Field Trial Results: Investigating the Benefits of Increased Fracture Conductivity in the Low-Permeability Sandstones of the Pinedale Anticline, Western Wyoming
Authors: Vincent M. C. 
Published: December 2007

A field trial was initiated to investigate the effect of additional fracture conductivity upon production from the low-permeability sandstone of the Pinedale Anticline in western Wyoming. Forty million pounds of proppant were pumped into 221 stages within 14 new wells. Seventeen production logs were performed during the three-year trial to determine the productivity of fractures propped with sand, resin-coated sand, or ceramic proppants.

The trial was specifically designed to maintain identical stimulation strategy with the exception of proppant type used in each stage. Proppant selection by stage was adjusted to provide reliable offset comparisons.

In addition to comparing individual stage production, total well rates and estimated ultimate recovery (EUR) all indicate that production rates from fractures receiving upgraded proppant are significantly higher than production from similar stages in offset wells.

Detailed analyses have been conducted to normalize data for reservoir quality, pay, pressure, and drawdown. Regardless of whether raw production data or normalized results are considered, upgraded stages are found to provide 180% to 400% of the gas rate from identical stages receiving lower conductivity proppant.

Four trial wells which received upgraded proppant in 35% of the stages provided 44% higher peak rates from the composite well, and sustained higher rates after reaching 250 and 500 MMCF cumulative production milestones. Based on current production profiles, these four trial wells are expected to recover significantly higher EUR than offset wells completed by this or other operators.

Where possible, productivity of these wells will be compared to 47 additional wells in the area completed by other operators. Although the results achieved by other operators have not been normalized for reservoir thickness or permeability, production from the four trial wells receiving the highest proportion of ceramic proppant provided 155 to 214% the peak rates and 158 to 178% the EUR of wells completed by other operators in this area.

Results from this trial demonstrate benefits from improved fracture conductivity far beyond what is predicted by current production models, even after incorporation of non-Darcy, multiphase flow, and cyclic stress impacts. At a gas price of $4.50/mscf, the productivity of a stage only needs to be increased by ~5% to repay the full cost of upgrading proppant with the first year's incremental production. The incremental cost to upgrade a typical frac stage (175,000 lbs.) of resin-coated sand (RCS) to Economy Light Weight Ceramic (ELWC) was $17,500. The large production improvements observed in this study far exceeded economic hurdles, repaying the incremental investment within 20 days and generating $300,000 incremental cash flow in the first year for each upgraded stage.

Results indicate that treatments are not yet optimized, and further increases in fracture conductivity are merited. Based on these encouraging results, a well completed seven miles northwest of this trial area received ceramic proppant in all 19 stages, with no use of sand or resin coated proppants. This new "Gannett" well had only one offset, and therefore did not provide ideal well control. Nonetheless, this location was selected because it was on private land and not subject to the seasonal drilling closures stipulated by the BLM in the central Mesa area. This new well was drilled to a greater total depth, and presented more total pay than the single offset. However, based on the first 70 days of production, rates are inferior to the parent well and have not substantiated the large increase in production observed in the trial area. It was noted that the frac gradient exceeded 1.0 psi/ft in all stages in the new Gannett well. Additionally, this new well is producing 200-300 barrels of water per day, while the parent well treated at lower pressures, and produced a more typical 30-40 bwpd. Although the recent Gannett well has failed to replicate the success seen in this trial, two additional wells in the Mesa area have been designed to receive ceramic proppant in every stage.