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A Petrophysical Method to Estimate Fractures From Standard Open-Hole Logs

Abstract

For a realistic approach of the potential of unconventional reservoirs, it is important to assess the degree of fracturing – both open and healed (cemented) fractures. The standard and most accurate approach is from interpretation of image logs. However, there are many areas where image logs have not been run, although there will probably be an abundance of standard open-hole logs. The procedure described here involves the interpretation of standard open-hole logs and consists of examining rates of change of curve magnitudes with depth. If the change to apparent high porosity cannot be reasonably explained as a consequence of depositional variations, an open fracture is assumed. Conversely, if the change is to low porosity, a healed (cemented) fracture is assumed. All log traces, as well as the caliper and density correction curves are examined. For each log, the interpreter defines a minimum change in curve magnitude from one depth increment to the next. Additionally, a minimum change over a defined depth window is defined. Results of fracture identification from individual logs are stacked to identify potential fracture clusters. By comparing this analysis with image logs over the same intervals, we have found that there is good correlation, especially if clusters occur. It is understood that log resolution does not allow identification of individual fractures. However fracture swarms can be recognized. Examples of the technique from a variety of reservoirs are included.