Moving Beyond the “Drill Here” Map: Interpreting Seismic Based Fracture Information Using Geologic Analogues and Discrete Fracture Network Models
GMG-AXIS Geophysics, Denver, CO
New developments in seismic acquisition, processing and interpretation can reduce the uncertainty when developing naturally fractured reservoirs. However, approaches which assume that seismic attributes can be translated from fracture intensity into “drill here” maps implicitly assume that high fracture intensity equals high productivity – and as a result frequently fail. In fractured reservoirs connectivity is the critical issue. Well performance is not determined by the fractures intersected by a well, but by what those fractures are connected to.
Fractures influence seismic by changing the response in isotropic velocity, amplitude vs. offset, and pressure induced velocity anomalies. Sometimes these changes enable direct imaging of pressure and fracture attributes. However, simply understanding these attributes are not sufficient to develop an effective drilling strategy in most naturally fractured reservoirs. Recently examples in the Rockies have shown areas where fracture intensity increases close to faults. However, in the area immediately adjacent to faults the high fracture intensity was associated with lower production, while the best production occurred at intermediate fracture intensities (Miskimins and Knight, 2003). Analysis of this type of reservoir would fail if the seismic attributes that predicted fracture intensity were interpreted under the assumption that fracture intensity = productivity.
A case study is presented showing varying predictions that would be made using the same fracture indicators from seismic data, with and without applying recently developed geologic models and mapped analogs to fractured Rocky Mountain reservoirs (Flodin 2003 and Sternlof et. al. 2003). The influence of fractures on seismic data and the influence of geologic models on the final interpretation will be characterized for a tight gas sand reservoir in the Rockies.