Evaluation of After-Closure Analysis Techniques for Tight and Shale Gas Formations
- Ibrahim Mohamed Mohamed (Texas A&M University) | Ramez Masoud Azmy (Texas A&M University) | Mohammed Ali Ibrahim Sayed (Texas A&M University) | Matteo Marongiu-Porcu | Christine Economides
- Document ID
- Society of Petroleum Engineers
- SPE Hydraulic Fracturing Technology Conference, 24-26 January, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.1 Tight Gas, 5.8.2 Shale Gas, 5.6.3 Pressure Transient Testing, 5.6.4 Drillstem/Well Testing, 4.6 Natural Gas, 2 Well Completion, 2.5.1 Fracture design and containment, 4.1.2 Separation and Treating, 2.5.2 Fracturing Materials (Fluids, Proppant), 3 Production and Well Operations
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Optimized hydraulic fracture design requires formation permeability as an input, but it is difficult to quantify in tight gas and shale gas reservoirs. After closure analysis (ACA) following a minifrac or fracture calibration test may offer a means to determine the formation permeability in cases for which both a formation test and a conventional pressure buildup test are impractical and/or unable to provide the permeability. However, ACA techniques use a variety of specialized plots, and there is a risk that apparent straight lines may lead to erroneous results.
This paper proposes a technique that provides a simple way to calculate formation permeability, initial reservoir pressure, fracture length, and closure pressure from a single specialized plot. The proposed technique is compared with the G-function method for the estimation of the closure pressure. In addition, it is compared with 3 ACA techniques (Benelkadi, Gu, and GFunction) used in the literature to calculate formation permeability for tight gas and shale gas wells.
Three field examples of pressure fall-off tests (in tight sand gas wells and shale gas well) are analyzed. The results show that the proposed technique provides a clear and rigorous analysis procedure for determination of permeability and other parameters required for the hydraulic fracture design. This proposed technique uses only a single plot comparing to the multiple plots required by the other techniques.
In hydraulically fractured wells, minifrac treatments and calibration tests are frequently performed before the main fracture treatment to determine the parameters required for the stimulation design. Typically the fracture calibration test is performed without proppant. Fracture-pressure analysis was pioneered by Nolte (1979, and 1988), and the basic principles for such analysis are analogous to those for pressure analysis of transient fluid flow in the reservoir. While the primary test objectives are quantification of the leakoff coefficient of the fracturing fluid and the closure pressure, in some cases the formation permeability can also be estimated (Baree et al., 2009). These parameters can be used to determine optimal values for the pad volume, the best fluid-loss additives to be used, and the pumping schedule that would achieve the optimum fracture from a productivity standpoint.
The Unified Fracture Design (UFD) approach indicates the optimum fracture geometry (fracture half-length and conductivity) for a given treatment proppant mass (Economides et al., 2002), provided that a reliable value for the formation permeability is known as well as the approximate well drainage area. When the well design incorporates transverse fractures created from a horizontal well in tight gas and, especially, in shale gas reservoirs, formation permeability is important for
establishing an optimal spacing between the transverse fractures (Song et al., 2011).
Pressure buildup and falloff tests are traditionally used for permeability determination, but in very low permeability reservoirs this is difficult because without stimulation the formation flow is minimal. The fracture calibration test offers a workable mechanism for permeability determination in very low permeability reservoirs. Several methods for determining permeability and other calibration test parameters are found in the literature. The common techniques depend on specialized plots that are designed to show a straight line for a portion of the data from which parameters are determined either from the slope of the line or from its endpoints. As an alternative, this paper offers a unified approach that avoids the need for many different plots of the data on many different plot axes. Instead, a single log-log diagnostic plot enables quantification of the parameters of interest.
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