Effect of Stimulation on the Performance of Devonian Shale Gas Wells

Abstract

Effect of Stimulation on the Performance of Devonian Performance of Devonian Shale Gas Wells 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 permeability anisotropy and induced fracture wing length permeability anisotropy 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 natural gas in the U.S. Typically, a Devonian shale gas well must be stimulated to achieve economical production levels. Historically, the stimulation 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 some operators. 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 are caused by reservoir heterogeneity and the lack of a scientific basis for stimulation selection and design. To optimize production from Devonia 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 Devonian shale reservoir types. Several recent papers have addressed this topic. Most recent and of particular importance to this paper is the report by Kuuskraa and Wicks. They presented a comparative analysis of tailored-pulse fracturing, conventional hydraulic fracturing, and borehole shooting. Their study used the SUGAR-MD simulators and a reservoir description common to Meigs County, OH. They concluded (1) that permeability anisotropy is the key geologic factor affecting the relative success of hydraulic fracturing and tailored-pulse 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 were proved 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 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 from stimulated Devonian shale gas wells. Theory Explosive stimulation (borehole shooting) refers to a technique for improving well performance by detonating an explosive in the formation interval of interest. The purpose of- this explosion is to remove any near-well damage resulting from drilling and to increase the size of the wellbore. Using the concept of effective wellbore radius, rwa, we modeled the effects of wellbore shooting by replacing, the actual wellbore radius, rw, with rwa, in an analytic dualporosity reservoir model. The effective wellbore radius, rwa, is defined by Eq. 1:(1) where s is the dimensionless skin factor. The concept of effective wellbore radius is illustrated in Fig. 1. 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 al, for constant-pressure production from a finite, dualporosity reservoir. Application of this model requires a numerical inversion of the Serra et al. solution in Laplace space. A more detailed description of this model can be found in Ref. I 1. Many hydraulic fracturing techniques have been applied to Devonian shale wells. The goal of these fracture treatments is to create a conductive, vertical hydraulic fracture that will enhance or create communication between the wellbore and existing high-permeability features in the reservoir. We used a two-dimensional, one-phase, finite-difference reservoir simulator, FRACSIM, to study the theoretical effects of vertical hydraulic fractures on Devonian shale wells. To do this. we modeled the orientation, frequency, and permeability of natural fractures using directional permeabilities in a single-porosity system, as suggested by permeabilities in a single-porosity system, as suggested by Ramey. The geometric average permeability, k, is related to these directional permeabilities as indicated in Eq. 2 13: (2) Fig. 2 is a schematic showing how this model was used to simulate permeability anisotropy,. Permeability anisotropy occurs in the Devonian shale in various degrees as a result of variation of natural fracture spacing, conductivity, and geometry and other heterogeneities. We modeled parallel and orthogonal sets of natural fractures by varying the degree of permeability anisotropy. Variations in fracture spacing were handled by a variation in the average permeability, k. The new technology in stimulation of Devonian shale wells is tailored-pulse fracturing. Tailored-pulse fracturing involves the detonation of a "controlled" explosion in the wellbore. This controlled explosion generates a tailored pulse, which has several beneficial effects on the adjacent formation: (1) the peak radial stress of the pulse is below the flow stress of the rock: (2) the peak stress is pulse is below the flow stress of the rock: (2) the peak stress is above the tensile strength of the rock (3) the initial loading rate is large enough to generate fractures; SPEPE P. 250