Structural failure mechanism and strengthening method of fracture plugging zone for lost circulation control in deep naturally fractured reservoirs

Focused on the lost circulation control in deep naturally fractured reservoirs, the multiscale structure of fracture plugging zone is proposed based on the theory of granular matter mechanics, and the structural failure pattern of plugging zone is developed to reveal the plugging zone failure mechanisms in deep, high temperature, high pressure, and high in-situ stress environment. Based on the fracture plugging zone strength model, key performance parameters are determined for the optimal selection of loss control material(LCM). Laboratory fracture plugging experiments with new LCM are carried out to evaluate the effect of the key performance parameters of LCM on fracture plugging quality. LCM selection strategy for fractured reservoirs is developed. The results show that the force chain formed by LCMs determines the pressure stabilization of macro-scale fracture plugging zone. Friction failure and shear failure are the two major failure patterns of fracture plugging zone. The strength of force chain depends on the performance of micro-scale LCM, and the LCM key performance parameters include particle size distribution, fiber aspect ratio, friction coefficient, compressive strength, soluble ability and high temperature resistance. Results of lab experiments and field test show that lost circulation control quality can be effectively improved with the optimal material selection based on the extracted key performance parameters of LCMs.

Reinforcement strength reduction in FEM for mechanically stabilized earth structures

The factor of safety of mechanically stabilized earth(MSE) structures can be analyzed either using limit equilibrium method(LEM) or strength reduction method(SRM) in finite element/difference method. In LEM, the strengths of the reinforcement members and soils are reduced with the same factor. While using the SRM, only soil strength is reduced during the calculation of the factor of safety. This causes inconsistence in calculating the factor of safety of the MSE structures. To overcome this, an iteration method is proposed to consider the strength reduction of the reinforcements in SRM. The method is demonstrated by using PLAXIS, a finite element software. The results show that the factor of safety converges after a few iterations. The reduction of strength has different effects on the factor of safety depending on the properties of the reinforcements and the soil, and failure modes.

Effect of mineralogy on friction-dilation relationships for simulated faults:Implications for permeability evolution in caprock faults

This paper experimentally explores the frictional sliding behavior of two simulated gouges:one,a series of quartz–smectite mixtures,and the other,powdered natural rocks,aiming to evaluate and codify the effect of mineralogy on gouge dilation and frictional strength,stability,and healing.Specifically,velocity-stepping and slide-hold-slide experiments were performed in a double direct shear configuration to analyze frictional constitutive parameters at room temperature,under normal stresses of 10,20,and 40 MPa.Gouge dilation was measured based on the applied step-wise changes in shear velocity.The frictional response of the quartz–smectite mixtures and powdered natural rocks are affected by their phyllosilicate content.Frictional strength and healing rates decrease with increasing phyllosilicate content,and at 20 wt.%a transition from velocity-weakening to velocity-strengthening behavior was noted.For both suites of gouges,dilation is positively correlated with frictional strength and healing rates,and negatively correlated with frictional stability.Changes in the permeability of gouge-filled faults were estimated from changes in mean porosity,indexed through measured magnitudes of gouge dilation.This combined analysis implies that the reactivation of caprock faults filled with phyllosilicaterich gouges may have a strong influence on permeability evolution in caprock faults.

Estimation of soil reinforcement by the roots of four postdam prevailing grass species in the riparian zone of Three Gorges Reservoir, China

Soil erosion and bank degradation is a major post-dam concern regarding the riparian zone of the Three Gorges Reservoir. The development and succession of vegetation is a main countermeasure,especially to enhance bank stability and mitigate soil erosion by the root system. In this study, the roots of four prevailing grass species, namely, Cynodon dactylon, Hemarthria altissima, Hemarthria compressa, and Paspalum paspaloides, in the riparian zone were investigated in relation to additional soil cohesion. Roots were sampled using a single root auger. Root length density（RLD） and root area ratio（RAR） were measured by using the Win RHIZO image analysis system. Root tensile strength（TR） was performed using a manualdynamometer, and the soil reinforcement caused by the roots was estimated using the simple Wu＇s perpendicular model. Results showed that RLD values of the studied species ranged from 0.24 cm/cm3 to20.89 cm/cm3 at different soil layers, and RLD were significantly greater at 0–10 cm depth in comparison to the deeper soil layers（＆gt;10 cm）. RAR measurements revealed that on average 0.21% of the reference soil area was occupied by grass roots for all the investigated species. The measured root tensile strength was the highest for P. paspaloides（62.26MPa） followed by C. dactylon（51.49 MPa）, H.compressa（50.66 MPa）, and H. altissima（48.81MPa）. Nevertheless, the estimated maximum root reinforcement in this investigation was 22.5 k Pa for H.altissima followed by H. compressa（21.1 k Pa）, P.paspaloides（19.5 k Pa）, and C. dactylon（15.4 k Pa） at0–5 cm depth soil layer. The root cohesion values estimated for all species were generally distributed at the 0–10 cm depth and decreased with the increment of soil depth. The higher root cohesion associated with H. altissima and H. compressa implies their suitability for revegetation purposes to strengthen the shallow soil in the riparian zone of the Three Gorges Reservoir. Although the soil reinforcement induced by roots is only assessed from indirect indicators, the present results still useful for species selection in the framework of implementing and future vegetation recovery actions in the riparian zone of the Three Gorges Reservoir and similar areas in the Yangtze River Basin.

Study on Properties of Anti-Gas Channeling Lightweight Temperature-Resistant Latex Slurry

There is low formation pressure coefficient and high formation temperature in ludong-wucaiwan area. Gas cut and gas channeling happen seriously during oil and gas well cementing. The existing anti-migration additive has only effects on single trait, so it is difficult to meet cementing requirement. According to this situation we could use latex slurry to anti-gas channeling. We have synthesised a set of anti-gas channeling lightweight temperature-resistant latex slurry and formed a new channeling preventing latex slurry through plenty of previous laboratory experiments. Finally the performance of latex slurry on temperature-resistant, anti-gas channeling and the anti-gas channeling of cement paste are studied. The experimental results show that this latex system has strong temperature-resistant and anti-gas channeling, which completely meet the requirement of cementing in this area.

Comparison of short-term and long-term performances for polymer-stabilized sand and clay

A series of tests were carried out on sulfate rich, high-plasticity clay and poorly-graded natural sand to study the effectiveness of a methylene diphenyl diisocyanate based liquid polymer soil stabilizer in improving the unconfined compressive strength （UCS） of freshly stabilized soils and aged sand specimens. The aged specimens were prepared by exposing the specimens to ultraviolet radiation, freeze-thaw, and wet-dry weathering. The polymer soil stabilizer also mitigated the swelling of the expansive clay. For clay, the observations indicated that the sequence of adding water and liquid polymer had great influence on the gained UGS of stabilized specimens. However, this was shown to be of little importance for sand. Furthermore, sand samples showed incremental gains in UCS when they were submerged in water. This increase was significant for up to 4 days of soaking in water after 4 days of ambient air curing. Conversely, the clay samples lost a large fraction of their UCS when soaked in water; however, their remaining strength was still considerable. The stabilized specimens showed acceptable endurance under weathering action, although sample yellowing due to ultraviolet radiation was evident on the surface of the specimens. Except for moisture susceptibility of the clay specimens, the results of this study suggested the liquid stabilizer could be successfully utilized to provide acceptable strength, durability and mitigated swelling.