Hydraulic Fracture Complexity: Diagnosis, Remediation, And Explotation

Abstract

Abstract Recent advances in hydraulic fracture mapping technologies have provided a wealth of information on fracture propagation in numerous geologic settings. Prior to such detailed measurements of actual fracture growth, fracture propagation was either assumed to be simple (single planar fracture) or the complexity was inferred based solely on fracturing pressure data. The nature or detail of this inferred fracture complexity and how it related to actual fracture growth (real fracture geometry) could not be determined. This resulted in significant uncertainty in fracture modeling, treatment designs, and many times, sub-optimum field development. This paper illustrates the application of the various methods and techniques available to diagnose fracture complexity, including simple pressure diagnostics such as G-function pressure decline analysis and sophisticated microseismic and tiltmeter fracture mapping technologies. After identifying complexity in hydraulic fracture growth, this information must be integrated with fracture, reservoir, and geologic models to properly evaluate stimulation, completion, and develop options; however, without properly identifying the nature and detail of the fracture complexity, the solution can many times be wrong - resulting in economic loss. This paper documents field observations of different mechanisms that result in fracture complexity and the corresponding physics that govern fracture growth in these reservoirs. These field observations of fracture complexity are supplemented by and related to results from mine-back and core-through experiments to better understand the relationship between fracture complexity, rock properties, and geology. Distinguishing between the various types of fracture complexity and properly modeling these complexities (in both reservoir and fracture models) can lead to significantly different treatment designs and field development strategies. The paper includes field case histories that document how the remediation of fracture complexity can lead to stimulation success, while in other cases it is the exploitation of fracture complexity that is the key to success. Introduction Complex growth of hydraulic fractures has been documented in mine-back experiments, providing direct observations of hydraulic fracture complexity in a variety of environments including tight sandstones and coalbed methane reservoirs. Unfortunately, data from mine-back experiments are very limited, and require other methods to diagnose hydraulic fracture complexity. Until recently, fracture pressure analysis was the only diagnostic available to estimate complexity.1–16 Fracture pressure analysis has been used to estimate both near-wellbore 1,17 and far-field fracture complexity; the focus of this paper is far-field fracture complexity. Hydraulic fracture complexity is usually associated with the interaction of the hydraulic fracture with a pre-existing rock fabric such as natural fractures or fissures, but it can also be linked to laminations and other geologic heterogeneities. In most cases, far-field fracture complexity is deemed detrimental due to excessive fluid leakoff and/or reduced fracture width that can result in early screenouts.18,19 In many cases, fracture complexity is reduced by adding particulates that likely plug secondary fractures and/or fissures 15,20,21; however, sometimes maximizing fracture complexity is the key to stimulation success and treatments are specifically designed to promote complexity.23,25,30 During the past ten years, thousands of hydraulic fracture treatments have been characterized using microseismic and tilt fracture mapping technologies. These measurements have shown a surprising diversity in hydraulic fracture growth, ranging from simple planar fractures to very complex fracture systems to extreme fracture height confinement (that are not explained by variations in rock properties and stress).22–39 The occurrence of complex fracture growth is much more common than initially anticipated and is becoming more prevalent with the increased development of unconventional reservoirs. The nature and degree of the fracture complexity must be clearly understood to select the best stimulation strategy. This paper focuses on techniques to diagnose fracture complexity and the selection of appropriate remediation or exploitation measures.