Abstract

This paper discusses multiple fracture creation, detection, and prevention. Multiple fracture creation will be discussed on the basis of rock mechanics theory, laboratory experiments, and field observations. In addition, several references from SPE papers will be outlined. The section on qualitative analysis will discuss several methods of detection, ranging from core and log analysis, production history matching, and pressure transient analysis to the use of a real-time, three-dimensional (3D) fracturing model that can possibly provide quantitative analysis. Mitigation methods with regard to perforation techniques will also be briefly discussed.

Introduction

Poor post-fracture well performance has long been attributed to such factors as proppant convecting out of zone or poor conductivity resulting from misapplied gel-break chemistry. Although evidence suggests that such effects could worsen poor fracturing treatment performance, many fracturing treatments that result in poor production may result primarily from the creation of multiple fractures. Many stimulation engineers have yet to accept this phenomenon fully because they believe that all multiple fractures result in screenouts. Screenouts may not always occur. This paper presents a successfully executed instance in which a 3D frac model revealed a surface treating pressure that indicated eight multiple fractures.

In a recent work, Mahrer et al. cited 285 articles, reports, and other documents that provided qualified observations of multiple fractures. A vigorous research of the literature was performed to discover citings of single-wing, planar fractures. With the exception of a theoretical reference by Howard and Fast, Mahrer found no references to such fractures. At the presentation of his work, however, Mahrer was besieged by several single-wing fracture constituents who would not accept his research. Mahrer proposed that the paradigm of single planar fractures should be the exception, not the norm. This paper complements Mahrer's work by exhibiting other facets of multiple fractures.

In rock mechanics theory, single-fracture planes are the given norm. This postulate is easy to address numerically and conceptually. However, many cases exist in which single planar fractures were not created during experimentation with hydrostone. Although this material is considered the most manageable rock available, it allows multiple fracture initiation in nonunique circumstances.

Table 1 outlines several symptoms of multiple fractures. This paper will discuss each of these symptoms and their possible causes:

  1. sand production without the placement of more than six sand grains of proppant,

  2. cyclic production performance after the fracturing treatment,

  3. shallow mineback studies,

  4. pressure behavior during stimulation, and other lesser known illustrations.

A detailed section is included that discusses how to use a 3D fracture simulator to qualify and possibly quantify multiple fractures. This section will describe the differences between single-plane tortuosity and a similar effect that occurs when multiple fractures are present.

Types of Multiple Fractures and Their Environments This paper will focus on multiple fractures that coalesce, overlap, or compete for the same pore space (Fig. 1). Most experts consider noncompeting fractures (Fig. 2), such as those generated when long pay intervals are fractured with several perforated intervals, as no threat to the fracture treatment's success. An example of this type of noncompeting fracture occurs in the Hugoton wells, where several members of the Chase group are being stimulated simultaneously.

Multiple fractures will most likely occur in (1) naturally fractured formations, (2) long intervals of perforations, with the perforating phase being 0 >to >180, and (3) strongly dipping planes and/or deviated wellbores to flat bedding planes.

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