Use of the SP Log in Waterflood Surveillance
Authors: Traugott M.O. 
Published: December 2007

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

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