Minimizing Multiple Fractures in Coalbed Methane Fracturing Operations

Abstract

This case history paper describes a technique that minimizes the creation of multiple fractures in deep coal seams during hydraulic fracturing operations. The technique enabled the operator and the service company to perform 24 nitrogen foam-fluid fracturing treatments in 33 days in the Upper Silesian basin (Poland). The creation of multiple fractures is a phenomenon often encountered during fracturing operations in coal seams and is the most probable cause of early screenouts. During this fracturing campaign, early screenouts were observed at sand concentrations less than 3 lb/gal. These screenouts were attributed to a lack of fracture width caused by the presence of multiple fractures, including possible horizontal fracture components or tortuosity. The operator and the service company developed a technique that involves pumping a small volume of highly viscous crosslinked gel before the main fracturing treatment. This technique prevented early screenouts and allowed the higher sand concentrations to be pumped. Introduction The operator won a concession to explore for coalbed methane in the Upper Silesian Basin of southern Poland in September 1993 (Figure 1). Following negotiation of various terms, the concession became effective in late August 1994 and drilling commenced in November 1994. The project targeted the coal-bearing strata of the Carboniferous era. Various seams of these strata have been actively mined for over one-hundred years in this area of Poland, with numerous shows of gas, fires and explosions. The concession consists of 487 km2 just south of the town of Katowice with active coal mines immediately to the east of the concession, and one active coal mine (Silesia Mine) surrounded by the concession area (Figure 2). The objective of this work was to sample the coalbed methane productivity of the concession in order to determine the economic potential for a development project that would consist of a couple hundred wells. The stratigraphy in the area consists of over 60 coal seams of 0.5-m or greater thickness over an approximately 1000-m interval. A statistical sampling was needed because of three reasons:–the mandate to evaluate the resource in a period of less than three years with a total of only 15 wellbores–the significant amount of coal–the anticipated variability of various coalbed methane parameters such as gas capacity, saturation, permeability, and production rate Based on the operator's experience in the San Juan and Warrior Basins, fracture stimulations were the preferred completion method for coalbed methane wells. Historically there had been very few fracture stimulations performed in Poland. and no fracturing equipment or experienced crews were available. With respect to perforating, there was no referencing of performance data to API standards. Thus the project needed to import these services with the associated high mobilization charges. Because of this high fixed-cost component, individual seam or zone completions were too costly and multiple-zone well completion campaigns were chosen. To reduce mobilization costs, a campaign approach for treating the selected intervals was chosen. This paper describes which problems were encountered during a campaign that involved treating 24 intervals in six of the eight test wells and how these problems were solved. The original plan for evaluation of the concession provided for seven coreholes to determine the basic gas capacity and isotherm information, and eight test wells for production testing. P. 161^