This paper describes an experiment to image hydraulic fractures in the Cotton Valley sands at approximately 9,500' depth. The purpose is to determine key geometrical attributes of those hydraulic fractures. A pilot study of one monitor well with a single geophone at normal well spacing and preliminary results from a more detailed study involving two monitor wells with arrays of geophones are presented. In the pilot study, it is shown that microseismic events induced by hydraulic fracturing at a depth of 9,500' were detected at distances exceeding 1,300' from the origin. Analysis of results indicated that a more complex geophone array was necessary. An industry consortium of major operators, service companies, government agencies and national laboratories was formed to perform a more detailed study using 96, 3-component geophone sondes distributed among two monitor wells. These monitor wells offset the treatment well using nominal 80 acre spacing rules and will be used as producers. A unique microseismic recording system specifically designed and manufactured for these operating depths (~9,500') is presented. Imaging is accomplished by using various data analysis techniques including real-time event location. Applications of the technology include optimization of hydraulic fracture designs and accurate mapping of fracture geometry. Fracture models will be developed to accurately depict fracture growth as defined by the finding of this project. Introduction Hydraulic fracturing is the primary completion technique for the Cotton Valley formation of East Texas. Operators in the East Texas area will make major investments in hydraulic fracturing during future years. Technology that accurately depicts geometrical attributes of hydraulic fractures will have a major impact on the future development of tight gas reservoirs. The fracture imaging study in the Carthage Cotton Valley Field, Panola County, Texas is focused on the determination of certain key geometrical attributes of a hydraulic fracture - height, length and fracture azimuth. Information regarding the internal structure and real time development of the created fractures is gathered. A consortium was formed with operators active in the Cotton Valley fields of East Texas. The project was driven primarily by the operations groups of each company and kept on track by cost control and process management. The primary data collected in this study are seismograms associated with micro earthquakes induced by hydraulic fracturing. Microseismic data provide the only source of information with the spatial resolution suitable for the imaging of fractures away from the borehole. Additionally, velocity data sets are being developed through a crosswell, tomography study to aid in the imaging process. Event location software is being utilized to analyze the above data and produce the required geometrical attributes of the fracture. The analysis consists of matching recorded arrival times with modeled times derived from a rate-traced velocity model calibrated locally with checkshot data. A detailed velocity model derived from the crosswell survey is utilized. Modeled travel times for both compressional and shear arrivals are matched in a least squares sense to recorded compressional and shear first arrivals. One of the benefits of having multiple data analyses is the ability to compare the reliability of the various analysis methods employed, and so determine the most efficient field procedure and event location technique combination for future operations in a normal production environment and at normal well spacings. Three wells were utilized in the experiment: one injection well and two seismic monitor wells. P. 131^

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Fracturing treatments using treated water and very low proppant concentrations ("waterfracs") have proven to be surprisingly successful in the East Texas Cotton Valley sand. This paper presents field and production data from such treatments and compares them to conventional frac jobs. We also propose possible explanations for why this process works. Introduction Hydraulic fracturing is the key technology to develop tight oil and gas reservoirs. Although millions of research dollars have been spent to date, much controversy remains about optimizing fracture design. Rock mechanics and fluid transport phenomena in hydraulic fracturing are still poorly understood. The processes are very complex with a host of unknowns. Measuring even one critical value such as net fracture treating pressure constitutes a difficult problem. Hydraulic fracture research and development has put a lot of effort into effective placement of propping agents to provide and maintain fracture conductivity. For this purpose the service industry has developed sophisticated fracturing fluid systems and an extensive recipe of chemical additives. The fluid system is engineered to change viscosity during its journey from the surface to the fracture and afterwards during fracture cleanup. The sole reasons for these fluid designs is to place proppant, minimize formation damage and ensure proper cleanup. In turn, the proppant has no function other than maintaining a conductive fracture during well production. What would happen though if the fracture actually retains adequate conductivity with very little or no proppant?–Rock fractures often have rough surfaces. After the fracture closes, the residual aperture distribution can be very heterogeneous in all three dimensions forming a very conductive path even at high closure stresses. - Proppant along with gel residue could actually impair fracture permeability and its ability to cleanup.–Fracture extension and cleanup is easier to achieve with low viscosity fluids. Fracture extension is the key design parameter in tight reservoirs. The above points may have a tremendous impact on the fracturing operation. Gelling agents, proppant and associated chemical additives comprise a large part of fracturing costs. In early literature, "self-propping" and "partial monolayers" of fractures has been discussed. In general though, the industry has discarded the idea. In the naturally fractured Austin Chalk the so-called "waterfrac treatments" are pumped with no propping agents. They are very successful. Why it works is still generally unknown. The hydromechanical response of natural fractures has been addressed in rock mechanics literature. It is an extremely important issue in the field of underground nuclear waste disposal. The effect of normal stress and shear stresses on a fracture (natural and artificial) dictate its conductivity. The ramifications of these forces on fracture propagation are just now beginning to be investigated (multiple fractures). Description of "Waterfracs" The following outlines the general pumping schedule (from here on, the treatments will be referred to as "waterfracs"). P. 457

In SPE 38611 "Proppants, We Don't Need No Proppants" data showed that fracture treatments using treated water and very low proppant concentrations (waterfracs) were very successful in the East Texas Cotton Valley sandstone. The paper presented limited initial results from one operator in one field. Following this paper a more comprehensive set of production comparisons of wells completed with standard frac jobs and waterfracs since 1996 for several different operators in the East Texas area are presented. Analysis of offset comparisons, economics, and other benefits are described from the aspect of several different operators. Conclusions will point out the cost savings and the ability to exploit marginal reserves with this technique. There will also be a perspective from each operator. The waterfrac technique has led to widespread discussions among many operators in various tight gas plays. Many operators are experimenting with the technique and experiencing excellent results. This technique is a major contribution to the reduction in completion costs in wells that must be hydraulically fractured. The industry has experienced a major inflation of well construction costs and this technique will be of paramount importance in our efforts to keep costs down in order to continue to develop tight gas reserves. Techniques such as these require many months of production in order to analyze to determine the actual results. Several techniques and the perspectives of several operators and how they make these important decisions will be presented here. P. 497

SPE Paper 38577 introduced the Carthage Cotton Valley Hydraulic Fracture Imaging Project and described the initial imaging work. This paper describes the most recent methodology and implications of imaging hydraulic fractures utilizing microseisms recorded by extensive sensor arrays. Initial imaging work utilized a forward modeling approach. This paper will report further on confirmation of the event locations by inversion methods and by using decimation studies. A source parameter analysis will also be presented. Cotton Valley fracture treatments were performed at depths near 10,000 ft and imaged with arrays cemented in offset wells approximately 1300' away. Several techniques are utilized in both event location analysis and in analyzing certain characteristics of the events. These methodologies provide interpretations of the actual fractures and therefore have implications in the design and modeling of fractures. This paper will also point out and verify the need for development of an economical and commercial service to image fractures in real time. Decimation studies utilizing fewer optimally spaced receptors will determine the ability to develop a commercial service. Imaging of the hydraulic fractures at the Cotton Valley test site provides conclusions not easily accepted. Issues such as asymmetry, dimensions of the fracture and growth characteristics all prove to be substantially different than expected. Findings such as these will affect modeling techniques, design parameters, and eventually well spacing and development plans. Accurate mapping of a fracture will provide conclusive information that can be utilized to develop optimized and efficient fracture treatments. More effective fracture treatments will result in reduced well costs which allow field extension and expansion opportunities. Any operator or service company should apply lessons learned and presented in this paper. Imaging of the fracture and analysis of the actual failure mechanisms will provide a basis from which to design and model fractures in the Cotton Valley formation. Ideas for technology and completion strategies could be applicable to tight gas sand completions in other areas. This paper will show technical contributions such as:State of the art technologies utilized for mapping of the fractures.Analysis techniques that will be paramount in the future development of a commercial fracture mapping technology.Event parameter analysis yielding deductions concerning actual failure types in the rock. These analyses will have a major impact on fracture design and modeling. P. 599