Streaming Potential Applications in Oil Fields

Abstract

Abstract Two successful field tests of streaming potential measurements in oil fields have been carried out: one in a horizontal oil production well and one in a vertical water injection well. Pressure transients were created and the streaming potentials generated by these transients were measured by arrays of permanent electrodes placed in the boreholes. The electrodes are partially insulated from the other completion components but nonetheless record high signal-to-noise ratio responses. These field experiments have demonstrated that the streaming potentials arising from pressure transients can be measured accurately under borehole conditions. Numerical methods have been developed to interpret these measurements to offer distributed characterization of matrix permeability at various depths of investigation and effective fracture transmissibility. While streaming potentials have been observed in many laboratory rock experiments, we believe these are the first streaming potential transients to be measured in an oil field. The field experiment results, together with forward and inverse modeling studies, indicate many possible applications of transient streaming potentials in oil and gas fields for reservoir characterization and reservoir monitoring. Streaming Potential In reservoir rocks there exists a thin charged double layer at the interface between the rock matrix and the water in the pores. The matrix surface is usually negatively charged, and the water is positively charged. When the water moves under a pressure gradient, an electrical current is generated. This electric current is the source of the streaming potential. Experimental Installations Two separate experiments have been undertaken in shallow producing oil fields in Indiana, U.S.A., to test the utility and reliability of electrode arrays to measure streaming potential. Vertical Well Cemented Arrays Electrode arrays were permanently installed in a water injection well and oil production well in a Mansfield sandstone reservoir in Indiana. 1,2 A 16-electrode array was cemented into the annulus of each well (Fig. 1). The lowest eight joints of casing were coated to insulate the casing and prevent short-circuiting of the electrode array through the casing. The electrodes were mounted outside the insulation. They were in electrical contact with the cement but not with the metal casing. The casing was perforated with oriented perforations so as not to damage the electrodes and the connecting cables. After perforation, electrical current leaked through the perforation holes to the metal casing. The electrical insulation of the casing was imperfect but functional, as indictated by field test results. Horizontal Well Sand Control Completion A second field experiment utilized a 21-electrode array deployed in a horizontal well that was drilled in a thin oil column within a Cypress formation sandstone reservoir. 3,4 The well was completed openhole with sand screens and a gravel pack. External casing packers subdivided the annulus into three zones. An electrical valve, which also recorded the annular and tubing pressure, controlled inflow to each zone (Fig. 2). The 21 centralizers acted as electrodes to form an electrode array that spanned the 694-ft-long completion. Each electrode was connected to a single conductor that was linked to the surface acquisition unit so that there was no requirement for downhole electronics. There were 7 electrodes in each zone at a spacing of 20 ft. A schematic drawing of the electrode array mounted on the outside of insulated joint sections of the sand-screen completion is shown in Fig. 3. Individual screens are joined together to form the completion string. The screen sections cannot be electrically insulated from the formation and the annular fluid. However, the joint sections between the screens were insulated. Mounted in the middle of each joint section was a centralizer. The weight of the completion string insured that each centralizer acted as an electrode that was always in good contact with the formation.