Resolving Created, Propped, and Effective Hydraulic-Fracture Length

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 12147, "Re solving Created, Propped, and Effective Hydraulic-Fracture Length," by C.L. Cipolla, SPE, E.P. Lolon, SPE, and M.J. Mayerhofer, SPE, Pinnacle Technologies, originally prepared for the 2008 International Petroleum Technology Conference, Kuala Lumpur, 3-5 December. The paper has not been peer reviewed. The full-length paper compares the strengths, weaknesses, and limitations of fracture modeling, production-data analysis (PDA), pressure-transient analysis (PTA), and numerical reservoir modeling in estimating effective fracture length and conductivity. The paper also evaluates how the complexities (in the hydraulic fracture) associated with non-Darcy flow, multiphase flow, and complex fracture geometries affect the results from the various techniques. The paper documents the significant differences in "effective" fracture length that, in many cases, can result from each technique. Introduction Reliable estimates of fracture length (i.e., created, propped, and producing or effective) are necessary to consider design changes in subsequent fracture treatments to optimize the performance of hydraulically fractured wells, particularly in low-permeability reservoirs. The created fracture length is the fracture length propagated during the fracture treatment, while the propped fracture length is the length supported by proppant after the fracture closes. The effective, or producing, length is the length that is open or contributing to hydrocarbon production after a fracture treatment. Increasing the effective fracture length usually means increased production. Incorrect estimates of the effective fracture length can lead to less-than-optimal gas recovery and often contribute to modifications of fracturing designs that may not result in improved well productivity. It has been known that the fracture lengths determined from fracture modeling, PDA, PTA, and numerical reservoir modeling are not in agreement with the created fracture lengths obtained from fracture mapping. Techniques for Determining Fracture Properties Fracture Modeling. Fracture modeling (i.e., net-pressure analysis) can provide information about fracture length, height, width, and conductivity. Fracture dimensions and conductivity can be estimated from fracture modeling by matching the observed fracturing net pressures (fracturing pressure minus minimum rock stress, or closure pressure). The limitations of this technique are that it typically provides nonunique solutions, can be unreliable if not calibrated, and requires baseline rock and stress data. Net-pressure history matching can be implemented by adding new physics to fracture models. With the right assumptions and physics, inferred fracture geometry can be more reliable; however, inferred geometry from net-pressure matching does not always agree with directly measured geometry.