Diagnostic Techniques to Understand Hydraulic Fracturing: What? Why? and How?

Abstract

Abstract In recent years there have been numerous advances in fracture mapping/diagnostic technologies. This paper details the state-of-the-art in applying both conventional and advanced technologies to better understand hydraulic fracturing and improve treatment designs. The initial portion of the paper describes the application and limitations of various diagnostic tools and methods, including well testing, net pressure analysis (fracture modeling), techniques that employ open-hole & cased-hole logs, surface & downhole tilt fracture mapping, microseismic fracture mapping, and production data analysis. The bulk of the paper is dedicated to case histories that illustrate the application of these various fracture diagnostic technologies. The case histories include examples of how several fracture diagnostics can be used in concert to provide more reliable estimates of fracture dimensions and allow better economic decisions. Introduction The process of hydraulic fracturing has always had a "black box" image. This has been partly because knowledge about fracture geometry is difficult to obtain with fractures growing thousands of feet below the surface, and partly because fracturing is proving to be vastly more complex than initially thought.1–3 While hydraulic fracture treatments continue to be designed using the best tools and techniques available, geometry estimates from fracture models have been difficult to verify. Numerous fracture diagnostic techniques have been developed to fill this knowledge gap, improving our understanding of hydraulic fracture behavior.4–10 The main purpose of fracture diagnostics is to help the producer optimize field development and well economics. This can include optimizing individual fracture treatments to obtain the most economic design and optimum interval/height coverage or optimizing the entire field development in terms of well spacing and location. Fracture diagnostics can be beneficial in numerous stimulation settings. Settings range from propped fracture stimulation of a new pay zone in a newly developed field to infill-drilling development, and from field development using hydraulically fractured horizontal wells to the evaluation of fracturing during steam-flooding or water-flooding. When executing fracturing operations in one of these settings, several questions can be answered in the design/evaluation process using fracture diagnostics, including:Do fractures effectively cover the pay zone?Are fractures confined to the pay zone?Does the fracture grow into an unwanted gas bearing or water-bearing zone?What is the optimum number of fracture treatment stages and treatment size to cover thick pay zones?How much more length/height/production is obtained if treatment size is increased?Is the final fracture conductivity sufficient to achieve the desired production? What is the optimum proppant?Is the hydraulic fracture oriented in the same direction as the primary set of natural fractures?What direction should a horizontal well be drilled to complete it with transverse (or longitudinal) multi-stage fracture treatments?Is the well pattern appropriate to maximize sweep efficiency in steam/water-flood areas?Do the injected waste and drill cuttings remain within the selected zone? Numerous fracture diagnostics are available (see Figure 1), including techniques that directly image "big picture" far-field fracture growth, dimensions, and orientation; tools that provide a local measurement of the fracture at the wellbore; and lower-cost indirect (model-dependent) diagnostic methods. There are three main groups of commercially available fracture diagnostic techniques, each with their own set of capabilities and limitations. A summary of the techniques, limitations and the parameters each technique measures is provided in Table 1.11