Typical hydraulic fracturing designs in shale utilize a predetermined fluid pump rate, which once achieved is held constant throughout the treatment, excluding situations when surface pressure limitations or other conditions disallow. We propose a method of pumping hydraulic fracture stages where the fluid pump rate is rapidly changed from the predetermined maximum rate, to some significantly lower rate, and then rapidly increased back to original maximum rate. This rapid change in the flow rate produces a pressure pulse that travels up and down the wellbore and has the capacity, together with the pump rate change, to open previously unopened perforations, while increasing fracture complexity through fluid diversion.
We observed increased microseismicity during hydraulic fracturing in stages with frequent pump rate changes. Regardless of their type and nature, seismic signals are indicative of fragmentation of the treated zone. This could be from shear shattering or dilatational opening. One can also assume that high signal density is a good measure of fracturing efficiency. To further investigate these observations, we implemented a variable pump rate fracture design in a Marcellus shale well. More specifically, we implemented the variable pump rate frac design in every odd stage, while implementing a constant rate design in every even stage. This was done in order to account for changes in the reservoir along the horizontal lateral.
Production log results showed on average a 19% increase in production for the variable pump rate stages versus the constant pump rate stages. A lower treating pressure was often encountered after the rapid rate changes, leading to the conclusion that unopened perforations were opened with the aid of the induced pressure pulses. Total well production decline was much slower for test well that included variable pump rate changes versus the offset horizontal well which did not include the variable pump rate frac design.
And finally water hammer frequency decay analysis shows a predictable trend in well with variable pump rate stages. Throughout the variable pump rate stages, no proppant transport issues were encountered and the frac stages were completed without any major issues.
Rapid rate changes applied throughout the fracture treatment enhance microseismicity, which could be interpreted as additional fracture complexity. Surface fracturing pressure data shows that rapid pump rate changes open additional perforations without physical flow diverters such as ball sealers or frac balls, while production log data shows higher production. Implementation of the Variable Rate hydraulic fracturing method results in no additional costs while it increases stimulation efficiency.