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SPE 135683
Analysis of Nitrogen Stimulation Technique in Shallow CBM Formations A. (Tony) Settari and R.C. Bachman, TAURUS Reservoir Solutions Ltd., and P. Bothwell, Energy Resources Conservation Board of Alberta (ERCB)
Copyright 2010, Society of Petroleum Engineers
This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Florence, Italy, 19–22 September 2010.
This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.
Abstract
Nitrogen (N2) stimulation has become the preferred technique for stimulation of coal seams in the Horseshoe Canyon play in
Alberta. It consists of stimulating each seam by pumping at very high rates for short time (2-4 mins). Because the coal is producible at shallow depths, ERCB has been developing and updating regulatory guidelines that aim to protect the freshwater supply.
This study was undertaken to improve the understanding of the process and provide recommendations on the regulatory guidelines for shallow depth (less than 200 m). The study was provided with extensive data from the industry (over 20,000 fractures in over
2000 wells) and has carried out several types of analyses in order to estimate fracture orientation and dimensions, and their dependence on N2 injection rate, duration and reservoir parameters. This included statistical analysis of large amounts of surface pressure data, pressure transient analysis of downhole pressure data, analysis of fracture mapping data, and conceptual simulations
of the injection process using coupled reservoir and geomechanical models. Coupled geomechanical modeling provided realistic
physical model of the process (in contrast to conventional models). Stress dependence of coal permeability and permeability anisotropy were shown to be the controlling mechanisms. This model was then used to investigate height growth mechanisms.
After considering the results of the analysis, its limitations, uncertainties in geological description of the coal and shale sequences, available case histories, and other factors, recommendations were made for modifications of the ERCB Shallow fracturing Directive 27.
Introduction
The CBM development play in the Horseshoe Canyon of Alberta consists of essentially dry coal and it is produced in wide range of depths, including very shallow (less that 200 m) wells. Following drilling, logging, and casing, wells are perforated fom top to
bottom over log picked coal seam intervals. Coal seams are vertically isolated from one another by shales.The majority of wells
are stimulated using a unique technique consisting of injection of liquid (supercritical) N2 at very high rates for short times (1-4 minutes). No proppants are pumped. Stimulations are performed from bottom to top. Each stimulated interval is isolated by packers both above and below the targeted zone. Following a brief shut-in after the treatment, the packers are unseated, moved upwards to the next targeted zone, and reset. The fracturing of shallow wells is regulated by EUB Directive 27 entitled “Shallow Fracturing Operations - Interim Controls, Restricted Operations, and Technical Review”, which was issued January 31, 2006, and regulates shallow fracturing in Alberta for depth shallower than 200 m. Although this Directive was put in place primarily on order to protect groundwater during nitrogen stimulation of shallow CBM wells, it applies to any fracturing operations. Under the current Directive, licensees are prohibited from conducting fracturing within a 200 m radius of water wells whose depth is within
25 m of proposed well fracturing depth, in order to ensure that water wells are not affected.
This study was initiated in to determine if these guidelines are reasonable and sufficient in light of the large amount of data now available, and if necessary provide recommendations to the Shallow Fracturing Steering Committee of ERCB. Its main tasks were:
- Provide better insight into the mechanics of CBM fracturing by high-rate nitrogen injection
