After-Closure Analysis To Identify Naturally Fractured Reservoirs: A Field Validation Study
- Simon T. Chipperfield (Santos Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 9-12 October, Dallas, Texas
- Publication Date
- Document Type
- Conference Paper
- 2005. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 3 Production and Well Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.5.2 Core Analysis, 5.6.2 Core Analysis, 5.6.4 Drillstem/Well Testing, 2.4.3 Sand/Solids Control, 5.1.2 Faults and Fracture Characterisation, 5.8.6 Naturally Fractured Reservoir, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 2.5.1 Fracture design and containment, 1.6.9 Coring, Fishing, 5.6.3 Pressure Transient Testing, 5.8.1 Tight Gas, 5.1 Reservoir Characterisation
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Economic optimization of fracture stimulation designs requires accurate reservoir description (ie. permeability and permeability thickness) and an understanding of the deliverability mechanism (i.e., natural fractures versus conventional matrix). This paper presents a technique which allows an engineer to evaluate a mini-frac injection in a similar manner to a conventional pressure transient test for the identification of productive natural fractures. Following a brief theoretical background to this new methodology, this paper provides a field validation study in oil and gas fields on two continents.
Identifying natural fractures drives certain changes in stimulation design. Techniques designed specifically to stimulate natural fractures include low gel loadings, energized fluids and 100 mesh sand for natural fracture preservation.[1,2]
A recently published mini-frac evaluation technique using the After-Closure pressure information3 provides a methodology which can identify productive natural fractures. Using this methodology, this paper delivers a retrospective evaluation of mini-frac information using data from tight gas fields in Central Australia and an oil field in Algeria. In addition, the results are validated with independent constraints such as pre-closure mini-frac analysis, conventional well testing, image logs and core analysis.
Finally, the results of this evaluation are distilled into a simple field implementation methodology which can be incorporated into a conventional pre-frac injection sequence.
Developing an understanding of critical reservoir parameters (such as k,kh and the presence of natural fractures) allows one to determine whether a reservoir should be stimulated and the most appropriate design. Until recently, this evaluation has been achieved through the use of classical reservoir engineering approaches such as Pressure Build-Up (PBU) tests. However, these tests can be costly, both in terms of additional equipment requirements and the associated delay in well on-line dates. Another disadvantage of these tests in tight gas fields, is that many of these wells do not flow pre-frac. The advent of After-Closure Analysis (ACA) however has allowed reservoir characterization to be conducted in a more cost effective manner through the merger of reservoir engineering and mini-frac analysis. ACA analyses the response of a mini-frac pressure fall-off after mechanical closure of the created fracture. As well as providing insight into the reservoir character, ACA can determine the ‘reservoir perspective' of fracture length (dependent on permeability, reservoir compressibility, porosity etc) such that this can be reconciled with the routinely calculated, pre-closure or ‘mechanical perspective' of fracture length (dependent on Youngs, Modulus, layer stress contrasts etc).
This paper shows an extension of ACA to identify and characterise naturally fractured reservoirs. This analysis approach identifies natural fractures that are open under in-situ conditions and assists the engineer in differentiating these natural fractures from those fractures that may only be open under high injection pressures.[4,5,6] Since naturally fractured reservoirs may require alternate design strategies,[1,2] this ACA approach provides not only a means of reservoir characterization but also drives stimulation design choices.
Theory HighlightsTable 1
The theory of ACA for the characterization of matrix reservoirs was first published by Gu et al. Since this time, ACA has been field tested and further refined by many authors[8,9,10,11,12,13,14,15] most notably by Nolte.[8,12] The response of ACA to the presence of open natural fractures has been published more recently.Highlights of the theory for matrix and naturally fractured reservoirs are provided below and presented diagrammatically in Table 1.
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