Abstract

Multi-stage hydraulic fracturing (HF) of horizontal wells is at the heart of successful oil and gas production performance from tight and shale reservoirs.

Fractures generated during the initial HF completion and natural fractures, tend to close as a well goes on production due to the increase of net stress on the fractures. The fractures closure reduces permeability and consequently productivity of the stimulated well. This study shows that under favorable conditions the production performance of the well can be revitalized with the use of a refracturing job. But there are key questions that need to be addressed:

  1. When is the optimum time for refracturing?

  2. What is the increase in permeability, production rate and cumulative production performance that can be expected from the refracturing job?

  3. Is it better to refracture the well or to drill an infill well?

This paper addresses those three questions by considering multi-porosities known to exist in shale reservoirs. This includes inorganic matrix porosity (ϕm), natural fractures (microfractures and slot porosity, ϕ2), organic porosity (ϕorg) and adsorbed porosity (ϕads_c). In addition, hydraulic fracturing generates porosity around the wellbore (ϕhf). These porosities form a quintuple porosity system that is further fed by gas dissolved in solid kerogen.

The porosities mentioned above are included in a material balance that is combined with fracture closure for generating a model that calculates the optimum time for refracturing. Production rates and ultimate recoveries from this model and observations of actual refracturing jobs are compared with results from infill drilling. By considering the same reservoir properties and exactly the same hydrocarbons in place the conclusion is reached that refracturing has the potential to be more cost effective as compared with infill drilling.

The novelty of the approach is the development of an easy to use production performance method that can be reproduced readily in a spread sheet for calculating optimum re-fracturing time, production rates, and cumulative recovery; and for making quick comparisons of the benefits of refracturing vs. infill drilling in shale reservoirs. Results of the easy to use material balance are corroborated with a state of the art commercial reservoir simulator.

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