BOREHOLE IMAGES AND PRODUCTION LOGS RELATE FRACTURES AND DISCONTINUITIES TO PRODUCTIVITY IN CALIFORNIA RESERVOIRS

Fractured reservoirs are unconventional because of their ability to produce at high rates from relatively low permeability matrix rock. This characterizes a number of offshore and onshore California reservoirs, particularly from the Monterey Formation. A common property of the high rate wells is connection to a significant fracture network.

Fracture studies have traditionally relied on outcrops and cores, but these data sources limit us to developing conceptual reservoir analogs or crudely quantifying fracture parameters, respectively. Our work integrates these data with subsurface information derived from borehole image logs. When calibrated with cores, images vastly improve our ability to describe elements of the fault and fracture network, and quantify specific parameters. Image logs from high deviation and horizontal wells are particularly valuable for revealing spatial relationships of fracture features and other reservoir discontinuities.

Interval formation tests and production logs document the contribution of individual reservoir elements to production. Large-scale fractures and highly-fractured chert intervals often dominate the fluid flow, with a few point sources contributing the majority of influx to the well. These are responsible for the high initial oil rates characteristic of some wells, but also can be pathways for water and gas. Faults often enhance fracturing locally, but occasionally act as reservoir barriers. Optimal completion strategies must take into account various physical parameters in addition to the fracture network: fluids and fluid properties, reservoir pressures, drive mechanisms, and mitigating damage caused by drilling.