Calibrating Coal Bed Methane Fracture Geometry in the Helper Utah Field Using Treatment Well Tiltmeters
Authors: Stutz H. L. , Victor D. J. , Fisher M. K. , Griffin L. G. , Weijers L. 
Published: December 2007

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. Coal bed gas is an emerging and important resource: over the ten-year period from 1990 to 2000, the number of wells producing coalbed gas in the U.S. Lower 48 has increased from 2,982 to 13,985 and annual production from these wells increased from 195 Bcf to 1352 Bcf (more than 7% of U.S. dry gas production).

The standard industry practice for the past 5 years in the Helper Utah Field has been to stimulate coals in multiple stages in order to ensure that all zones were adequately stimulated. Even though many indicators and models implied "uncontrolled" fracture height growth, there is still much discussion on whether or not height coverage is lost to a horizontal "slippage" component. The usefulness of threedimensional hydraulic fracture simulators to model and predict fracture geometry in coal-beds has been restricted by a general lack of knowledge about fracture growth and dimensions in these unconventional reservoirs.

There is also an historical paradigm about some coals being natural height "barriers". Additionally, uncertainty exists concerning the impact on fracture height growth due to the numerous lithologic changes found over the gross pay interval.

The Helper Field contains multiple sand and shale sequences between coal beds so multi-staging has been the only way to ensure complete coverage of the gross interval. Several technologies have been applied, some routinely, in an attempt to better understand fracture geometry in coals: including fiber optic temperature logging, radioactive tracers, dipole sonic logs, pump-in fall-off tests, and tiltmeter fracture mapping. The logging measurements normally measure rock or fracture properties only very near the borehole, while the far-field mapping techniques are sometimes hampered by lack of offset monitor well locations close enough to properly image the fracture.

Today, tiltmeter instruments are available which can measure fracture height and width in real-time from within the treatment well itself, eliminating the need for a nearby monitor well. This technology was applied on fracture treatments in the Ferron Coals in the Helper Field near Price, Utah and the measured fracture geometry results were then fed back into a 3-D fracture simulator which integrated measured geometries along with net pressure and other treatment parameters to build a calibrated fracture model honoring all real-data and realistically reflecting the fracture behavior in this reservoir. The results clearly indicate that adequate height is being created to cover large pay sections through a single, small perforated interval. This reduces stimulation and completion costs as compared to the multi-stage technique. Additionally, the limited interval perforating technique minimizes the number of fractures and reduces fracture complexity.

Measured fracture geometry and the procedures used to calibrate the fracture simulator will be presented along with results of how these fracture treatments were optimized.