Efectos de la simulacion en pozos perforados de shale gas

Effect of Stimulation on the Performance of Devonian Shale Gas Wells
M.D. Zuber, SPE, Texas A&M U.

W.J. Lee, SPE, Texas A&M U.
J.M. Galens III, SPE, SA Holditch & Assocs. Inc.

Summary. This paper presents the results of a study of the effect of various stimulation methods on Devonian shale reservoirs
and compares the economics of the stimulation alternatives. Formation permeabilityanisotropy and induced fracture wing length are the key factors that determine the optimal stimulation technique. Induced fracture conductivity and orientation' are also important.

Introduction
The Devonian shales are one of the largest potential sources of natur" al gas in the U.S. Typically, a Devonian shale gas well must be stimulated to achieve economical production levels. Historically, thestimulation method was to "shoot" the borehole with 80% gelatinated nitroglycerine. Hydraulic fracturing techniques have also been widely applied to stimulate Devonian shale gas wells. N2-foam fracturing methods currently are widely used, mainly because they minimize the amount of water contacting the formation. Straight N2 fracturing, with and without proppant, is also frequently used by someoperators. Tailored-pulse fracturing techniques are currently under development. The potential ability to generate radial fractures in the formation, and the absence of formation-damaging fluids, make tailored-pUlse fracturing attractive for stimulation of Devonian shale gas wells. The various stimulation techniques have been applied to Devonian shale gas wells with mixed results. These variations arecaused by reservoir heterogeneity and the lack of a scientific basis for stimulation selection and design. To optimize production from Devonian shale wells, the best stimulation method for each individual well must be chosen. To facilitate this choice, the effect of the different types of stimulation on Devonian shale well performance must be understood and compared over the wide range of Devonianshale reservoir types. Several recent papers have addressed this topic. 1-8 Most recent and of particular importance to this paper is the report by Kuuskraa and Wicks. 4 They presented a comparative analysis of tailored-pulse fracturing, conventional hydraulic fracturing, and borehole shooting. Their study used the SUGAR-MD simulator 9 and a reservoi~ description common to Meigs County, OH. Theyconcluded (1). that permeability anisotropy is the key geologic factor affecting the relative success of hydraulic fracturing and tailoredpulse fracturing techniques; (2) that borehole shooting is economically inferior to radial fractures of 30-ft [9-m] wing length; and (3) that induced fracture wing length is the key variable for selecting a fracturing technique. However, these calculations wereproved for only a single reservoir description. . The purpose of this paper is to present the results of a study of how various stimulation techniques affect Devonian shale gas well performance for a wide range of reservoir characteristics and to compare these stimulation techniques over this range of reservoir characteristics. This includes an analysis of actual field production data fromstimulated Devonian shale gas wells. the size of the wellbore. Using the concept of effective wel1bore radius, r wa ' we modeled the effects of wel1bore shooting by replacing the actual wellbore radius, r w, with rwa in an analytic dualporosity reservoir model. The effective wel1bore radius, rwa' is defined by Eq. I:
rwa=rwe- S ,
••••• ,., •••• , ••••••••• , ••••••••••••••

(1)

where s is thedimensionless skin factor. , The concept of effective wellbore radius is illustrated in Fig. I. A skin factor of zero represents the original wellbore, and a negative skin factor represents an enlarged wellbore. This analytic model is based on the solution presented by Serra et at, 10 for constant-pressure production from a finite, dualporosity reservoir. Application of this model requires a...