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Abstract:
Radial Jet Drilling (RJD) is a technique to stimulate wells by creating small-diameter laterals from vertical or deviated wells using hydraulic jets. The laterals, also called radials, can be up to 100 m in length. To analyze under which sub-surface conditions the radials improve the well performance most, a step-wise approach is followed in which first the performance of a single stimulated well is analyzed and in a second step, the performance of a doublet system is analyzed. Finally, case studies that are more detailed are simulated.
For the single well case, a good first estimate of radial stimulation performance for different reservoir conditions can be obtained from (semi-) analytical solutions. These results show that the anisotropy in the permeability and the thickness of the reservoir influence the relative increase in productivity/injectivity most. The permeability influences in particular the absolute performance of the stimulated well. Many aspects not included in the semi-analytical solution also influence the performance of the radial stimulation:
- Since the radials are open hole, stability for friable rocks or deep reservoirs is unlikely. This depends on the in-situ stress conditions. Collapsed radials probably have much lower performance or no effect at all.
- The uncertainty in the radial path and diameter decreases the expected benefits from radials significantly depending on the type of reservoir. For example for a layered reservoir, the expected increase may be tens of percent lower.
- Due to the small diameter (0.02-0.05 m) and rough surface of the radials and the high rates of geothermal wells, viscous pressure drop due to flow in the radials has to be taken into account for prediction of performance. For example for a radius of 0.04 m and well rate of 3600 m3/d, expected increase in performance is halved when taking into account pressure drop.
- Heterogeneity in the permeability has a strong impact on the performance of the radials. Performance of individual radials depends in first approximation on the local permeability. However, this is difficult to capture in general terms.
- Near well bore damage (positive skin) and prior stimulation (negative skin) have a large impact on the expected increase due to stimulation. In case the radials can be used to by-pass near well damage, performance can be much higher than predicted using the analytical equations.
- Heterogeneity due to fault and/or fractures, voids, sharp transitions or layering all make potential success more uncertain and predictability lower due to potential issues with jetting.
Whether increased performance for a single well can be translated to similar increased performance of a doublet depends on the doublet settings and subsurface conditions. For a fixed doublet distance or field size, an increase in rate due to improved performance of the wells will result in a reduced field life. The increased well performance can also be used to lower pumping cost at a fixed rate and thus improve performance of the doublet. It was found, that for most subsurface systems, the impact of the radials on production temperature was minor (for constant rate). Only for some fractured systems, short-circuiting can be increased due to radials. Overall, the ideal candidate for radial stimulation is a reservoir which is not too deep, in homogeneous, competent rock with a well with near well bore damage or in a not too deep anisotropic reservoir in which the main well is not drilled beneficially compared to the main direction of permeability.
