General criterion for intersection between hydraulic induced fractures and pre-existing natural fractures

Pre-existing natural fractures and other structurally weak planes are usually well-developed in unconventional reservoirs.When such fractures intersect with hydraulic induced fractures,they will redirect and propagate as an important mechanical principle of volume fracturing by the formation of complex fracture networks.Under the shadow effect of natural fractures and other structurally weak planes with hydraulic supported fracture stress,hydraulic fractures do not fully propagate in the direction of the maximum horizontal-principal-stress.This paper computed the stress intensity factors of hydraulic fracture types I and II by integrating the various interactions,established universally-applicable mechanical principles for the propagation behavior when a hydraulic fracture propagating in an arbitrary direction intersects with a natural fracture at an arbitrary angle,and demonstrated the mechanical principles of the intersection between hydraulic induced fractures and pre-existing natural fractures.This study proved the following conclusions:as the intersection angle between the hydraulic fracture and the maximum horizontal-principal-stress increased,the possibility of the hydraulic fracture being captured by the natural fracture with an identical approaching angle first increased and then decreased;as the net stress increased,the intersection behavior between the hydraulic fracture and the natural fracture transitioned from penetration to capture.

Overburden fracture evolution laws and water-controlling technologies in mining very thick coal seam under water-rich roof

Considering the danger of water inrush in mining very thick coal seam under water-rich roof in Majialiang Coal Mine,the universal discrete element(UDEC)software was used to simulate the overburden fracture evolution laws when mining 4#coal seam.Besides,this study researched on the influence of face advancing length,speed and mining height on the height of the water flowing fractured zones(HWFFZ),and analyzed the correlation of face advancing length and change rules of aquifer water levels and goaf water inflow.Based on those mentioned above,this research proposed the following water-controlling technologies:draining the roof water before mining,draining goaf water,reasonable advancing speed and mining thickness.These water-controlling technologies were successfully used in the feld,thus ensured safely mining the very thick coal seam under water-rich roof.

Crushing and embedment of proppant packs under cyclic loading: An insight to enhanced unconventional oil/gas recovery

Crushing and embedment are two critical downhole proppant degradation mechanisms that lead to a significant drop in production outputs in unconventional oil/gas stimulation projects. These persistent production drops due to the non-linear responses of proppants under reservoir conditions put the future utilization of such advanced stimulation techniques in unconventional energy extraction in doubt. The aim of this study is to address these issues by conducting a comprehensive experimental approach. According to the results, whatever the type of proppant, all proppant packs tend to undergo significant plastic deformation under the first loading cycle.Moreover, the utilization of ceramic proppants(which retain proppant pack porosity up to 75%), larger proppant sizes(which retain proppant pack porosity up to 15.2%) and higher proppant concentrations(which retain proppant pack porosity up to 29.5%) in the fracturing stimulations with higher in-situ stresses are recommended to de-escalate the critical consequences of crushing associated issues. Similarly, the selection of resin-coated proppants over ceramic and sand proppants may benefit in terms of obtaining reduced proppant embedment.In addition, selection of smaller proppant sizes and higher proppant concentrations are suggested for stimulation projects at depth with sedimentary formations and lower in-situ stresses where proppant embedment predominates. Furthermore, correlation between proppant embedment with repetitive loading cycles was studied.Importantly, microstructural analysis of the proppant-embedded siltstone rock samples revealed that the initiation of secondary induced fractures. Finally, the findings of this study can greatly contribute to accurately select optimum proppant properties(proppant type, size and concentration) depending on the oil/gas reservoir characteristics to minimize proppant crushing and embedment effects.

Estimation of the horizontal in-situ stress magnitude and azimuth using previous drilling data

Oil exploration and production,well stability,sand production,geothermal drilling,waste-water or CO_(2) sequestration,geohazards assessment,and EOR processes such as hydraulic fracturing,require adequate information about in-situ stresses.There are several methods for analyzing the magnitude and direction of in-situ stresses.The evaluation of tensile fractures and shear fractures in vertical oil and gas wellbores using image logs is one of these methods.Furthermore,when image logs are run in boreholes,they can be extremely costly and possibly stop the drilling.The data for this study were gathered from seven directional wells drilled into a strike-slip fault reservoir in southern Iran.Vertical stress,minimum horizontal stress,pore pressure,Poisson's ratio of formations,and 233 mud loss reporting points make up the entire data.This is the first time maximum horizontal stress direction has been calculated without referring to image log data.In addition,the points of lost circulation were categorized into natural and induced fracture.The results revealed that,the maximum horizontal stress direction of the reservoir was calculated at 65
northeast-southwest.The error rate is roughly 10
when comparing the results of this investigation to those obtained from the image log.The maximum horizontal stress direction is calculated precisely.In terms of tensile fracture pressure,the in-situ stress ratio identifies the safest as well as the most critical inclination and azimuth for each well.