Drilling and Fracturing Improvements for Low-Permeability Gas Wells in Western Canada

Abstract

Summary Laboratory and field research have developed better drilling and fracturig fluids for extremely water-sensitive gas-producing formations in western Canada. Other improved techniques include perforating in dry hole, stage fracturing using baffles and balls, higher rates, higher sand concentrations, larger volumes, and immediate flowback with carbon dioxide assist. Introduction A Map of the area involved in this study of gas fields in the Alberta basin of southern Calberta is shown in Fig. 1. The first commercial gas production in Alberta began in the Medicine Hat area in 1890.1 Since then, many advances in completion and stimulation techniques have been made in Canada. The intent of this study is to present some of these recent improvements. A study2 of the geology of the Alberta basin shows a stratigraphic column with sediments ranging from Pleistocene through Cambrian covering more than 7,000 ft (2133 m), including the most productive Upper Cretaceous. The Upper Cretaceous, from the base of the Fish Scale formation to the Milk River formation, is about 1,200 ft (366 m). The lithology consists of a thick marine shale containing several sandstone and siltstone zones. These zones provide the majority of the gas production. Three of the main producers - the Milk River and Medicine Hat formations, the Second White Specks sands, and productive zones such as the Glauconite and Cardium - are shown in Fig. 2. The Medicine Hat formation and the Second White Specks sands are offshore bar types deposited in a high-energy environment. Reservoir qualities are fair to good, with permeabilities ranging from 0.1 to 5 md. The Milk River formation, which is usually thicker and more widespread than the other zones, consists of fine-grained sediments. These sediments were transported beyond the high-energy environment and deposited in quieter waters offshore, resulting in thin-bedded sands with high silt content interbedded with shale. The Milk River sand porosities vary from 10 to 15% in the better sand stringers. The net thicknesses of productive Milk River formation vary from about 1 to 30 ft (0.3 to 9 m) thick, with as much as 300 ft (90 m) gross thickness. Gas formations in Alberta usually contain high clay content, as shown in the petrographic and X-ray diffraction analyses (Tables 1 and 2) of typical cores from the Milk River, Medicine Hat, and Fish Scale formations. The gas formations in Alberta may contain 20 to 30% mixed-layer clays containing smectite and other clays (Table 2), causing these formations to be extremely water sensitive. Clays can be observed in the scanning electron microscope micrographs shown in Figs. 3 and 4. Fig. 3 is a micrograph of a typical Milk River formation core showing a pore space infilled with and surrounded by mixed-layer clays and chlorite with a pocket of siderite at the bottom left corner. The trace of siderite, an iron carbonate, was not detected by the X-ray diffraction analysis. A micrograph of Medicine Hat formation core, Fig. 4 shows a high concentration of mixed-layer clay with some kaolinite, illite, chlorite, calcite, and dolomite scattered throughout. The sand grains appear to be heavily coated with clays. The pore space is partially filled with mixed-layer and kaolinite clays.