LFracture Acidizing With Foamed Acid: Success In the Crossfield D-l Carbonate
Authors: Petryk R.P. Bob , Goruk B.W. Bernie 
Published: March 2010

Fracture stimulations with foamed acid have been instrumental in significantly improving the producing rates of gas wells in the Crossfield D-1 formation. Because of improved cleanups after the treatment, well stimulations with foamed acids tend to give better results than conventional acid treatments, especially in reservoirs with declining pressures, such as the D-1. This paper discusses various aspects of job design and summarizes treatment results.

Introduction

Amoco Canada Petroleum Company Ltd. is Unit Operator of the East Crossfield D-1 Unit, located 25 miles (40 km) north of Calgary near the town of Crossfield (Fig. 1). The East Crossfield D-I field was discovered in 1960 and currently produces from 35 sour gas wells (Fig. 2). In 1968, the East Crossfield Plant was constructed and the field was Drought on stream. Although the plant is capable of processing 120 MMSCFD (3.4 × 106 m3/d) of sour gas, the field cannot support this inlet rate due to declining pressures and wellhead deliver abilities (Fig. 3). As a result, an extensive stimulation program is being carried out to increase deliverability and op? timize the drainage of economic reserves.

Geology and Reservoir Characteristics

The East Crossfield D-1 reservoir is made-up of the Crossfield member of the Stettler Formation in the Wabamun (D-1) Group and is of Upper Denovian Age. The Crossfield member is a sour gas producing zone which averages 100 ft (30.5 m) in thickness and varies in width from 2 to 12 miles (3.2 to 19.3 km). This geological feature extends in a north-south direction close to the Fifth Principle Meridian of Alberta from aids to Okotoks, a distance of about 90 miles (144.8 km).(1)

The average producing depth of the East Crossfield D-1 reservoir is 8800 ft (2682 m). The gas stream contains 39% H2S, 10% CO2 and 2% N2 (Table I). Approximately 4 barrels of condensate per MMSCF of gas (22.6 × 10−6 m3/m3) is also produced. The average reservoir temperature and pressure are 185 °F (85 °C) and 2050 psi (14,100 kPa), respectively.

The Crossfield member is a shelf deposit between intertidal and supratidal anhydrite deposits to the east and an open marine basin to the west(1) (Fig. 4). Gas comes from porous dolomitized stromatoporoidal and algal bank rock. Pores consist of vugs of algal origin and original stromatoporoidal organic porosity (vugular and pinpoint porosity) produced by ground-water solution.(2) Partial filling of the original pore spaces with calcite cement has greatly reduced total porosity and, in certain areas of the field, has reduced effective porosity. Average porosity and permeability(3) are 6.1% and 5.2 md respectively, with an average water saturation of 18%. Porosity footage and permeability footage isopachs of the D-1 for the East Crossfield field are shown by Figures 5 and 6 respectively.

Although the Crossfield D-1 reservoir has localized areas of high porosity and permeability due to vugs and natural fractures, drainage of a large portion of the reservoir is largely dependent upon matrix permeability which is generally low.