Hydraulic fracturing is the overwhelming completion method in the international petroleum industry. It is estimated, that today it has become a $20 billion industry. It is also one of the most challenging enterprises, incorporating scientific information from geology, reservoir and production engineering, rock mechanics, complex fluid rheology and field economics. More than a decade ago we introduced the concept of Unified Fracture Design (UFD) as a means towards physical optimization. Since then UFD has been adopted by a great number of practitioners and hundreds of papers have been published. Not all approaches have been successful, nor have they been done appropriately. In this work we relook at the whole issue in a systematic way. We have identified and describe here a 9-step sequence to mitigate the gaps and help in the optimized field application of modern hydraulic fracturing. These include:Required minimum reservoir characterization: Typical log description.Estimate of k, h, interlayer stress contrast, distance from water.Optimization of fracture size using UFD. Maximum dimensionless PI at optimum conductivity. Optimization of drainage shape and stimulated reservoir volume (SRV).Unique monitoring execution, other than the traditional net pressure or injection variables. New benchmark indicators are used, cross-plotting descriptive variables such as: level of pressure vs. volume of fluid injected vs. operational characteristics, such as tip screen-out.4. Fracture injection tests, designed for fluid efficiency, closure pressure, leak-off coefficient and the newly developed capability for after-closure analysis for well and reservoir variables.For multifrac applications such as shale gas, horizontal wells in very low-permeability formations, zonal isolation and perforation strategies are essential issues.Design of injection schedule to accomplish the desired fracture geometry is the next concern. What happens if things go wrong? The strategy of contingencies must be in place before the execution starts.On-the-fly redesign responding to contingencies, such as net pressure constraints and formation barriers. Ability for alternative plans quickly.Once the job is completed the next step is whatever is necessary to close the fracture and flow the well back. What needs to be done in different geologic settings?Finally, to close the circle, performance evaluation is indicated, which involves comparison between actual performance and expectations and the definitive reconciliation of any discrepancies.
For more than 50 years a vision of a hydraulic fracture that is vertical with two symmetrical wings has been accepted by two highly diverse sciences in petroleum production: fracture mechanics and production/reservoir engineering models. The fracture in both sciences has a height, a length tip-to-tip, and an average width. A fracture propagation model is usually employed to determine fracture dimensions. Linear elastic fracture mechanics points towards a relationship between fracture length (and, implicitly, height) and fracturing pressure. Therefore, pressure analysis during execution is supposed to 1) determine the generalized fracture geometry and 2) to quantify fracture dimensions such as length and width. Once the well is put on production, the fracture geometry can be determined either through a well test or through long-term production data analysis or both. The models employed for this exercise have been in wide use and have been credited with considerable success in hydraulic fracture treatment evaluation. Pressure patterns would lead to the determination of the apparent fracture half-length and fracture permeability-width product. Unfortunately, often there is a discrepancy to a severe discrepancy in the fracture dimensions obtained from the two methodologies. We present here several theories describing the discrepancy and we quantify the impact of reservoir and fracture parameters. These include reservoir areal permeability anisotropy, damage to the proppant pack, discontinuity in fracture conductivity and, of course, turbulence effects. We are applying this approach to 23 hydraulically fractured wells in a less-than 1 md oil and gas reservoirs in Western Siberia and achieve reasonable reconciliation between the results of the diverse methods of fracture geometry determination and the impact of reservoir and fracture variables. Fracture treatments, designed for other wells in the field take these conclusions into account.
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