There are now multiple geophysics- and engineering-based methods for the evaluation of hydraulic fracture stimulation effectiveness associated with multi-fractured horizontal wells (MFHWs) completed in unconventional gas and light oil reservoirs. These methods, which can be classified according to the timing of data collection relative to the main hydraulic fracture stimulation (see Figure 1), will provide different types of information for fracture characterization. Diagnostic fracture injection tests (DFIT), performed prior to the main stimulation treatment, are used to derive information about in-situ stress (closure pressure), leakoff mechanisms, reservoir pressure and formation permeability, which is useful for the main hydraulic fracture treatment design. However, the DFIT data may not be used to evaluate the results of the main stimulation treatment. Microseismic data, collected during the main stimulation treatment, may be interpreted for created fracture length or area, but may not necessarily provide information about what fraction of the created fracture(s) are conductive and producing formation fluids to the well. Pressure and rate data collected during the main treatment may be interpreted with frac models to derive a created and propped fracture half length. Rate-transient analysis, which is analogous to pressure-transient analysis (well-testing), can be used to evaluate short and long-term production data for hydraulic fracture properties such as total effective length (or effective area) and fracture conductivity. Techniques for long-term production analysis are relatively well established, while analysis of short-term (flowback) data is relatively new. Flowback data consists of (usually) high resolution rate and pressure data collected immediately after fracture stimulation, and is often dominated by production of hydraulic-fracturing fluids. This short-term data, dominated by flow properties of the fractures themselves, provide a unique opportunity to characterize the hydraulic fractures immediately after they are created. Comparison with properties derived from long-term production analysis provides an opportunity to evaluate fracture performance degradation or enhancement over time.
In this work, fracture property estimates from these multiple sources are compared for a multi-fractured horizontal well producing from a tight oil reservoir to gain insight into dynamic changes in hydraulic fracture properties over time. A loss of effective producing length is observed through comparison of flowback and long-term production analysis; the possible causes of this reduction include gas breakthrough to the fractures from the formation once flowing pressure drops below bubble point pressure, reduction of fracture conductivity, and secondary fracture contribution.
The main focus of this study is the evaluation of hydraulic fracture properties before and after breakthrough of formation fluids during the flowback period. The rate-transient analysis principles used in the analysis are explained in detail, including a discussion of how complex fracture, reservoir and fluid behavior are incorporated into the analysis. Flowback analysis is a new and evolving field – future areas of research will be highlighted.
About the Author(s)
Christopher R. Clarkson is a professor and the Encana-AITF Chair in Unconventional Gas and Light Oil research in the Department of Geoscience and an adjunct professor with the Department of Chemical and Petroleum Engineering at the University of Calgary. His work focus in industry was on exploration for and development of unconventional gas (UG) and light oil (ULO) reservoirs. His research focus since coming to U of Calgary in 2009 has been on advanced reservoir characterization methods for UG-ULO, such as rate- and pressure-transient analysis, flowback analysis, and core analysis. He is also interested in simulation of enhanced recovery processes in UG-ULO, and how these processes can be used to reduce greenhouse gas emissions. Clarkson leads an industry-sponsored consortium called “Tight Oil Consortium”, focused on these research topics for unconventional light oil reservoirs in Western Canada.
Clarkson holds a Ph.D. degree in geological engineering from the U. of British Columbia, Canada. The author of numerous articles in peer-reviewed scientific and engineering journals, Clarkson received the Rossiter W. Raymond Memorial Award from AIME and the Alfred Noble Prize from ASCE for his paper “Application of a New Multicomponent Adsorption Model to Coal Gas Adsorption Systems” published in the Society of Petroleum Engineers Journal (September, 2003). Clarkson was an SPE Distinguished Lecturer for the 2009/2010 lecture season.
Clarkson, C.R., Qanbari, F., and Williams-Kovacs, J.D., 2014, Innovative use of rate-transient analysis methods to obtain hydraulic fracture properties for low-permeability reservoirs exhibiting multi-phase flow. The Leading Edge, October 2014