Fracturamientohidraulico

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Chapter

8

Fracture Treating Pressure Analysis

8.1 Introduction To Fracturing Pressure Analysis
Hydraulic fracturing, as with other drilling, completion, and reservoir behavior problems, is complicated by the fact that processes cannot be directly observed. For describing reservoir behavior, this deficiency has been overcome by the development over the past 50 years of analyses based onwellbore pressure and flow rate. But, only in the last few years has similar analyses for fracturing been introduced and successfully applied. History Shortly after the introduction of hydraulic fracturing and its acceptance by the industry, the importance of fracturing pressure data was recognized, as evidenced by a quotation from Godbey and Hodges1 “By obtaining the actual pressure on theformation during a fracture treatment, and if the inherent tectonic stresses are known, it should be possible to determine the type of fracture created.” Later, fracturing pressure and the relation between pressure and in-situ stresses were inherently included in pioneering model development work of Khristianovic and Zheltov,2 Perkins and Kern,3 and Geertsma and de Klerk4 during the 1950s and 1960s.However, it was still several years later before the analysis of fracturing pressure data started to become an accepted industry practice. In 1978, Amoco Production Company initiated a coordinated program of field data collection5 and analysis to improve the understanding of the mechanics of the fracturing process. Much of this understanding had not changed since the early 1960s and was beingseverely tested by ever larger and more expensive treatments. A series of papers at the annual meeting of SPE in 1979 presented results from this program, including a paper by Nolte and Smith6 which first introduced a basis for the interpretation of pressure behavior during a fracture treatment, and one by Nolte7 for interpreting pressure decline after the treatment. The paper by Nolte and Smithpresented a means for inferring periods of confined-height extension, uncontrolled height growth, and, more importantly, identification of a “critical pressure.” When a treatment reaches the critical pressure, fracture extension is reduced significantly and a pressure (screenout) condition or undesired fracture height growth can follow. Nolte and Smith demonstrated in the paper that a log-log plot of netfracturing pressure (above closure stress) vs.

July 1993

8-1

Hydraulic Fracturing Theory Manual

8

Fracture Treating Pressure Analysis

treating time could be used to identify periods of unrestricted extension, confined height, excessive height growth, and restricted penetration. This plot and technique has been used extensively since its introduction by both operators and servicecompanies to determine fracture characteristics and geometry, and as an evaluation tool for optimizing treatment designs. Nolte7 also presented analyses permitting some of the parameters that quantify a fracture and the fracturing process to be estimated from the pressure decline following fracturing. At the time this work was presented, there was no direct or simple procedure for evaluating thebasic parameters controlling a fracture treatment. Procedures were presented for quantifying fluid loss coefficient, fracture length and width, fluid efficiency, and time for the fracture to close from the fracturing pressure decline. The “minifrac” procedure was introduced for obtaining these parameters for use in designing the actual fracture treatment. The analysis procedures from these twopapers6,7 have been used extensively by the industry to evaluate fracture treatments related to tight gas massive hydraulic fracturing,8-10 waterflood wells,11 moderate permeability oil wells,12 and geothermal formations.13 The work by Nolte and Smith was extended to include analysis for determining proppant and fluid schedules from the fluid efficiency when little or no information is available,14...
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