Xiangchen Li

Laboratory measurement and interpretation of nonlinear gas flow in shale

Gas flow mechanisms in shale are urgent to clarify due to the complicated pore structure and low permeability. Core flow experiments were conducted under reservoir net confining stress with samples from the Longmaxi Shale to investigate the characteristics of nonlinear gas flow. Meanwhile, microstructure analyses and gas adsorption experiments are implemented. Experimental results indicate that non-Darcy flow in shale is remarkable and it has a close relationship with pore pressure. It is found that type of gas has a significant influence on permeability measurement and methane is chosen in this work to study the shale gas flow. Gas slippage effect and minimum threshold pressure gradient weaken with the increasing backpressure. It is demonstrated that gas flow regime would be either slip flow or transition flow with certain pore pressure and permeability. Experimental data computations and microstructure analyses confirm that hydraulic radius of flow tubes in shale are mostly less than 100 nm, indicating that there is no micron scale pore or throat which mainly contributes to flow. The results are significant for the study of gas flow in shale, and are beneficial for laboratory investigation of shale permeability.

Laboratory Measurement and Interpretation of the Changes of Physical Properties after Heat Treatment in Tight Porous Media

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.

Mitigating Borehole Instability And Formation Damage With Temporary Shielding Drilling Fluids In Low Permeability Fractured Reservoirs

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. Introduction Recent decades have seen the increasingly important role of low permeability oil and gas reservoirs in increasing reserve and production supply. Proved low permeability oil reserves are 99.4×10 tons, 36% of the total oil reserves, and proved tight sandstone gas reserves are 3.1×10m by the end of 2010 in China. Borehole instability and formation damage contribute a lot to poor economic benefits of development of low permeability oil and gas reservoirs, which need to be minimized and effectively controlled for the play to be economical exploitable, so the significance of preventing the borehole instability and formation damage can never be overestimated. Borehole instability, followed by a series of problems, might induce the increase of a considerable amount of nonproductive time, and may indirectly or directly trigger formation damage, so the full consideration should be taken to how to maintain perfect wellbore quality during well drilling and completion. Sha 3 member in B depression is highly fractured low permeability reservoirs, one of the recent important exploration targets. Most of the wells were drilled under underbalanced conditions to improve exploration success rate, but the wells were designed with the barefoot interval of over 2000 m or even about 3500 m long in consideration of the cost and economic benefits. Different kinds of drilling fluid techniques, including the nano-emulsion drilling fluid, polymeric alcohol and potassium formate, were tried to settle borehole instability problems, but they did not work well. This paper focuses on finding the reasons for borehole instability and formation damage, and then presents the measurements for mitigating both borehole instability and formation damage of the low permeable fractured reservoirs by optimizing the drilling fluids.