Off-Balance Growth: A New Concept in Hydraulic Fracturing

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized to be experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Abstract This paper offers a new fracturing concept that explains how and why actual hydraulic fractures deviate from the commonly accepted single-fracture growth models. It demonstrates that the fracture follows the local path of least resistance, not the global path, and this leads to substantial branching, presence of extensive shear fractures, and a growth pattern that is dominated by conditions at the tip of the propagating fracture, thus making its growth haphazard and off balance. Within each fracture, branching and shear fracturing create a complex path for the fluid flow and proppant transport, as well as production of reservoir fluid. As a result, most hydraulic fractures are substantially shorter and narrower than the intended design and yield a suboptimal production increase because of inadequate fracture length and conductivity. Often, common well-completion designs also lead to creation of multiple fractures at the wellbore, a situation further aggravated by prevailing fracture designs. Off balance growth can obstruct proppant flow, which may lead to screenout. Introduction The oil and gas industry has long recognized the inadequacy of existing theories to predict the behavior and outcome of hydraulic fracturing treatments. Treatments require higher pressures than predicted by theory. After-fracture pressure buildup tests often behave more like those of radial flow from wells with negative skin than of fractured wells.1 Actual production increases from fractured wells are lower than predicted from fracture design computations. Several field observations show very complex fracture paths, presence of multiple fractures, and random-appearing proppant distribution. Warpinski et al.2–5 provide a comprehensive review of some of the major experiments conducted by Sandia Natl. Laboratories at the Nevada Test Site and by the Gas Technology Inst. and U.S. Dept. of Energy at M-Site. These reports detail important features of these fractures, together with an explanation of their cause. These results are used extensively as background and support for the new off-balance fracture growth concept. The concept of fracture tortuosity was introduced to account for a nonplanar near-wellbore fracture path. Although this term is not scientifically defined, its use is very widespread in fracturing literature and generally means any wellbore effect that complicates the near-wellbore creation and extension of the fracture. The focus of most existing papers is more on recognition of the problem than on the underlying cause. The general approach has been to relate tortuosity to near-wellbore events and conditions. Most literature refers to two specific causes for tortuosity: near-wellbore change in fracture direction and initiation of multiple fractures by use of perforations. Mahrer et al.6 give a comprehensive account of events and publications leading to the current industry understanding and history of tortuosity. Aud et al.7 attribute most screenouts to near-wellbore events most likely caused by tortuosity. Cleary et al.8 provide a description of some of the events that can cause tortuosity. Weijers et al.9 consider natural fracturing as the main cause of multiple fracturing. Shlyapobersky and Chudnovsky10 attributes the complex fracture behavior to the presence of a near-tip "process zone" that increases the dissipation of energy during hydraulic fracturing. In this paper, the concept of off-balance fracture growth is introduced and used to show how it dominates the growth pattern of a large majority of hydraulic fractures, both near the wellbore and away from it. Two distinct fracture characteristics are defined and described. These are multiple fracturing(a near-wellbore effect caused mainly by well-completion details) and branching and shear fracturing (which exist within the main body of the fracture and control its global growth pattern). Off-balance growth occurs every where along the fracture and is not restricted to the near-wellbore region. Its consequences are very narrow fracture widths, short fracture lengths (created and propped), severe branching, high pressure drops along the fracture, and preferential fluid and proppant movement dominated by shear fractures. Fundamental Fracturing Concepts Fractures in rocks are created by any one or a combination of three mechanisms: opening, sliding-mode, and tearing-mode fractures.11