A Quantitative Approach To Analyze Fracture-Area Loss in Shale Gas Wells During Field Development and Restimulation
Authors: Sahai Vivek , Jackson Greg , Lawal Hamed , Abolo Nnamdi , Flores Cecilia P. 
Published: August 2015

The economic development of shale gas fields requires the reservoir volume to be stimulated by use of multiple hydraulic-fracture stages along the horizontal wellbore. To optimize the gas reserves, the stimulated reservoir volume needs to be maximized; this requires that offset wells be placed as close to the nearby wells as the fracture-stage lengths will allow. When offset or infill wells are drilled adjacent to existing wells, the new well's hydraulic fractures can interfere with or intersect the existing well's fractures; this reduces the existing well's production rate. This apparent fracture-area-loss event has been called a “frac hit(s)”. For various reasons, the operator may need to evaluate the economic impact of the frac hit(s) on the offset well. Likewise, the operator may need to evaluate the refracturing [refract(s)] of a well caused by providing an additional stimulation treatment to a well that should increase the well's gas-production performance. The presented analysis technique applies to either case.

A single well-analysis technique that quickly determines the economic impact of frac hit(s) or refrac(s) is presented. Sufficient production data before and after the frac hit or refrac need to be gathered to establish the internal linear transient flow periods on an inverse productivity-index (PI) vs. square-root-of-time plot. This rate-transient-analysis (RTA) technique has a straight line during the early portion of the data after well storage and well cleanup, which indicates an internal linear transient flow period for the prefrac-hit/prerefrac-hit and post-frac-hit/post-refrac data sets. These straight lines ensure that one can apply the RTA to provide the PI and, thus, the effective fracture areas, Af1 and Af2 or the before and after data sets, as is explained.

To verify the accuracy of the RTA results for the fracture-area reduction/gain, a numerical model is made with the known wellbore, fracture stage, and fluid-properties information, and a pressure history match is performed on the prefrac-hit data with historical production data to constrain well-production rates along with the fracture length that provide the determined effective fracture area. By use of the Af1 and Af2 values from the RTA analysis, a pressure match is achieved for the before and after frac-hit data. This verifies that the analytic RTA model provides good results for the fracture-area change caused by the frac hit.

One can forecast either a stochastic analytic model (Miller et al. 2010; Miller 2014) or the history-matched numerical model for several years with the field-operating conditions. With this matched model, one can add the post-frac-hit data, and one can achieve a pressure history match by adjusting the effective fracture area. One also can forecast this history-matched model for several years, and one can use the prefrac-hit and post-frac-hit/refrac forecast results to estimate the financial impact of the frac hit/refrac.

The single-well-analysis work flow is demonstrated with smoothed data that are based on a real frac-hit well. This simplifies the work-flow demonstration.

A sensitivity analysis was run to determine the effect of reservoir permeability on a frac hit. The results show that it has a very large impact at low permeability (&lt;25 nd). At 10 nd, a percentage loss of net present value (NPV) is the same as a percentage loss of fracture area (i.e., a 75% loss in frac area will give approximately a 75% loss in NPV). At high reservoir permeabilities (&gt;100 nd), the impact of a frac hit is tempered because of more flow from the matrix.

It is shown that fracture geometry (variation in the fracture-stage lengths) does not greatly affect the results to determine the fracture-area reduction caused by a frac hit.

A huge benefit of following the suggested work flow is that the RTA results provide limiting estimates of the reservoir permeability and fracture area that will save significant time history matching the prefrac-hit- and post-frac-hit data sets.