SP signals to determine if their measurement, using permanently installed downhole electrodes, might be useful in monitoring fluid flow. SP Components The SP acts to maintain overall electro-neutrality when a separation of electrical charge occurs in response to gradients in pressure, chemical composition, or temperature. In a water-wet reservoir rock, charge separation occurs at the mineral/water interface because the water reacts with the mineral surfaces to leave an excess of (typically) negative charge on the mineral surface and an excess of positive charge in the water adjacent to the mineral surface. This arrangement of charges at the mineral/water interface is known as the electrical double layer. The negative charge on the mineral surface is immobile, but some of the excess positive charge in the adjacent water is mobile and will move with the fluid. If the water is subjected to a pressure gradient, which causes it to flow relative to the mineral surfaces, then some of this positive charge is transported with the flow. The net excess of positive charge moving with the flow gives rise to a so-called streaming current. To balance this streaming current, a conduction current is established and the electrical potential required to maintain this conduction current is the EK, or (more specifically) streaming, potential. The term EK is used here for consistency with the terminology used to describe the EC and thermoelectric (TE) potentials.

Summary Spontaneous potential (SP) is routinely measured using wireline tools during reservoir characterization. However, SP signals are also generated during hydrocarbon production, in response to gradients in the water-phase pressure (relative to hydrostatic), chemical composition, and temperature. We use numerical modeling to investigate the likely magnitude of the SP in an oil reservoir during production, and suggest that measurements of SP, using electrodes permanently installed downhole, could be used to detect and monitor water encroaching on a well while it is several tens to hundreds of meters away. We simulate the SP generated during production from a single vertical well, with pressure support provided by water injection. We vary the production rate, and the temperature and salinity of the injected water, to vary the contribution of the different components of the SP signal. We also vary the values of the so-called "coupling coefficients," which relate gradients in fluid potential, salinity, and temperature to gradients in electrical potential. The values of these coupling coefficients at reservoir conditions are poorly constrained. We find that the magnitude of the SP can be large (up to hundreds of mV) and peaks at the location of the moving water front, where there are steep gradients in water saturation and salinity. The signal decays with distance from the front, typically over several tens to hundreds of meters; consequently, the encroaching water can be detected and monitored before it arrives at the production well. Before water breakthrough, the SP at the well is dominated by the electrokinetic and electrochemical components arising from gradients in fluid potential and salinity; thermoelectric potentials only become significant after water breakthrough, because the temperature change associated with the injected water lags behind the water front. The shape of the SP signal measured along the well reflects the geometry of the encroaching waterfront. Our results suggest that SP monitoring during production, using permanently installed downhole electrodes, is a promising method to image moving water fronts. Larger signals will be obtained in low-permeability reservoirs produced at high rate, saturated with formation brine of low salinity, or with brine of a very different salinity from that injected.

The complex distribution of electrochemical sources in the borehole environment has historically limited the quantitative interpretation of spontaneous potential (SP) logs. In addition, SP models based on characterizations of these sources have been poorly validated because of the difficulty of generating realistic SP signals in controlled environments. In this article, we report on our SP experiments and models to address these problems. We describe a novel experimental technique for simulating SP signals in the laboratory. By placing synthetic, ion-selective membranes between aqueous saline solutions of different concentrations, we created source distributions that approximated those encountered by borehole logging tools in oilfield formations. Configurations of cation-selective and anion-selective membranes simulated cases of piston4ike invasion, sawtooth-shaped invasion, and no invasion of a sand zone flanked by two shale layers. We used these data to validate a forward model that represents SP sources as electrical double layers. Also, we devised an inversion scheme to estimate the transference numbers of the membranes (and hence their strengths as SP sources), assuming that their geometric distribution is known. Results from both the forward and inverse models agree with the experimental data. This suggests that these simple models could be successfully adapted to the analysis of SP field logs.

The PDF file of this paper is in Russian. In layers of limited thickness and high resistance, the amplitude of the spontaneous potential (SP) logs differs significantly from the amplitude corresponding to the layer of unlimited thickness. For a more accurate determination of reservoir properties by the SP method it is necessary to move from the apparent values of the curve to the static potential of the reservoir, that is, to solve the inverse problem. The article presents an analytical solution for a direct problem of the SP method in case of rocks in a well crossing an electrically inhomogeneous layer of limited thickness with a zone of drilling mud penetration. The analytical solution of a similar problem proposed by Schlumberger-Doll Research (M.R. Taherian, at al.) for an impenetrable formation in the absence of penetration zone is discussed. It is shown Schlumberger's solution is a subcase of the analytical solution regarded in present article. On the basis of the analytical solution of the direct problem the inverse problem of the SP method was solved taking into account the potentials of rock matrix. Solving the inverse problem in conjunction with the electric logging data is shown on the example of middle Cretaceous reservoirs (Achimov deposits) in Western Siberia. For this purpose we used algorithms of numerical solution of the direct problem in case of well crossing an electrically inhomogeneous layer of limited thickness with a zone of drilling mud penetration by the integro-interpolation method, and the analytical solution of the direct problem for a layer of limited thickness with regard to the potential of rock matrix. The results of numerical and analytical solutions of the inverse problem are almost identical. Proposed algorithms are intended to use in the Rosneft corporate software for petrophysical modeling.

Most engineers and geoscientists participating in petrophysics training programs dislike the chart book technique for deriving R, from the spontaneous potential log. This process can be done on a computer or calculator quite easily if the chart for converting R, to R,, (and the inverse conversion R,, to R,) is represented mathematically. Equations are presented in this paper that describe R, as function of R,, and temperature (and the inverse transformation R,, as a function of R, and temperature) which very closely match the curves in the Schlumberger chart SP-2. These equations can easily be utilized in a computer to calculate R, from the static spontaneous potential, temperature, and R,/ values for a formation. Sample functions written in Visual Basic are provided and can be used in an Excel spreadsheet or as an independent program on a Windows PC.

New nuclear magnetic resonance (NMR) measurements-such as inincralogy-free porosity, direct hydrocarbon volume and type, and clay-bound water and its tic to cation Exchange capacity (CEC)-have brought a new level of quality to log-derived water saturation, both in the flushed zone and in Virgin formation. However, the inversion of resistively logs Into water-filled porosity remains heavily dependent on formation water resistively on many occasions, the log analyst has no direct measure of and the R,,, estimate that the analyst must make can introduce a high degree of uncertainty in Subsequent calculations. Suchasituation led to a search for a method that could be used to help ensure high-quality R, selections. A method was found that integrated NMR and spontaneous potential (SP) measurements.

SP logs recorded in a West Texas waterflood exhibited enough sensitivity to indicate zones taking fluids in open-hole wells and in wells lined with fiber glass. These surveys are logged both when the wells are shut in and during injection, and the difference in values is the electrokinetic component of the measurable spontaneous potential, which is proportional to the rate of flow into each zone. Introduction In waterfloods such as the Wasson flood in West Texas, uniform injection of water into all productive zones is essential to efficient oil recovery. Wells in the Wasson San Andres field penetrate more than 200 ft of stratified dolomite section. Premature flood breakthrough in a small part of this section can cause producing wells to water out after recovering only producing wells to water out after recovering only a fraction of the total oil reserves. Therefore, profile logs such as flowmeter logs, temperature logs and radioactive-tracer surveys are periodically, run in injection wells so that zones of high intake can be isolated and those of low intake can be selectively pumped into at higher rates, pumped into at higher rates, Conventional profile surveys, however, are generally not reliable in those old wells completed open hole, shot with nitroglycerin, and later cased with an uncemented fiber glass liner. Uncemented liner and hole-size effects confuse the profile interpretations. These problem wells are best surveyed using an unconventional profile tool, the Spontaneous Potential (SP) Log. As implied by Gillingham, zonal differences between SP logs run in wells while they are shut in and during injection are related to flow rate. This technique has the sensitivity to profile most open-hole or fiber-glass-lined fresh water injection wells, but lacks the quantitative accuracy of radioactive-tracer and flowmeter surveys. Field Operation The SP device is the simplest logging tool available. It is basically a recording voltmeter with one electrode lowered inside the wellbore and the other electrode grounded at the surface. In profile logging, the downhole electrode is usually lead, weighted with 10 to 20 ft of insulated sinker bars and machined to pass through 2-in. tubing. The surface equipment includes a standard SP logging panel. The surface electrode consists of a simple electrical connection to the iron wellhead. This round connection is stable and common to most injection wells. Detectable galvanic potentials associated with unlike electrodes (lead and potentials associated with unlike electrodes (lead and iron) are constant on all logging runs in a particular well and are unimportant. The first step in SP profile logging is to survey the well while injecting at a normal stable injection rate. Use of a lubricator allows entering the well without interrupting the flow. The well is surveyed while logging both up and down until the curves repeat within about 3 mv. Runs in one direction occasionally have less noise and drift than in the other direction. In addition, fast logging speeds - greater than 100 ft/ min reduce drift caused by the relative water motion past the down-hole electrode and also reduce the past the down-hole electrode and also reduce the probability of recording, spurious noise. In practice, probability of recording, spurious noise. In practice, the bottom few feet of hole are not logged. Sometimes electrically charged sediment on bottom attaches to the sonde, causing drift as it later washes off. The next step requires surveying the well while it is shut in and stablized. The well is sufficiently stable when the SP signal from a stationary sonde, positioned opposite a zone that was taking fluid, positioned opposite a zone that was taking fluid, stablizes. This zone is chosen from known data or picked by trial and error. picked by trial and error. JPT P. 151

The pdf file of this paper is in Russian. The problem of estimation of low-resistivity reservoirs volumetric data is considered. Low-resistivity reservoirs are oil and gas saturated ones, which true resistivity is below its critical value at the oil-water interface. During tests it is possible to obtain substantial oil or oil with water inflow from low-resistivity reservoirs, which by the results of well logging materials interpretation are defined as water-saturated. Bigradient (divergent) method of the spontaneous polarization logging is developed in KogalymNIPIneft Branch of LUKOIL-Engineering LLC in Tyumen. The method differs from the existing method of measuring the spontaneous potential by higher sensitivity to the resistance of part of layer, not changed by penetration of drilling fluid, and by higher resolving capacity along the axis of the well (interlayer stratification). Hardware-methodical complex allows to measure spontaneous potential with new scheme, the first and the second differences of the spontaneous potential - with divergent logging method of L.M. Alpin. The original method of an oil and gas saturation index estimating of structurally complicated, including low-resistivity, reservoirs is developed. Also the variant of oil and gas saturation index estimating according to the standard spontaneous polarization logging in the complex with the electric methods of borehole survey according to proposed method is provided for. The example of interpretation of low-resistivity oil-saturated reservoirs of Jurassic deposits of Maloklyuchevoye field of LUKOIL-West Siberia LLC is given.

As a continuation of the earlier paper on the general subject of the SP log, amore complete analysis of certain features of the SP log in shaly sands isgiven. The pseudo-static SP in front of shaly sands is compared, on atheoretical basis, to the static SP in front of clean sands, as a function ofthe respective amount of shale and sand in the formation, and of the relativeresistivities of the shale, of the uncontaminated part of the sand, and of theinvaded zone of the sand. As a conclusion, the advantage of using reasonably conductive mud in this caseis shown. The discussion is illustrated by field examples. Introduction The discussion reported in the present paper is based on a theoreticalanalysis, and not on experiment. The field examples, joined to the text, areshown only as qualitative illustrations of the essential results of thisanalysis. Although the hypotheses made in the theoretical developments mayperhaps be somewhat improved, it seems, nevertheless, that the results obtainedaccount reasonably well for the actual phenomena, and give a fair approximationof their order of magnitude. The paper contains a mathematical analysis of a tri-dimensional distribution ofpotentials and current lines, due to spontaneous electromotive forces arisingat the contact of shales and free electrolytes, as a function of the geometryand of the respective resistivities of the different media involved. It isassumed, although this hypothesis is not proven, that the emf's remain the sameeven if the shale occurs in very thin layers or in dispersed particles. It has already been pointed out that, all other conditions being the same, thedeflection of the SP log in front of a shaly sand is smaller than opposite aclean sand. When the thickness and the conductivity of a clean sand are largeenough, the deflection of the SP log reaches a limiting value which is equal tothe "static SP" of the clean sand. It is generally convenient to takethe static SP of shale as the reference value or "base line." As aconsequence, and for the sake of abbreviation, the expression, "static SPof a clean sand," is often used to designate the difference between thestatic SP of that sand and that of the shales, which difference is a measure ofthe total electromotive forces involved in the chain mud sand-shale. T.P. 2912