Proppant Placement Using Reverse-Hybrid Fracs
- Yajun Liu (University of Texas at Austin) | Phani Bhushan Gadde (University of Texas at Austin) | Mukul Mani Sharma (University of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- August 2007
- Document Type
- Journal Paper
- 348 - 356
- 2007. Society of Petroleum Engineers
- 1.8 Formation Damage, 5.8.1 Tight Gas, 5.6.5 Tracers, 4.2 Pipelines, Flowlines and Risers, 2.4.5 Gravel pack design & evaluation, 3.2.6 Produced Water Management, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.2 Reservoir Fluid Dynamics, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.6 Natural Gas, 5.6.3 Pressure Transient Testing, 3.2.4 Acidising, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control
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In conventional gel-fracturing treatments, the damage induced by the gel can have a significant impact on well performance, particularly in low-permeability gas formations. Slick-water fracs have been shown to be more successful in some tight gas formations because of reduced gel damage and limited height growth. Proppant placement is a major concern in water fracs. Hybrid water fracs (i.e., using slick water as the pad fluid and gel to place the proppant) provide improvements to the performance of water fracs.
This paper proposes a new method, reverse-hybrid fracs (RHF), for the efficient placement of proppant deep into created fractures while minimizing gel-induced damage. Experiments were conducted in a simulated fracture (i.e., slot cell) to study the transport of proppant. Slick water was injected first into the slot, followed by gel. Finally, slick water containing proppant was injected to displace the gel. The water that carried the proppant quickly formed viscous fingers through the gel. The gel was observed to form long, thin layers that effectively hindered proppant settling and helped transport the proppant further into the slot. This resulted in the formation of proppant packs above the gel layers.
In this paper, experimental results are presented to show how the gel layers distribute in the slot and how proppant distribution is affected by the gel layers. A transparent cell composed of rough walls was set up to investigate the effect of fracture wall roughness. The effects of fluid viscosity ratio, fracture wall roughness, and gel pad volume were investigated. Based on the experiments and scaling relations, recommendations are made for the pumping sequence and the size of the gel and slick-water stages in reverse-hybrid fracture treatments.
This method of proppant placement requires less gel than conventional gel fracs. Other possibleadvantages include: (1) less gel damage to the proppant pack, (2) limited height growth, and (3) less penetration of the pad fluid into the formation resulting in shorter cleanup time following fracture treatment.
In a fracturing treatment, the effective fracture lengths achieved (e.g., measured by matching the production response or from pressure-transient tests) can quite often be significantly smaller than the created fracture lengths (e.g., measured by fracture-mapping techniques). This difference can be attributed to inadequate proppant transport and/or insufficient fracture cleanup.
In hybrid fracs, low-viscosity and slick water are used to create the fracture, while a high-viscosity gelled fluid is used in the proppant stage. In some fields, the use of hybrid fracs has resulted in a significant increase in effective fracture lengths and well productivity (Sharma et al. 2004). The use of hybrid fracs can, under certain conditions, result in the increased possibility of tip screenout. Because the fracture is being created with a high leak-off, low-viscosity fluid, the smaller fracture widths can result in premature tip screenout.
This is the primary motivation for using reverse-hybrid fracs. As the name suggests, the sequence of fluid injection is the reverse of what is used in hybrid fractures; a high-viscosity polymer (e.g., linear or cross-linked) fluid is used to create the fracture while the proppant is pumped, behind this high viscosity pad, into a low-viscosity fluid. This kind of treatment is referred to as reverse-hybrid fracs (RHF) in this paper.
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