Abstract

Coalbed methane development continues to increase accounting for 8% of U.S. gas production in 2002. Many coalbed methane wells must be stimulated by hydraulic fracturing in order to achieve economic production rates. However, optimizing hydraulic fracture treatments in coalbed methane is difficult and advanced diagnostics can aid in the process. This paper discusses completion optimization for two coalbed methane reservoirs in the Rocky Mountains. Hydraulic fracture mapping was performed with treatment well tiltmeters on several wells in each reservoir. Treatment well tiltmeter fracture mapping is an emerging technology to optimize development of low permeability reservoirs. The fracture height measured with the treatment well tools was used to calibrate a fracture model for each field. The fracture mapping, calibrated model and on-site diagnostics were used to modify completion and stimulation strategy in these fields.

The completion strategy for both fields was fairly similar early on. Fracture mapping showed height growth through several coal stringers in both areas. There was some degree of containment in the laminated lithology bounding the pay packages but one field was more prone to height growth at higher pumping rates. Job size and injection rate were drastically reduced in this field because of the height growth tendencies and the presence of nearby water sands. This paper shows how the application of fracture diagnostics and engineering can reduce costs and help avoid costly errors. One example is shown with advanced diagnostics applied early in the development of a field and another example is shown with diagnostics applied after some development has already occurred. The differences in hydraulic fracture geometry that can occur between areas are also shown and how they create changes in completion strategy.

Introduction

Coalbed methane (CBM) development continues to expand in the U.S.1 Hydraulic fracturing is often necessary for commercial production of natural gas from coal beds. In recent years, much effort has been spent in the area of completion design, fluid chemistry, proppant selection and job procedures in an effort to optimize our understanding of stimulation behavior in coal bed environments.2–7.

Fracture diagnostics have drastically improved their capabilities over the last decade8–17. They now enable real-time measurements of how fractures actually grow. The newest technology is treatment well tiltmeter mapping8, where an array of downhole tiltmeters is lowered into the actual well being treated to measure fracture height, and in some cases fracture width. Once the model is successfully calibrated, it is used to develop relationships between fracture treatment size, height and length growth. Experience has shown that fracture modeling without proper calibration from actual growth measurements and net pressure behavior can lead to completely erroneous fracture geometry estimates11–14. This measurement of fracture height can be used to calibrate the hydraulic fracture model for each field.

Helper Field is located in Utah near the town of Price as shown in Figure 1. Full-scale development of this field began in 1997 following a multi-year pilot program. The primary completion target in the Helper Field is a series of coal seams that are found over a 250' thick interval at depths of 3000 ft to 4000 ft. This interval was typically stimulated in two or more stages in order to insure that the entire payzone was treated. Hydraulic fracture mapping and fracture engineering was performed in Helper in 2001 and 2002.

Copper Ridge Field is in Wyoming near the town of Rock Springs as shown in Figure 1. In 2002 Anadarko began evaluating development of the Almond coals in this field with a multi-well pilot program. The coals in the Copper Ridge field are usually present as multiple stringers over a 100 ft to 300 ft thick interval at depths of 2600 to 3000 ft. The coals in Helper and Copper Ridge were originally considered to be quite similar and therefore, completion and stimulation practices in Copper Ridge were based on the lessons learned at Helper. Hydraulic fracture mapping and fracture engineering was performed in Copper Ridge in 2002 and 2003.

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