A borehole-stability model that uniquely couples the mechanical and chemical aspects of drilling-fluid/shale interactions was developed. The model allows the user to determine the optimum drilling parameters (e.g., mud weight and salt concentration) to alleviate borehole-stability-related problems with oil- or water-based drilling-fluid systems. Chemically induced stress alteration based on the thermodynamics of differences in water molar free energies of the drilling fluid and shale is combined with mechanically induced stress. These two potentials are coupled by use of the framework of poroelasticity theory to formulate the physiochemical basis of this borehole-stability model

Borehole-stability model to couple the mechanics and chemistry of drilling-fluid/shale interactions

Discrete-Element-Method/Computational-Fluid-Dynamics Coupling Simulation of Proppant Embedment and Fracture Conductivity After Hydraulic Fracturing

Geomechanical issues in the exploitation of natural gas hydrate

Influence of different thermal cycling treatments on the physical, mechanical and transport properties of granite

This review paper summarizes recent advances and challenges in the assessment of rock behavior and performance in deep low-permeability and high-temperature geothermal reservoirs. Geothermal energy systems for electricity production target deep rock between ca. 2 km and 5 km depth to obtain sufficiently elevated temperatures. Rock permeability enhancement faces many challenges, and therefore the development of Enhanced Geothermal Systems (EGS) still represents a pioneering effort. The potential and advantage of EGS above conventional geothermal reservoirs is its independence of the location that supplies sufficient heat and fluid. Several issues prevent the successful application of EGS technology. First, the effects of non-uniform in-situ stresses and loading history on rock fracturing are not well understood. Second, the role of rock anisotropy, heterogeneity and thermal effects on rock properties in the design of hydraulic fracturing operations is not clear. Third, the reduction of induced seismicity effects raises safety and public acceptance issues. This manuscript formulates outlines for future research directions. Specifically, the recommendations focus on the development of tools for better understanding and mitigating problems, which occur during stimulation of deep geothermal reservoirs.

A review on challenges in the assessment of geomechanical rock performance for deep geothermal reservoir development

Considering the influence of casing, analytical solutions for stress distribution around a cased wellbore are derived, based on which a prediction model for hydraulic fracture initiation with the oriented perforation technique (OPT) is established. Taking well J2 of Z5 oilfield for an example, the predicted initiation pressure with the OPT of our model is about 4.2 MPa higher than the existing model, which neglects the influence of casing. In comparison with the results of laboratory fracturing experiments with OPT on a 400 × 400 × 400 mm3 rock sample for a cased well with the deviation of 45°, the fracture initiation pressure of our model has an error of 3.2 %, while the error of the existing model is 6.6 %; when the well azimuth angle is 0° and the perforation angle is 45°, the prediction error of the fracture initiation pressure of the existing model and our model are 3.4 and 7.7 %, respectively. The study verifies that our model is more applicable for hydraulic fracturing prediction of wells with OPT completion; while the existing model is more suitable for hydraulic fracturing with conventional perforation completion.

Hydraulic Fracture Initiation and Propagation from Wellbore with Oriented Perforation

In oil and gas drilling or geothermal well drilling, the temperature difference between the drilling fluid and formation will lead to an apparent temperature change around the borehole, which will influence the stress state around the borehole and tend to cause borehole instability in high geothermal gradient formations. The thermal effect is usually not considered as a factor in most of the conventional borehole stability models. In this research, in order to solve the borehole instability in high-temperature formations, a calculation model of the temperature field around the borehole during drilling is established. The effects of drilling fluid circulation, drilling fluid density, and mud displacement on the temperature field are analyzed. Besides these effects, the effect of temperature change on the stress around the borehole is analyzed based on thermoelasticity theory. In addition, the relationships between temperature and strength of four types of rocks are respectively established based on experimental results, and thermal expansion coefficients are also tested. On this basis, a borehole stability model is established considering thermal effects and the effect of temperature change on borehole stability is also analyzed. The results show that the fracture pressure and collapse pressure will both increase as the temperature of borehole rises, and vice versa. The fracture pressure is more sensitive to temperature. Temperature has different effects on collapse pressures due to different lithological characters; however, the variation of fracture pressure is unrelated to lithology. The research results can provide a reference for the design of drilling fluid density in high-temperature wells.

Borehole Stability in High-Temperature Formations

Characterization of laboratory-scale hydraulic fracturing for EGS

Heat production from lab-scale enhanced geothermal systems in granite and gabbro

The interaction between a natural fracture (NF) and a hydraulic fracture (HF) has been studied extensively, both experimentally and numerically, to better understand the potential for crossing and arrest of a hydraulic fracture intersecting a natural fracture. However, the actual mechanical interaction between a hydraulic and a natural fracture or a bedding plane has not been studied, particularly under triaxial stress and injection conditions. Analysis of field microseismic data recorded during hydraulic fracturing shows that the bedding plane could slip due to the approaching hydraulic fracture. In this paper, we present the results of some lab-scale experimental work, demonstrating HF/NF interaction with an emphasis on the slippage of a discontinuity surface. Injection pressure, stress applied, and the sample deformation are monitored during the tests. Acoustic Emission (AE) technology is employed to record the AE signals generated during fracture initiation, propagation, and during the sliding of the joint. In addition, strain gauges are used to measure the slippage on the natural fracture. The tests are carried out on 101.6 mm diameter cylindrical samples of PMMA, shale, and granite. The calculated displacement based on the recorded strain clearly shows a jump at the breakdown point, which is accompanied by increased AE activity and stress drop. Analysis of the data clearly shows the occurrence of slippage on the joint in response to an approaching hydraulic fracture. Expectedly, the degree of shear slip varies with natural fracture dip and friction angle.

Lianbo Hu

Papers

Hydraulic Fracture Initiation and Propagation from Wellbore with Oriented Perforation

Borehole Stability in High-Temperature Formations

Characterization of laboratory-scale hydraulic fracturing for EGS

Heat production from lab-scale enhanced geothermal systems in granite and gabbro

Laboratory-Scale Investigation of the Slippage of a Natural Fracture Resulting from an Approaching Hydraulic Fracture