Hydraulic Fracture Design Optimization for Deep Foothills Gas Wells
Authors: Reynolds M. , Shaw J. , Pollock B. 
Published: March 2010

The Deanne Glauconitic A pool is located approximately 80 km northwest of Rocky Mountain House, in the Alberta foothills. The reservoir is a thin, tight sandstone, located at a depth of approximately 3,500 m. In situ stress conditions in this area have been strongly influenced by the orogeny of the Rocky Mountains, with significant tectonic stresses imposed on the rock matrix. This high stress regime poses a major challenge to the very costly hydraulic fracturing treatments.

A review of off-setting fracture treatments in the area showed that approximately 50% of previous treatments suffered from premature screenout, with poor post-stimulation productivity. A comprehensive study was conducted to understand the cause(s) of previous failures, and to make recommendations for an optimal fracture design. Once the key fracturing design parameters were identified, a recompletion design was carried out, including a refracture treatment, which successfully placed all of the fracture sand.

This paper describes recommended procedures to optimize hydraulic fracture design for a deep gas reservoir in the foothills. These procedures include:Review off-setting fracture treatments;Identify the key drivers for successful sand placement;Examine core samples and design the fracture fluid, with emphasis on minimizing rock-fluid interactions;Model the hydraulic fracture using a 3D simulator;Size the fracture to optimize economic returns;Design the recompletion program and estimate costs;Implement the fracture treatment with stringent QA/QC processes; and,Post-fracture analysis by means of net pressure and radioactive logs to validate the fracture design.

Introduction

Field History

The Deanne Glauconitic A pool was discovered with the completion of the well NUL el al Deanne 11-03-39-11 W5M in April 1983. The reported initial pool pressure was 55,360 kPaa at an MPP depth of 3,446.5 mKB giving a reservoir pressure gradient of 16.1 kPa/m.

The well was tied in and placed on production in October 1987 at an initial rate of 100 103m3/d. Subsequently, a total of 10 wells have been completed in the Glauconitic zone in the immediate area, with a second pool (Glauconitic B) identified to the west of the original pool. The cumulative production from the Glauconitic A pool to February 2002 was 713.9 106m3 from a total of five producing wells. The reservoir has been produced by depletion drive, and the most recent available static reservoir pressure measurement was 28,160 kPaa as of June 2001.

The reservoir natural gas is sweet, condensate rich with high heating value. The gas specific gravity is approximately 0.68, consisting of 83% methane, 11% ethane, and 3% propane. The average condensate gas ratio is approximately 0.14 m3/103m3 (25 Bbl/MMscf).

Rock Properties and Reservoir Quality

The Glauconitic formation in the Deanne area has variable gross pay thickness between 4 and 8 m. The open hole log derived porosity in the pay section varies between 9 and 12% with some streaks up to 15%. The reservoir is under-saturated with average water saturation values in the 15 to 25% range. The reservoir permeability, derived by in situ methods, was in the range of 0.1 to 1 mD.