Evaluating Stimulation Effectiveness in Unconventional Gas Reservoirs
Authors: Cipolla C. L. , Lolon E. P , Dzubin B. 
Published: October 2009

This paper presents production evaluation criteria that can be used to compare the overall stimulation effectiveness in unconventional gas reservoirs. Characterizing the "relative" conductivity of the fracture network and primary fracture are critical to evaluating stimulation performance. Due to the uncertainty in matrix permeability and network fracture spacing (i.e. – complexity), it is difficult to find unique solutions when modeling production data in unconventional gas reservoirs. However, it may be sufficient to identify qualitative behaviors that can distinguish between key production mechanisms. This paper utilizes numerical reservoir simulation combined with advanced decline curve analyses to identify expected production signatures that can be used to evaluate stimulation effectiveness in unconventional gas reservoirs. The paper documents production signatures for low conductivity and high conductivity networks and primary fractures and discusses non-uniqueness issues with production data analysis in unconventional reservoirs. There are issues with non-unique production signatures and similar signatures may result from different combinations of matrix permeability, network block size and conductivity, and network size. However, the production signatures for high and low conductivity networks and primary fractures may be distinguished using advanced type curve graphing techniques. In addition, insights into total network size and block spacing may be possible.

The paper concludes with the evaluation of production data from 41 Barnett shale fracture treatments, including fracture and re-fracture treatments. The dataset included vertical and horizontal wells stimulated using water-fracs and cross-linked (XL) gel treatment designs. Four treatments included microseismic mapping to further constrain the interpretations. More than 60% of the XL gel treatments in vertical wells exhibited production signatures that could be interpreted as either (1) a high conductivity primary fracture connected to a low conductivity network or (2) a uniformly high conductivity network. The remaining XL gel treatments in vertical wells exhibited mostly moderate conductivity fracture network signatures. Most of the vertical wells re-fractured using slick-water treatments (water-fracs) exhibited low conductivity fracture network production signatures, with over 60% of the wells evaluated showing this signature and the remaining wells exhibiting moderate conductivity fracture network signatures. Although the initial XL gel treatments showed production signatures indicating higher fracture conductivity than the subsequent water-frac re-fracs, all of the water-frac re-fracs significantly improved gas production. Microseismic mapping has shown that water-frac restimulation treatments create larger and more complex fracture networks in the Barnett shale, contacting much more reservoir surface area, which explains the superior production despite the lower fracture conductivity. The majority of the horizontal wells, over 70%, showed moderate conductivity production signatures with the remaining 30% showing low conductivity signatures. The water-frac treatments in the horizontal wells appear to provide better relative fracture conductivity on average than the vertical well water-fracs. The improved network fracture conductivity could be due to design changes in the horizontal well treatments, smaller network block sizes resulting from the proximity of perforated intervals along the lateral, and/or a smaller network size being drained by each primary fracture along the lateral.

The ability to differentiate, even qualitatively, between various stimulation and completion approaches can result in much more reliable economic decisions and identification of deficiencies in current stimulation designs. The work to date indicates that improvements in network fracture conductivity and/or the conductivity of the primary hydraulic fracture could significantly improve well performance and gas recovery. This work will aid in identifying stimulation and completion techniques that create high relative conductivity fractures and/or fracture networks. These evaluations can be coupled with economic analyses to determine the most cost-effective method to stimulate production and increase gas recovery in unconventional gas reservoirs.