Research On Fracture Genetic Mechanics And Parameter Design Of Radial Lateral Fracturing In Coalbed Methane

In order to solve the low single well productivity issue in China coalbed methane development,a novel stimulation approach,namely radial lateral fracturing(RLF),is proposed,which requires radial drilling one or more laterals in a vertical or horizontal well first and then operating the fracturing through the laterals.Based on the experimental analysis and numerical simulations,the fracture genetic mechanics and fracturing parameter design process for RLF's implementation in a vertical well are studied.The research contents and accomplishments are as follows:1.An experimental simulation of RLF is conducted with natural coal samples,from which two fracture profiles are obtained: one is the T-shaped profile with vertical initiation,the other is the “one-horizontal-multiple-vertical” shaped profile with horizontal initiation from one bedding.Under the experiment conditions,the first profile is likely to happen if the lateral direction is far from the max horizontal stress while the second profile tends to be formed if the lateral is parallel to or close to the max horizontal stress direction.From the fracturing curves of the specimens with horizontal initiation,the initiation and injection pressures are reduced with the increase of lateral length and count.In the specimens with “one-horizontal-multiple-vertical” shaped profile,the distribution of vertical fractures are not affected by the radial laterals' direction and the horizontal fracture can substantially cover an area with a radius of the lateral length.The rock property difference may lead to the initiation type disparity between two specimens even though their lateral layouts are the same.2.The break process of RLF is simulated based on the seepage-stress-damage coupling model and the affection of coal rock characteristics on the initiation type are analyzed.The results show that: the anisotropy of coal tensile strength andpermeability and coal heterogeneity can greatly influence the initiation type of RLF.Under the simulation conditions,lower anisotropy and higher heterogeneity,which are reflected as a higher degree of cleat system development in coals,are in favor of vertical initiation for RLF.Otherwise the RLF tends to generate horizontal initiation from a bedding.Larger vertical stress is more beneficial for RLF to generate vertical initiation.3.A productivity analysis on the two fracture profiles obtained from the experiments is carried out.By comparing the productivity with perforation fracturing,the “one-horizontal-multiple-vertical” shaped profile is selected as the target profile of RLF because of its greater improvement of the single well productivity.The influence of the horizontal fracture's half-length on productivity is greater than the lateral length,therefore the extension of horizontal fracture should be enlarged as much as possible.To ensure horizontal initiation,the target coal seam should be comprised with undeformed coals or blocky coals whose cleat systems are not well-developed.The layer's overburden stress should be less than or close to the max horizontal stress.4.A discrete element numerical model of the “one-horizontal-multiple-vertical” shaped profile is established and the sensitivity analysis of stress state,operation parameters and rock properties on the stimulation reservoir volume(SRV)is presented.The results show that: the stress state of overburden stress as the maximum stress is most favorable for RLF to generate a large-scale fracture network while the stress state of overburden stress as the minimum stress is least suited for the RLF operation.The SRV and single well productivity are growing with the increase of lateral length and fracturing fluid viscosity.For the coal seams with low brittleness(low Young's modulus),low solidness(low joint stiffness)and weak cementation(low joint cohesion and friction angle),if the well density is low,short length laterals and low viscosity fluid can be used in multiple wells to operate synchronous fracturing for large-scale fracture network generation and total productivity enhancement of all wells.The layer number of laterals can be designed based on the formation thickness.4 branches with the phase of 90°and each lateral is at 45° from the horizontal stress is proposed as the optimal lateral pattern for singlelayer layout.Our research presents a new stimulation method for coalbed methane development and provides the design support for its application.