Numerical simulation on borehole breakout and borehole size effect using discrete element method

Estimation of horizontal stress magnitudes from borehole breakouts has been an attractive topic in the petroleum and mining industries,although there are critical research gaps that remain unfilled.In this paper,numerical simulation is conducted on Gosford sandstone to investigate the borehole breakout and its associated borehole size effect,including temperature influence.The discrete element method(DEM)model shows that the borehole breakout angular span is constant after the initial formation,whereas its depth propagates along the minimum horizontal stress direction.This indicates that the breakout angular span is a reliable parameter for horizontal stress estimation.The borehole size effect simulations illustrated the importance of borehole size on breakout geometries in which smaller borehole size leads to higher breakout initiation stress as well as the stress re-distribution from borehole wall outwards through micro-cracking.This implies that the stress may be averaged over a distance around the borehole and breakout initiation occurs at the borehole wall rather than some distance into the rock.In addition,the numerical simulation incorporated the thermal effect which is widely encountered in deep geothermal wells.Based on the results,the higher temperature led to lower breakout initiation stress with same borehole size,and more proportion of shear cracks was generated under higher temperature.This indicates that the temperature might contribute to the micro-fracturing mode and hence influences the horizontal stress estimation results from borehole breakout geometries.Numerical simulation showed that breakout shape and dimensions changed considerably under high stress and high temperature conditions,suggesting that the temperature may need to be considered for breakout stress analysis in deep locations.

Thermal performance analysis of borehole size effect on geothermal heat exchanger

Thermal performance was the most important factor in the development of borehole heat exchanger utilizing geothermal energy. The thermal performance was affected by many different design parameters, such as configuration type and borehole size of geothermal heat exchanger. These eventually determined the operation and cost efficiency of the geothermal heat exchanger system. The main purpose of this work was to assess the thermal performance of geother^nal heat exchanger with variation of borehole sizes and numbers of U-tubes inside a borehole. For this, a thermal response test rig was established with line-source theory. The thermal response test was performed with in-line variable input heat source. Effective thermal conductivity and thermal resistance were obtained from the measured data. From the measurement, the effective thermal conductivity is found to have similar values for two- pair type （4 U-tubes） and three-pair type （6 U-tubes） borehole heat exchanger systems indicating similar heat transfer ability. Meanwhile, the thermal resistance shows lower value for the three-pair type compared to the two-pair type. Measured data based resistance have lower value compared to computed result from design programs. Overall comparison finds better thermal performance for the three-pair type, however, fluctuating temperature variation indicates complex flow behavior inside the borehole and requires further study on flow characteristics.

In-situ stress inversion in Liard Basin, Canada, from caliper logs

This paper proposes an integrated method of analytical calculation,artificial intelligence,and probabilistic analysis to cost-effectively determine geomechanical properties and in-situ stresses from borehole deformation via caliper logs.It's also demonstrated in this paper that the actual borehole size can not be simply taken as the bit size by default,and adjusted borehole size has to be used to find the reasonable borehole deformation.In the proposed method,an artificial neural network(ANN)is applied to map the relationship among in-situ stress,adjusted borehole size,geomechanical properties,and borehole displacements.The genetic algorithm(GA)searches for the set of unknown stresses and geomechanical properties that match the objective borehole deformation function.Probabilistic analysis is conducted after ANN-GA modeling to estimate the most possible ranges of the parameters.The hybrid method has been demonstrated by a field case study to estimate the adjusted borehole size,Young's modulus,and the two horizontal in-situ stresses using borehole deformation information reported from four-arm caliper logs of a vertical borehole in Liard Basin in Canada.