Abstract
One of the most common field development plans in shale plays involves drilling lease/acreage retention wells in different areas followed by coming back and drilling infill wells. In majority of these shale plays, job sizes for hydraulic fracturing treatment are getting bigger over time in order to achieve more volume as well as value. However, due to depletion, there exists a pressure sink around the older existing producers and that significantly increases the possibility of older well getting "frac-hit" by new stimulation, and receiving large volume of frac fluid. Frac-hits can easily be seen using pressure gauges in older wells, and other surveillance techniques including chemical/RA tracers, microseismic, etc. One of the other "interference" effects that change the behaviour of parent well is refracturing. Operators are identifying candidates that were either poorly stimulated initially or have lost productivity over their life, and are refracturing those wells. In both frac-hits and refracs, there is a change in well productivity; and understanding and quantifying this loss/gain in production still remains challenging.
In this paper, both analytical and numerical modeling techniques were used to explore the existing workflows and techniques in the literature to study frac-hits and refracs. Rate transient analysis (RTA) was used to complement the numerical reservoir simulation models. Flow regimes were identified on superposition time plots using RTA (linear flow regime, boundary-dominated regime), and numerous sensitivities were run on the frac-hit/refrac timing, reservoir matrix permeability. Frac-hit examples from Eagle Ford shale were examined.
This paper studies the existing RTA techniques to model frac-hits/refracs and compares them with the new technique proposed herein. It was observed that RTA models needed to be re-initialized to model post frac-hit or refrac behaviour to correctly quantify the changes in SRV. It is shown that although the existing techniques work reasonably well at low matrix permeability, the error margin goes up as the permeability increases. In Eagle Ford, the permeability is high enough to warrant using this new analysis method.
Existing analyses methods primarily use diagnostic plots (superposition time) and are only applicable to frac-hits or refracs prior to boundary dominated flow (BDF) regime. This proposed method is valid over different flow regimes & larger permeability ranges. The analysis method recommended in this study allows the operators to better analyse the efficiency and benefits of their refracs, as well as detrimental impact of frac-hits from infilling and downspacing the wells.
