Stimulation of a naturally-fractured, low permeability, low-pressure 2000-foot horizontal well in a low permeability reservoir and in-situ stress environment requires careful stimulation fluid design to minimize the capillary retention of treatment fluids. Therefore, a systematic approach to stimulation design using N2, C02, and N2-foam was used to select one which is most efficient. Stimulation modeling was used to evaluate fracture geometry with particular concern for the minimum pressure rise above parting pressure required for height growth during frac fluid injection. Up to seven zones along the horizontal wellbore are available for stimulation. Each zone was ranked and pre-frac tested to establish pre-frac permeabilities. A N2 and N2-foam data frac was performed in one zone to establish leakoff characteristics. Subsequently, N2, C02, and N2-foam treatments were performed on a 400-foot zone to evaluate the effectiveness Of C02 versus N2 frac fluids. Both the data frac and subsequent stimulations were evaluated in the two least productive intervals in order to use the preferred fluids in the best zones in the reservoir. The post-treatment decline curves for N2 and C02 indicate a C02-based fluid treatment should be performed in the most productive interval to achieve maximum success. Results of the stimulation conducted are presented along with discussion of improvement ratios and potential utility to other horizontal drilling projects. Background The stimulation aspects of horizontal drilling represent a technical challenge in tight formations where the horizontal placement of a horizontal wellbore may not always provide adequate economic production. Little or no published literature exists on the mechanics of hydraulic fracturing of horizontal wells. Typically, long horizontal wells are completed with preperforated liners to preserve hole integrity. The disadvantage of this type of completion is the associated risk of pulling the liner at a later stage of production history and re-running and cementing a casing string such that selective placement of fracturing of fluids can be accomplished. An alternative approach is zone isolation accomplished by the installation of external casing packers and port collars as an integral part of a casing st ring run along the horizontal section. Such a completion arrangement provided stimulation intervals ready-made perforations injecting fracturing fluids into an open hole fracturing condition behind pipe. This was the method of completion used in this 2000 foot horizontal well to avoid problems of formation damage associated with cementing and to eliminate the need for tubing-conveyed perforating of numerous treatment intervals. The U.S. Department of Energy's Morgantown Energy Technology Center has been investigating the merits of drilling high angle wells for more than 20 years. Two high angle wells were completed in the Devonian Shale at 43 and 520 from vertical. Recent emphasis has been on the use of horizontal wellbores to enhance gas recovery efficiency -in tight formations. Initial study of horizontal drilling in fractured Devonian have in the Appalachian Basin involved selection of a geographic area followed by full-field reservoir simulation and initial well design. Once the cite was selected, computer software was used to examine drill string loads, design bottomhole assemblies, track well trajectory, and to provide daily reporting during drilling. P. 451

A series of stimulations were designed to open and propagate natural fractures known to exist along a 2000 foot horizontal well in Wayne County, West Virginia. The stimulations were also designed to induce fractures in the formation as well as propagate the natural fractures by manipulating pressure and injection rates. A number of radioactive tracers were used to determine where fractures were opened and propagated at different injection rates. The tracers were found in fractures in zones other than the one pumped into, a fact considered prima facie evidence that natural fractures with two or more orientations had been opened and propagated. Pressure testing and gas sampling of the isolated zones confirm that fracture communication was accomplished along nearly 1000 feet of borehole by stimulation of one 400 foot long section. A technique for inducing multiple hydraulic fractures with multiple orientations was demonstrated. Background Horizontal wells are drilled to solve production problems or to improve hydrocarbon production problems or to improve hydrocarbon recovery efficiency from a particular reservoir. It is believed that the original concept of drilling horizontal wells was to contact more formation in a reservoir which was not a particularly outstanding producer, or which had other production problems. One of the earliest known attempts problems. One of the earliest known attempts at horizontal drillin in th United States was that done in the Venango sandstone from a shaft drilled near Franklin, Pennsylvania in 1944. The particular problem being addressed was how to produce the heavy crude oil which had lost its solution gas because of the shallow depth of the formation (less than 50 feet) and thus was produced at slow rates. Later efforts addressed themselves to tight, low permeability shales and sandstones which were productive only as a function of the natural productive only as a function of the natural fractures which are conduits for the reservoir hydrocarbons. The ideal wa to improve recovery efficiency is to increase the total number of natural fractures which can be connected to a single wellbore for drainage. Thus the concept of drilling a well in a particular direction and attaining an inclined angle (40 to 90 degrees) was a logical means of improving upon the recovery efficiency of a vertical wellbore. A method of improving the gas recovery efficienc of inclined of horizontal wellbore in a naturally fractured reservoir is to extend the geometry and flow capacity of existing fractures as well as create new induced hydraulic fractures. This can best be accomplished by stimulating the natural fractures which exist in the reservoir by inflating them with a non-damagin fluid and propping the inflated fractures to maintain the propping the inflated fractures to maintain the enhanced flow capacity and to induce additional fractures by increasing the injection rate. This was the technical approach used to stimulate gas recovery from the Devonian shales in the horizontal well. This type of operation can be accomplished quite readily if the right geologic condition can be found. That condition is generally associated with normal or block faulted areas where multiple fracture directions are generated in association with the faulting. Other conditions where multiple fracture sets are generated are associated with thrust faulted areas. Several geologic settings were selected in Wayne and Lincoln Counties, West Virginia, which were known to have multiple fracture sets as a function of the pre-Cambrian rift-type basement faulting which produced the Rome Trough, and the test well was drilled in one of them. P. 27

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