ABSTRACT

Most service companies that provide dipmeter logging services also provide special log presentations to facilitate fracture identification. These presentations usually display the individual microresistivity curves overlaid in pairs. Steeply-dipping conductivity anomalies associated with fractures normally result in separation of the curves. Unfortunately, not all separation observed on these presentations can be attributed to fractures. Based on numerous comparisons of dipmeter logs with core and borehole televiewer data a set of guidelines was developed for collecting data and interpreting fractures using dipmeter logs. Data collection criteria include the use of expanded-scale monitor logs (25in/100 ft) and variation of the emitter-exciter voltage (EMEX) to maintain a high degree of curve activity, especially in highly-resistive units. For interpretation of conventional four-arm, four-button dipmeters, we prefer to present all six possible curve pairs on the log. With the four-arm, eight-button dipmeter, we prefer interpreting only the dipmeter fracture logs generated by overlaying the microresistivity curves from the same pad. In interpreting it is important to pay close attention to the borehole caliper data. We use the borehole caliper to identify the azimuthal orientation of borehole breakouts and to discount separation on tool pads that are aligned with these breakouts. Where separation is associated with pads located at right angles to the borehole breakouts, it is highly probable that a near-vertical extension fracture has been traversed. Curve separation can result from causes other than fractures; these causes include borehole spalling or irregularities, tool pulls, floating pads, electrode insulator failures, incomplete response to thin beds, thinning beds, clay clasts, and pebbles. Because curve separation is not a unique signal of fracturing, the most effective use of the dipmeter is as a means to condemn those zones where no fractures (no separation) are evident and to high-grade those zones that are possibly fractures (separation evident) for further testing.

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