In recent years, there have been significant developments in the optimization of hydraulic fracture treatment of wells with one pay zone, but studies concerning treatment in wells with multiple zones are virtually nonexistent. Because a significant portion, it not a majority, of hydraulic fracture treatments are done in wells with multiple pay zones, discussions are presented to show (1) the use of limited technology leads to uneconomical depletion of the formation and (2) a model for effectively designing fracture treatment for multiple pay zones.
Three field examples are presented to illustrate the various uses of the model in optimizing hydraulic fracturing treatment of wells with multiple zones.
INTRODUCTION AND BACKGROUND
Approximately 35 to 40 percent of all new wells drilled in the United States require hydraulic fracturing for commercial production. Consequently, a demand exists for reliable techniques of hydraulic fracturing, in recent years, significant developments have taken place in the optimization of hydraulic fracture treatment of wells with one pay zone. Topics pertaining to fracture migration, adequate proppant transport, and three-dimensional hydraulic fracture geometry simulation of single zones have been extensively discussed and published. Similar studies concerning treatment in wells with multiple zones, however, are virtually nonexistent. The difficulty of using conventional single-zone treatment design on wells with multiple zones is also well documented.
Current methods of designing hydraulic fracture treatment for wells with multiple pay zones include limited entry, staging (with the use of diverting agents and ball sealers), and packing off zones (including techniques employing bridge plugs and sand plugback). A common assumption used in these methods considers all zones to open up and the fracture to propagate in a similar fashion. This assumption typically stems from not propagate in a similar fashion. This assumption typically stems from not knowing the variations in fracture gradient pressure (in-situ stress) of the various zones.
Limited entry permits fracture to be placed in all desirable parts of a reservoir and gives maximum control of stimulation fluids at each fracture point. In essence, it provides for the equal distribution of treating fluids through all perforations by (1) limiting the number and size of perforations, and (2) by controlling the differential pressure across the perforations. Correctly applied under the proper well conditions, limited entry can be extremely effective. Although this "basic definition" illustrates what limited entry is, the proper design and application of the technique needs to be specifically outlined.
All the multiple zone treatment techniques mentioned lack a very important aspect - the capability of quantitatively predicting the created fracture characteristics in each zone. Recent publications state that this limitation primarily is due to the lack of technology in fracture migration prediction of wells with multiple zones. Also, the unavailability of prediction of wells with multiple zones. Also, the unavailability of reliable and continuous fracture gradient pressure and mechanical properties data has hindered the commercial development of a fracture properties data has hindered the commercial development of a fracture migration predictive tool.
The multiple-zone fracture migration model reads in fracture gradient pressure, Poisson's ratio, and moduli data every 6 in. (15.2 cm) over the pressure, Poisson's ratio, and moduli data every 6 in. (15.2 cm) over the intervals of interest.