Investigations of the Destruction Mechanism of High Pressure Water Jetting

Abstract

As the demand for green energy is growing, geothermal energy will play an important part in the future energy mix. Geothermal energy is widely used for both direct-use and electricity generation. A viable geothermal reservoir must have a sufficiently high temperature, a fluid pathway through the rock (permeability) and a fluid that can collect and transport the heat to the surface. Deeper reservoirs often have high temperatures and a low permeability, whereas shallow reservoirs often have a high permeability but low temperatures. When the permeability is too low, the reservoir can be stimulated. This is conventionally done by fracking the reservoir rock; that is, creating fractures to enhance the flow. This is a very expensive and potentially hazardous operation. Another technique is Radial Jet Drilling (RJD), which uses a high pressure focused fluid jet in order to drill small diameter horizontal holes (laterals) from the vertical borehole. Laterals can be drilled in multiple directions of the borehole and can be as long as 100 metres, potentially increasing the production 3-8 times (Blöcher et al. 2016). However, the destruction mechanism of rocks during jetting is not yet fully understood.

Experiments and simulations were conducted in order to investigate the destruction mechanism of a rock during jet impingement. First, experiments and simulations were carried out which had the aim to find a relation between stagnation pressure (exerted pressure on the rock by a water jet) and jetted cavity depth. The experiments provided information about the maximum jettable depth (with a static nozzle), while the simulations provided a continuous pressure depth relationship. Combining the simulation and experimental results leads to the conclusion that pressure fluctuations are key for the destruction process, which is consistent with the theorem of hydraulic fracturing.

Furthermore, a Finite Difference Method (FDM) solver was developed in order to investigate the pore pressure inside a porous rock during jet impingement. The solver was used to investigate (1) the difference in pressure between the first layer of pores (adjacent to impinged area) and the fluid pressure of the jet and (2) the influence of an increased pore pressure in the whole sample (i.e., back pressure). It became clear that the pressure fluctuations in time between the pores and jet are significant and sufficient enough to induce hydraulic fracturing. Increasing the back pressure resulted in higher pressure fluctuations, but only when the back pressure is higher than the average jetting pressure.

There are still some doubts in the literature in whether or not cavitation erosion is the governing destruction mechanism. Cavitation is the formation of vapour bubbles in a fluid due to fluid pressure drop below the vapour pressure. Once the bubbles travel to a higher pressure regime, they implode, creating either a shock wave or a mini jet, resulting in erosion of material. Hahn et al. (2019) and (Kumagaiet et al. (2011) evaluated the location of cavitation erosion on a flat surface and concluded that cavitation erosion is not the governing erosion mechanism as there was no damage at the impinged area. But those studies did not prove that cavitation erosion does not occur inside a hole. Experiments and simulations were therefore performed to investigate cavitation erosion inside a cavity. It is found that cavitation erosion does occur inside a cavity, even at cavity depths of >25mm. However, experiments on one of the rocks showed that the maximum jetting depth was 25mm. From these results, it was concluded that although cavitation erosion occurs inside a cavity, it cannot be the governing destruction mechanism.

Based on the results in this study, it is concluded that hydraulic fracturing is the governing erosion mechanism, because of the proved dependency of destruction on pressure fluctuations and the disprove of surface erosion due to shear forces (in literature, Buset et al. (2001)) and cavitation erosion being the governing mechanism. Another, in literature, proposed mechanism is pore-elastic tensile failure, which is partly interconnected with the theorem of hydraulic fracturing and not further investigated in this study. Based on this conclusion, a 'jetting correlation' was developed which is able to predict whether a rock is jettable or not. Combined with the pressure-depth relationship, it can also predict the maximum jetting depth. The correlation matched with the experimental results, which is another indication that hydraulic fracturing is the governing destruction mechanism.