Refracture candidate selection and treatment execution has always presented four main challenges to the industry when multiple pay zones are involved. The first challenge is determining the underlying cause of the poor production performance (poor reservoir quality, an ineffective original stimulation treatment, or both). The second challenge is the identification of the specific zones with significant remaining reserves that have been poorly stimulated. The third challenge is determining the current reservoir pressure in all of the prospective refracture candidate zones. The fourth major challenge is selectively stimulating the target zones in the wellbore when there are existing perforations above the target zones.
The first and second challenges are addressed using the "Completion Efficiency" (CE) or "Recovery Factor" (RF) techniques discussed in SPE 90483.1 These techniques use an integrated petrophysical, reservoir, and completion model to evaluate well and zone performance. The techniques has been employed on over 3000 zones to date in a wide variety of reservoirs with excellent results. The CE and RF analysis can be supplemented by rate-transient production decline analysis if adequate producing time has elapsed. The third challenge is addressed using selective zone buildup and/or injection tests to obtain reservoir pressure and permeability. The recommended technique uses a service rig and a low rate pump truck, tubing, packer, bridge plug, and a surface readout pressure gauge with downhole shutin. This technique has recently been employed on over 60 zones in the last two years in the Hugoton field in Kansas with excellent results. The fourth challenge is selectively isolating individual perforated intervals for the refracture treatments. The most efficient technique involves the use of openhole packer hardware set inside the cased hole. The openhole packer technology has been primarily deployed in horizontal openhole wellbores to date, however the hardware is readily adaptable to cased holes and has been used on a number of vertical cased holes to date (Table 1). The integration of these proven techniques (CE, RF, downhole shutin surface readout testing, and openhole packer hardware) provides the means to meet all of the main challenges of refracturing multiple zone wells. Field examples are provided to demonstrate the viability of the concept, and a methodology is proposed for "best practices" in the implementation of the techniques.