A Double-Electrode Method of Spontaneous Potential Logging
Authors: Koerperich E.A. 
Published: December 2007

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