Impact of flow field of hydrothermal jet drilling for geothermal wells in a confined cooling environment

Significance Statement

Geothermal energy is a resource potential with low carbo emission as well as widespread distribution. Geothermal resources are divided into hot dry rock and hydrothermal sources consistent with their naturally occurring states. Hydrothermal sources are exploited by extracting water, steam and gases contained in the reservoir. However, the hot dry rock is geothermal energy stored in underground hot and low permeable crystalline rocks generally located 3-10km deep.

The hot dry rock presents a potential indigenous resource that can provide heat and electric power. However, a feasible exploitation of geothermal energy counts on smooth drilling. The typical rotary drilling methods use bottom on the drill bit to crush rocks and a jointed drill string to elongate to the required depth. Unfortunately, in considerable deep hard rock formations, the contact between the rock and the bit, continuous tripping and making strong connections has resulted to drill bit abrasion, and consequently time consuming and costly drilling.

High-pressure water jet method uses water destructive energy to enhance rock removal rates. It has been adopted for assisting mechanical action to improve the rate of penetration in mining and petroleum recovery. However, the method is insufficient in breaking hard rock formations such as granite. Thermal spallation drilling on the other hand uses high temperature air or flame via combustion to heat rock surfaces causing high thermal stresses, which in turn induce fragmentation.

For deep wells drilling via a number of complicated formations, many rock assemblages do not spall, therefore impeding advancement in the course of thermal spallation drilling. To offset this shortcoming, researchers led by Professor Xianzhi Song from China University of Petroleum, China, proposed a new high temperature, high velocity jet drilling method based on high-pressure water jet drilling as well as thermal spallation approach. The approach is a contact-free technology, which implements a coiled tubing for continuous penetration. Their work is published in peer-reviewed journal, Geothermics.

In a bid to simplify the numerical simulation of the axisymmetric circular drilling model, the authors adopted 2-dimensional cross sections to represent the actual 3-dimensional setup. Consistent with the cooling configurations, the overall geometry model was divided into two regions. They included the well bottom along with the annulus.

The researchers set the wellbore length at 300mm in order to make sure the outcomes reflected the distribution regularities of the hydrothermal jet. The borehole and coiled tubing diameter were 50.8mm and 25.4mm respectively. Cooling water exit diameter was 10mm.

The authors analyzed the effects of jet velocity, cooling water velocity, jet temperature and standoff distance. They observed that impacting flow field of the jet on the bottom of the well was divided into the jet zone, return zone, and the eddy zone. Each of these zones had varying flow patterns. The temperature at the jet zone remained constant, but dropped in return and eddy zones. Temperature at wellbore was higher as compared to that around the tubing owing to return flow of water-cooling.

Jet velocity, standoff distance, and cooling water velocity posed significant effects on the bottom hole temperature and pressure. The researchers observed that the length of the potential core was correlated to the value of maximum axial velocity. Increasing jet velocity caused great pressure and temperature drop at the bottom of the well. Increasing cooling water velocity moderated bottom hole pressure drop. For this reason, the cooling water velocity needs to be designed in keeping with the optimal bottom hole pressure distribution requirements.

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Reference

Xianzhi Song, Zehao Lv, Gensheng Li, Xiaodong Hu, Yu Shi. Numerical analysis on the impact of the flow field of hydrothermal jet drilling for geothermal wells in a confined cooling environment. Geothermics, volume 66 (2017), pages 39–49.

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