Successfull Acid Stimulation Of Deep Dolomitic Gas Reservoirs In The Canadian Foothills
Authors: Milligan M.R. , Roodhart L.P. , Van Hoof G. , Wong C.T. 
Published: March 2010

The Western Canadian Foothills contain deep gas bearing dolomitic reservoirs in the Devonian Wabamum and Mississippian Turner Valley formations. The reservoir matrix permeabilities can be low (less than 0.1 mD) and the in-situ rock stresses high, with fracture gradients up to 24 kPa/m. In 1989, three wells, completed in these reservoirs, were fracture-stimulated with massive pad-acid treatments which significantly increased well productivity. Production increases up to five times the pre-frac flow rates were achieved. That success was the result of comprehensive laboratory and engineering studies to select and design the optimum treatments, strict on-site quality control, careful planning and execution of the jobs.

Introduction

The Western Canadian Foothills gas trend is located on the eastern slopes of the Rocky Mountains and extends from the Alberta/ Montana border in the south up and into northeastern British Columbia. The Rocky Mountains and Foothills were formed during a time of major east-west compression of the sedimentary deposits or the western Canadian basin which also resulted in a series of northeast-southwest-trending folded and thrust-faulted structures at depth.

The Foothills play is characterized by relatively deep (3000 m to 6000 m), sour (H2S) natural gas reservoirs found in thrust faulted carbonate structures, predominantly dolomites of Mississippian and Devonian age in Alberta and of Triassic age in British Columbia. Shell has traditionally been and remains today an active player in the Foothills and has on production sour natural gas fields at Water-ion, Sorge, Jumping Pound, Burnt Timber, Limestone, Moose Mountain/Whiskey Creek, Panther River and Clearwater. In addition, recent Foothills discoveries have been made by Shell at Ram River in Alberta and Boulder in British Columbia. The productive formations in those fields are generally the Mississippian Turner Valley (Mt) and Pekisko (Mp), The Devonian Wabamun (Dwa), Nisku (Dn) and Reef (Dr) in Alberta, and the Triassic Baldonnel (TRb), Pardonet (TRp) and Charlie Lake (TRcl) in British Columbia.

The porous rocks comprising Foothills productive formations are primarily dolomites and have low in-situ matrix permeabilities from less than 0.1 mD to 10 mD, with an average of around 2 mD. Natural fractures appear to play an important role in enhancing the productivity of these low permeability formations. The degree of enhancement varies from reservoir to reservoir and in some fields from well to well. An interesting aspect of the natural fractures is that they are often in filled with calcite which is dissolved at a much faster rate by hydrochloric (HCI) acid than the dolomite matrix. The result is enhanced permeability along the etched natural fractures. This is important during acid fracturing treatments as it results in high acid leakoff from the created fracture faces and the formation of acid wormholes(1–2). Massive Acid Fracturing (MAF) treatments using alternating stages of non-reactive pad fluid and acid have proven successful in reducing leakoff into natural fractures(3).

Most wells in the Foothills respond well to conventional acid fracturing treatments(4). They generally comprise between 50 gal. to 250 gal. of HCI acid per foot of vertical pay(5).