Fracture mapping and modeling optimize CBM fracture treatments: use of in-well tiltmeters and fracture modeling enabled the operator to achieve significant, per-well cost savings at two CBM fields in the western US

World Oil, June, 2004 by Lloyd Stutz, Kevin Fisher

Anadarko Petroleum Corp. employed advanced fracture mapping technology by using new, in-well tiltmeters and fracture modeling to optimize hydraulic fracture stimulation treatments in two coalbed methane (CBM) plays. The plays are in Utah's Helper field and the Copper Ridge field of southwestern Wyoming.

At Helper, data from tiltmeters proved that single-stage treatments could stimulate the entire multi-seam interval. Savings of $35,000 to $50,000/well were realized, compared to prior multiple-stage treatments. At Copper Ridge, treatments were optimized to stay away from permeable water sands. Savings from reduced water production were $1.3 million in the first year. In addition, individual well treatment costs are lower, ranging from $100,000 to $150,000 less per well. Anadarko's cost savings in stimulations and disposal for the 16-well, Copper Ridge pilot program exceeded the actual costs incurred for fracture treatments.

TILTMETERS FOR MAPPING FRACTURES

Several diagnostic technologies are commercially available to map hydraulic fracture growth. Surface tiltmeters measure fracture orientation, but there can be surface access issues. Tiltmeters or microseismic arrays can be used in offset wells, but those wells are often not available.

Fortunately, tiltmeter technology has been adapted, so that tools can be deployed in the well being treated. An array of downhole tiltmeter instruments is run into the wellbore prior to beginning the treatment. The tools are coupled to the wellbore with magnetic decentralizers. Pumping begins, and induced tilt is measured at each tool, allowing fracture height to be measured in real time. These real-time data are particularly critical for the complex fracturing often seen in CBM development. Fracture geometry is directly measured as a function of actual treatment parameters. The result is a calibrated fracture model that enables quick optimization of treatment designs.

HELPER FIELD

At Helper field, the targeted Ferron coal seams are interspersed with tight gas sands over a 250-ft interval. Early in its history, the standard industry practice for completing CBM wells at Helper had been multiple-stage treatments, to ensure that the entire interval was stimulated. Anadarko employed fracture mapping from Treatment Well Tiltmeter (TWT) data and fracture modeling, to evaluate and optimize perforation and hydraulic fracture stimulation treatments. The obvious driver was to obtain equivalent or better performance at lower cost by treating the interval with a single frac stage.

Anadarko employed TWTs on several wells, where proppant-free, crosslinked gel fracs were pumped through perforations in the interval's center. Data clearly show that the fracture on this well grew 180 ft (to 3,960 ft from 3,780 ft), to cover the majority of the pay interval from this single stage, Fig. 1. The log shows five to seven coal seams that would have required several stages to stimulate, using earlier frac design strategies. This well was perforated only in the middle seam (at 3,880 to 3,890 ft) and was fracture-stimulated in a single stage. The orange curve shows tilt magnitudes from the TWT array on this frac, demonstrating conclusively that the fracture height effectively covered all coal seams. The fracture top was measured at 3,780 ft, and the bottom at 3,960 ft.

Subsequent, other treatments successfully stimulated the entire interval in one stage. Production results are as good, or are slightly better, than neighboring wells fractured in multiple stages. Cost savings average $35,000 to $50,000/well.

COPPER RIDGE FIELD

Anadarko's CBM production at Copper Ridge field comes from the Almond coals. The Almond coals are present in 2- to 12-ft stringers spread over a 100-to 300-ft gross interval at depths ranging from 2,600 to 3,000 ft. The most prolific seams are usually perforated and fracture-treated in one or two stages using crosslinked gel with a combination of 16/30 and 20/40 sand.

The Ericsson sand, used for water disposal, underlies the coal sections, while higher-permeability water sands overlay the sections. An early pilot program to optimize hydraulic fracture stimulation treatments had several goals, including learning how to:

* Stay out of water zones

* Minimize the number of stages while still achieving complete interval stimulation

* Create long fractures with adequate conductivity.

Fracture mapping occurred on six wells, and fracture engineering and modeling were performed. Where fractures were mapped, growth was measured in real time. The measured fracture growth was then incorporated into the software program, FracproPT, along with pressures, volumes and injection rates. This allowed on-site adjustments to prevent fracturing into the adjacent water sands. The fracturing model was quickly optimized and used on all subsequent wells.

In a technology first, tiltmeter data were gathered while proppant was pumped. Previously, data had only been gathered during proppant-free stages, such as with mini-fracs, acid fracs and water-only fracs. To ensure that it could be done, the tools were first tested in Halliburton's flow loop. This process determined the safe operating envelope of pumping rate, viscosity, sand concentration and job duration, within which the tools and wireline could safely operate.