Experimental Study of Hydraulic Fracture Conductivity Demonstrates the Benefits of Using Proppants
- C.N. Fredd (Schlumberger Oilfield Services) | S.B. McConnell (Schlumberger Oilfield Services) | C.L. Boney (Schlumberger Oilfield Services) | K.W. England (Schlumberger Oilfield Services)
- Document ID
- Society of Petroleum Engineers
- SPE Rocky Mountain Regional/Low-Permeability Reservoirs Symposium and Exhibition, 12-15 March, Denver, Colorado
- Publication Date
- Document Type
- Conference Paper
- 2000. Society of Petroleum Engineers
- 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.6.9 Coring, Fishing, 5.5 Reservoir Simulation, 5.8.3 Coal Seam Gas, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 5.4.2 Gas Injection Methods, 5.5.2 Core Analysis, 2.4.3 Sand/Solids Control, 2.5.2 Fracturing Materials (Fluids, Proppant), 3 Production and Well Operations
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Hydraulic fracturing treatments using treated water and very low proppant concentrations ("waterfracs") have been successful in stimulating low-permeability reservoirs. However, the mechanism by which these treatments provide sufficient conductivity is not well understood. To understand the effects of hydraulic fractures on conductivity, a series of laboratory conductivity experiments were performed with hydraulically fractured cores from the East Texas Cotton Valley sandstone formation. Jordan sand and sintered bauxite proppants were used at concentrations of 0, 0.1 and 1.0 lb m/ft2, and the conductivity was measured at effective closure stresses ranging from 1,000 to 7,000 psi.
The results of this study demonstrate that fracture displacement is required for surface asperities to provide residual fracture width and sufficient conductivity in the absence of proppants. However, the conductivity may vary by at least two orders of magnitude depending on formation properties such as the degree of fracture displacement, the size and distribution of asperities, and rock mechanical properties. In the presence of proppants, the conductivity can be proppant or asperity dominated, depending on the proppant concentration, proppant strength and formation properties. Under asperity dominated conditions, the conductivity varies significantly and is difficult to predict. Low concentrations of high-strength proppant reduce the effects of formation properties and provide proppant dominated conductivity. At conventional proppant concentrations, conductivity experiments performed with flat, parallel core faces tend to overestimate the conductivity observed with hydraulic fractures. Actual hydraulic fracture conductivity may be as much as an order of magnitude lower in the presence of low strength proppant. An important implication of this study is that the success of a "waterfrac" treatment is difficult to predict because it will depend significantly on formation properties. This dependence can be overcome by using high strength proppants or proppants at conventional field concentrations.
Although proppants are routinely used to achieve conductivity during hydraulic fracturing treatments, recent fracturing treatments using treated water and very low proppant concentrations ("waterfracs") have been successful in low-permeability reservoirs1-4. The mechanism by which these treatments provide sufficient conductivity is not well understood. The presence of residual fracture width caused, for example, by surface asperities and proppant bridging and the lack of damage associated with the use of gels in conventional proppant treatments are possible explanations2,5. Residual fracture width has been observed during laboratory experiments6 and field tests7 and can be attributed to the combined effects of surface roughness and fracture displacement8. The surface asperities are thought to withstand high formation closure stresses and create sufficient conductivity for wells completed in very low-permeability formations. The low concentrations of proppant are added to supplement the asperities and improve overall fracture conductivity.
Factors affecting the conductivity of hydraulic fractures and proppant packs have been reported in the literature. The importance of parameters such as fracture displacement, fracture roughness, mechanical properties, and closure stress on fracture conductivity have been demonstrated in the absence of proppants9-12. When proppants are present, parameters such as proppant strength, proppant concentration, and closure stress have been shown to be important13-14. However, these studies were performed with hydraulic fracture in the absence of proppants or with proppant and flat, parallel core faces. No study has addressed the effects of hydraulic fractures on conductivity in the presence of low concentrations of proppant (i.e., conditions that may exist during waterfrac treatments).
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