A method has been developed for the analysis of pressure falloff data following a single-stage treatment in a multi-stage fracture stimulation. The basic premise is that the greater the permeability contacted by the fracture stimulation, the greater the rate of pressure falloff will be. This can be done with as little as 15 minutes of falloff data, but with a “zipper” style completion, the surface pressure falloff of a given fracture stage may be monitored for several hours for no incremental cost while an offset well on the same pad is being stimulated. The initial falloff data is collected well before fracture closure, so proppant is not yet a factor – the pressure decay is influenced by the total fracture system of that stage.
This analysis has been performed on approximately 30 wells, each with about 20 stages, including two wells equipped with fiber optic sensing. The pressure decay follows a straight line on a plot of pressure versus logarithm of time. The slope of that line is the decay exponent, and a large exponent is indicative of greater connected permeability or fracture complexity.
The development of this technique is in its early phases, but thus far a good correlation has been observed between the pressure decay exponent and microseismic activity, as well as between pressure decay and the Young's modulus of the rock being stimulated. In a multi-cluster “plug and perf” completion equipped with fiber optic cable, a positive correlation was observed with the number of clusters being treated. When the same hydraulic fracture stimulation was executed in similar rock types, very consistent results were obtained, suggesting a valid and repeatable relationship. The final validation of this technique will be possible when compared against production logging results.
The prospect of a low cost, or even free, analytical technique in an environment where anything beyond a gamma ray curve is often a luxury, is particularly exciting. This assessment technique could be used for optimization of perforation cluster design and location, landing zone, and fracturing fluid optimization. The authors invite other operators to try this technique and discuss their observations.
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