Fracture models provide the ability to predict how changes to a fracture treatment should alter fracture geometry, but suffer from a tenuous and generally unknown relationship with
Figure 1. Calibrated model matches microseimic mapping results. The east frac wing was not fully observed — fracture is assumed to be symmetric. (All graphics courtesy of Pinnacle Technologies) |
Overton field
The first example discusses the integration of microseismic fracture mapping with fracture modeling, well production results and offset well interference data in the Overton field of East Texas operated by Southwestern Energy (SPE Paper 95508). Target formation is the low-permeability gas-producing Taylor Cotton Valley at depths of about 11,500 ft (3,708 m). The fracturing program in this field included different types of waterfrac and linear gel hybrid frac treatments.
Microseismic fracture mapping indicated that created fractures in this area are very long, with half-lengths of about 1,500 ft (458 m). Figure 1 shows the mapping results along with the calibrated fracture model. Due to observation well position it was not possible to see the entire east wing of the fracture, so based on other mapping data in the Cotton Valley the fracture was assumed to be symmetric. Fracture modeling was performed using FracproPT, a calibrated 3-D fracture model. The model was calibrated with the microseismic mapping results by matching both the measured fracture geometry and the treating net pressures.
After determining the created fracture length, it is important to evaluate how much of the created length is actually effectively producing. There is compelling evidence in this area that
Figure 2. Example of production interference between two wells along the fracture orientation (N71E) indicates long conductive fracture lengths. |
Copper Ridge
The second example is from a coal reservoir in Wyoming and shows how calibrated fracture modeling can help with on-site fracture treatment design changes and optimization of future
fracture treatments (SPE Paper 84490). The Copper Ridge field is in southwest Wyoming near
Figure 3. Fracture height coverage and length for a two-stage completion strategy shows the final result of fracture optimization with the calibrated fracture model. |
In this project, measurements from treatment well tiltmeters which were deployed in the treatment well during the fracture treatment were integrated to build a calibrated fracture model. Treatment well tiltmeter mapping (TWTM) was performed on six wells, and fracture engineering and modeling was performed on all 16 wells in the pilot program. In addition to a general assessment of how these coals treat, the objective of the study was to optimize treatment designs in order to: 1) stay out of prolific water sands, 2) provide payzone coverage of all coal stringers, and 3) create long fractures with adequate conductivity. Fracture growth was monitored in real-time, and frac volume and injection rates were adjusted on site to avoid fracturing into any water sands.
After performing TWTM in the first well, calibrated model predictions indicated the potential danger of significant frac height growth into the overlying shore-face water sands with the planned 150,000-lb frac job. The 150,000-lb design was subsequently reduced by 50% on location after the minifrac. In addition, height growth also accelerated as pump rate was increased from 25 bbl/minute to 35 bbl/minute (original designs had planned to use a 40-bbl/minute pump rate). Therefore, pump rates were reduced to 25 bbl/minute for all subsequent treatments. Subsequently, the resulting calibrated model was generally able to predict fracture growth in future treatments. Variations from the design predictions were usually associated with variations in frac complexity and increased leakoff throughout the treatment, which is typical for complex coal formations with intermixed sands and shales. The fracture complexity used for future fracture designs was based on the average complexity observed in previous wells, since complexity can vary from well to well. Based on the initial calibrated modeling results, the completion strategy was modified on subsequent wells to include two-stage treatments in order to provide coverage of all prospective coal stringers while staying out of the overlying water sand. Figure 3 shows the height coverage of a two-stage completion, indicating that all coal stringers can be stimulated while avoiding the overlying shore-face water sand.
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