Effect Of Fracture Azimuth On Production With Application To The Wattenburg Gas Field

Abstract

Abstract The application of MHF stimulations to create deeply penetrating fractures in low permeability formations penetrating fractures in low permeability formations will require knowledge of fracture azimuth for optimum field development. A great deal of attention has been focused on field methods for determining azimuth but as yet there has been no quantitative estimation as to how critical this data will be. This need is addressed here and an approximate method is presented for calculating the potential for pressure interference due to variations in fracture azimuth. This method, which is based on theoretical pressure contours around a fractured well, is used to study the Wattenberg Gas Field and the effect of azimuth on recovery can be significant. Introduction Since its inception, some thirty years ago, the process of hydraulic fracturing has become an accepted process of hydraulic fracturing has become an accepted practice for well stimulation in the oil and gas practice for well stimulation in the oil and gas industry; and theoretical work has been advanced, and generally accepted, that this process creates (except in very shallow wells) a vertical parting in the earth, which propogates from the wellbore in two directions along a single azimuth line. The orientation and azimuth of these fractures has, in general, only been of academic interest as a tool for measuring the magnitude and orientation of the in-situ earth stresses. The actual azimuth of the fractures had little bearing on production, since the combination of relatively high permeability, and relatively short fracture lengths made permeability, and relatively short fracture lengths made the stimulation basically a very "local" wellbore effect, and did not significantly alter reservoir drainage. However, in recent years, this process has been extended to the creation of extremely long fractures in an effort to achieve enhanced recovery from low (micro-dary) permeability formations, and questions have been raised permeability formations, and questions have been raised as to how this will affect reservoir drainage. This report will attempt to partially quantify this effect, however the possibility of its importance its easy to visualize if the reservoir drawdown is pictured as seen Figure 1. This figure depicts model generated pressure contours surrounding wells in formations with two different permeabilities, and it is easy to see that the low permeability reservoir is draining a more-or-less elliptically shaped area, as opposed to the higher permeability case, where the drainage pattern is permeability case, where the drainage pattern is essentially unaffected by the fracture. In this second case, the drainage is virtually radial, and traditional well spacings and locations would be used. In recent years much work has been done attempting to predict or measure in-situ fracture azimuth and these attempts can be divided into four general categories:1)geological or tectonic analysis,2)oriented core analysis to determine mechanical properties or anisotropies,3)borehole measurements, properties or anisotropies,3)borehole measurements, and4)geophysical measurements or monitoring. Several techniques have now been developed to the point where fracture azimuth can be measured at least to moderate depths of 6-8000 feet (2–3 km). These include tiltmeter surveys, surface electric potential measurements, downhole passive seismic potential measurements, downhole passive seismic surveys, and oriented core analysis. Details of these procedures will not be discussed here since the results, confidence levels in these results, and their application of field planning is the primary goal. primary goal. Obviously, the goal of any reservoir planning is to extract the maximum hydrocarbon resource within economic limits. Such planning is complicated for low permeability reservoirs due to the eccentricity of permeability reservoirs due to the eccentricity of the drainage areas. This report will discuss a rational procedure which can be used to study the effect of this accentricity on field development, although final answers will require sophisticated multiwell modelling. However, an analysis such as the one presented here for Wattenberg can be used to study behavior and place bounds for various well patterns. Also, the general lack of knowledge of patterns. Also, the general lack of knowledge of interwell properties, permeability variations, etc. over large areas will make such modelling very difficult.