Prevention of water blocking and optimization of multiscale flow channels will increase gas production of tight reservoirs. Physical properties of samples from representative tight gas reservoirs were measured before and after high temperature treatment. Results show that, with the increase of treatment temperature, mass decreases, acoustic transit time increases, and permeability and porosity increase. Permeability begins to increase dramatically if treatment temperature exceeds the threshold value of thermal fracturing, which is 600~700°C, 500~600°C, 300~500°C, and 300~400°C for shale, mudstone, tight sandstone, and tight carbonate rock, respectively. Comprehensive analyses indicate that the mechanisms of heat treatment on tight porous media include evaporation and dehydration of water, change of mineral structure, generation of microfracture, and network connectivity. Meanwhile, field implementation is reviewed and prospected. Interpretations indicate that, according to the characteristics of multiscale mass transfer in tight gas formation, combining heat treatment with conventional stimulation methods can achieve the best stimulation result.

Complex geology and fracturing operations have led to frequent sand production problem in the shale gas well. Sand production brings huge engineering risks and seriously affects the normal production of the shale gas well. In order to study the property and source of the yielded sand, sand samples in three production periods of flowback, production test and gas production are collected from Sichuan Basin of China. Combining the methods of particle size analysis, microscope observation, scanning electron microscope, CT scanning, infrared spectroscopy and energy dispersive spectrum analysis, the multi-scale structure and composition characteristics of the yielded sand from different production periods were investigated. Results show that the sand size is the largest in the production test period and the smallest in the gas production period. The large-size sand is blocky in the flowback period, while it is flaky in the period of production test and gas production. The roundness of sand becomes worse as the sand size decreasing. Sand composition has the characteristics of fracturing proppant and shale mineral. Cementing material between large-size sands has the network structure and the higher content of aluminum and iron. Organic chemicals are found to be adhered to the sand surface in all three periods. Both shale fracture and proppant failure can generate particles that provide the material source for sand production. This research provides the source of the yielded sand and a theoretical guidance for the sand production mechanism.

Along with the increasingly deep wells and long horizontal wells, the wellbore instability and formation damage undeniably becomes the biggest concerns during well construction. Sha 3 reservoirs in faulted B Depression, a low permeability exploration target zones, are highly fractured. Most of the wells with the barefoot interval length of over 2000m or even about 3500m were drilled under underbalanced conditions to improve exploration success rate. The exploration results were disappointing. Shale sloughing and borehole enlargement frequently occurred during the process of previous drilling, followed by some troubles including serious formation damage. Different kinds of drilling fluid techniques, including the nano-emulsion drilling fluid, polymeric alcohol and potassium formate, were tried to settle these problems with mixed results. Mineral analysis, shale dispersion and swelling test, and dynamic leakoff tests show that the solid particles mismatching well with the sizes of pore throats and the width of natural fractures and the high pH value of drilling fluids are two key factors triggered the borehole instability and formation damage. Based on the temporary shielding theory, the modified drilling fluids are developed by adding fibrous bridging agents and by decreasing pH value. Dynamic leakoff tests manifest that the modified drilling fluids are able to rapidly seal the micro fractures and pore throats and the percentage of regained permeability can reach more than 85%. Two experiment wells, a vertical well and a sidetracked well, located in an area of severe shale sloughing, were drilled with the modified drill fluids under micro overbalanced conditions. The results indict that the diameters of two boreholes are near-gauge, that the coring, well logging and completion operations are conducted very successfully, and that the well construction time is cut down by 50% compared with that of previous drilled wells.

1. Unconsolidated sandstone reservoirs hold a significant position in global oil and gas resources. However, their long-term development faces the persistent challenges of sand production and a sha...