Fracturing curve types and their formation mechanism of coal seam 3 in Shizhuang block, Qinshui basin

Fracturing curve of coalbed methane (CBM) wells offers lots of dynamic information of fracture extension, which is able to reflect fracturing effect.Based on the fracturing data of 45 CBM wells where coal seam 3 was only fractured and drained in Shizhuang CBM block, fracturing curve types were classified and the relationship between fracturing curve types and gas production of CBM wells were discussed.Most importantly, the formation mechanism of fracturing curve types were first time discussed.The study shows that the fracturing curves of coal seam 3 in Shizhuang CBM block can be divided, according to the morphological characteristics during sand-bearing stage, into four types including stable, descending, rising and fluctuating.The stable and descending curves always lead to high gas productions.However, the rising and fluctuating curves are usually corresponding to low gas productions.The in-situ stress types and related hydraulic fracture types have a controlling effect on fracturing curve types.In a field of in-situ stress type II (σ_H > σ_h > σ_v), horizontal hydraulic fractures accompanied with “T”-shape or “工”-shape complex fractures are developed and results in rising and fluctuating fracturing curves.However, in a region where the in-situ stress type is either type III(σ_H > σ_v > σ_h)or type I (σ_v > σ_H > σ_h), vertical hydraulic fractures or vertical hydraulic fractures accompanied with “T”-shape or “工”-shape complex fractures are developed and results in stable and descending fracturing curves.The distribution of fracturing curve types in Shizhuang CBM block shows a certain regularity.In the eastern part of the block where in-situ stress type II (σ_H > σ_h > σ_v) or type III(σ_H > σ_v > σ_h)was identified, the stable, descending, rising and fluctuating fracturing curve types were observed.In the western part of the block where in-situ stress type I (σ_v > σ_H > σ_h) were identified, only the stable and fluctuating fracturing curve types were observed.