The Jonah field in Wyoming is a giant gas field producing from the low-permeability (0.024 md) Lance formation, a 2,800+ ft stacked sequence of 20 to 50 lenticular fluvial channel sands interbedded with associated overbank siltstone and floodplain shale deposits. Sandstone bodies occur as individual 10- to 25-ft thick channels and stacked-channel sequences that can be more than than 200-ft thick, in some cases. The sands are generally discontinuous and sandstone-body geometries are largely uncertain. Within this interval, the net-to-gross ratio varies from 25 to 40%. One of the most important challenges is to obtain effective fracture- height coverage over the entire Lance formation.
A study of fracture-height growth in hundreds of propped-fracture treatments was conducted using data from surface and downhole tiltmeter measurements and microseismic fracture mapping, for hundreds of wells. Thus data, plus results from numerous diagnostic fracture-injection tests (DFITs), demonstrated that fracture growth in the Lance formation is complex (not conventional bi-wing fractures), with fractures propagating primarily in a northwest direction and secondarily in a perpendicular northeast direction. Effective propped half-lengths are significantly smaller than measured hydraulic half-lengths and the production response suggests that only a fraction of the created fracture half-length is effectively contributing to the production response. Fracture length-to-height ratios average about 4, meaning that fracture half-lengths are twice as long as the fracture height. The combined evaluation of the microseismic and well-log data revealed a strong correlation between the effectiveness of the intervening shales as barriers to fracture-height growth and shale resistivity.
Fracture models predict how changes to a fracture treatment alter fracture geometry and fracture mapping provides a direct measurement of the fracture geometry resulting from a given treatment. The combination of conventional fracture models with direct measurements allows creation of calibrated fracture models that have superior predictive capabilities. A calibrated fracture model that ties the log analysis to the mapped fracture-growth behavior and to the net-pressure response measured during propped fracture treatments, was developed for the Jonah field. This model uses a “composite layering effect”, which is based on the log-measured deep-resistivity value, to limit fracture-height growth. A composite layering effect is consistent with the observation of complex fracture growth from the microseismic mapping and surface tiltmeter data. Log, core, and DFIT data were used to define model layers and their properties, e.g., Young’s modulus, permeability, and fracture closure stress.
Based on this model, a new staging strategy that uses the high-resistivity shale barriers as natural dividers between the stages, was developed. This strategy shows significant improvement in treating each interval separately because it results in reduced stage overlap, less height growth, and more growth in fracture half-length, for similar-sized treatments. A 3D fracture-growth model was modified to determine perforation strategy and fracture-treatment schedules needed to obtain effective coverage of the Lance formation in new Jonah wells, in a semi-automated process. A calibrated fracture-growth model can provide more predictable estimates of fracture-height growth and can optimize staging in wells with large pay intervals. The entire fracture-design process can now be performed in an integrated software package.
Fracture geometry prediction with standard model settings (left) and calibrated model settings (right).
SPE 116304 Development of a Calibrated Fracture-Growth Model and Automated Staging Routine for the Jonah Field
Scott Malone, SPE, and Mark Turner, SPE, EnCana Oil and Gas (USA), Inc.; Mike Mayerhofer, SPE, Pinnacle; Neill Northington, SPE, Carbo Ceramics; and Leen Weijers, SPE, Pinnacle