Optimizing infill drilling in a challenging environment

The Pinedale reservoir in Wyoming, USA, is characterized by a series of highly discontinuous, ultratight, stacked sand bodies. The reservoir interval averages approximately 5,000 ft [1,500 m]. Effectively producing from these sand bodies requires optimizing the well density to the effective drainage radius. From core studies substantiated with well pressure data, Ultra Petroleum Corporation estimated that the reservoir sand bodies range in height from 9 to 15 ft [3 to 5 m] and in width from 80 to 280 ft [24 to 85 m]. Sand body width represents approximately 14% of the initial-drilling 40-acre pattern. Optimizing the drilling pattern required imaging the sand bodies, but conventional seismic methods are limited by subsurface layer attenuation and low density contrast in highly compartmentalized sand-shale sequences.

Imaging reservoir compartments in high resolution

To meet these imaging challenges, DeepLook-CS service was selected to obtain seismic images between wells. DeepLook-CS service projects the seismic signal directly into the reservoir, so it does not travel through the highly attenuating unconsolidated near-surface layers. The result is much higher frequencies at the downhole receivers than can be achieved with surface seismic surveys. These higher frequencies (up to 3 kHz) deliver seismic imaging with 3- to 5-ft [0.9- to 2-m] vertical resolution.

In south Pinedale field, two crosswell seismic profiles (Well A–Well B and Well A–Well C) were recorded before hydraulic fracture stimulation in Well C. DeepLook-CS service surveys were performed over a 3,280-ft [1,000-m] interval in the wells, which are separated by 1,002 ft [305 m]. The reservoir heterogeneity of the imaged sand bodies was superimposed with fracture wing images from hydraulic fracturing monitoring to determine how the induced fracture is bounded within the reservoir.

Targeting infill drilling at optimal spacing

The results of each of the 20 stages of the hydraulic fracture stimulation job were superimposed onto the high-resolution seismic image from DeepLook-CS service. The resulting composite image clearly identifies the dimensions of both the reservoir sand body and hydraulic fracture wing extension (green).

Better understanding of the Pinedale reservoir from the superimposed images was used to identify an optimal infill drilling spacing of 10 acres, which improved the recovery factor from 17.5% for the initial 40-acre well density to 58.8% for the new 10-acre well density.