Fracture Design and Validation With Uncertainty and Model Limitations

Abstract

Standard methods of fracture design and evaluation use idealized assumptions that result in differences between the design length and the apparent length during production. These limitations provide economic risks, which can be reduced by quantifying the limitations and including their effects in an economic analysis before selecting the optimal design length. IntroductionThe basic elements of fracture design were developed between 1955 and 1961. These developments include fracture-geometry models 1,2; in-situ stress and fracture orientation 3 ; characterization of fluid loss4; coupling of geometry, fluid loss, and material balance for predicting penetration 4 ; and fractured-reservoir response. 5 These contributions provided an adequate foundation for the relatively small treatments (e.g., average of 25,000 Ibm sand in 1971 **) performed before the advent 6 of massive hydraulic fracturing in the late 1970's.These larger treatments were more costly and had an increased likelihood of disappointing results. Consequently, a period of significant R&D began. This led to improved descriptive models, diagnostic methods for deviations from the idealized models used in standard practice, and analysis procedure for parameter definition. Economic analyses 7,8 coupling treatment costs with posttreatment production performance were also introduced to the design process. These advances provided a basis for the evolution of improved fracturing practices. Applications of this advanced technology have been limited, however, because of the additional cost necessary to obtain and interpret the required data. The reluctance to make this investment is fostered by the failure to quantify and consider the economic risks inherent in standard practice. The advanced methods and economic risk are discussed in this paper.Engineering by nature is a compromise between reduction of the sources of risk, control of the additional costs and resources associated with the application of advanced analyses, and the use of relatively simple models within standard practice. The compromise is generally achieved in the design process through safety factors. Effective application of safety factors requires identifying and quantifying the effects of uncertainties and other limitations on the total design process through a risk analysis.

Examination of a Cored Hydraulic Fracture in a Deep Gas Well

“…The recommended emphasis for extending fracture design technology, based on these considerations, is to follow the path of more mature engineering disciplines by developing (1) relatively simple placement and production models with well-defined limitations, (2) rational assessments for the level of data required for a specific formation, (3) safety factors based on the specific conditions 22 for an economic analysis leading to the optimum treatment design, and (4) effective evaluation tools to permit identification of any missing pieces after a minimum number of treatments for a specific formation.…”

“…This simplistic, model-driven paradigm for hydraulic fracturing has biased our view of many other aspects of the process as well. Height growth, for example, is treated as an elastic process, involving equilibrium models,24,30,31 time-constant approximations, 22,32 or fully 3D finite-element models. 23,24 Little attention has been given to the inefficiencies of fracture growth across bedding and nonelastic behavior, 33 factors that may be as important as the easily analyzed elastic components.…”