Complex Fracture Design For Hydraulic Fracturing Treatments In Coalbed Methane Reservoirs

Coalbed mehtane is featured by its widespread distribution and long-term stable gas production,which is an important component of unconventional natural gas resources and currently one of the hot spots of new energy research.Coalbed methane reservoirs are typically low permeability formations,and most of the wells can hardly obtain gas production without hydraulic fracturing.Thus hydraulic fracturing is one of the dominant stimulation methods of coalbed methane development.Coal rocks are soft rocks with low Youngs’ modulus,abundant natural fractures such as cleat and high stress sensitivity.Due to natural cleat,complex fracture propagation is likely to occur.With complex fractures such as T-shaped fractures and irregular fracture networks developing,the opening degree of the main hydraulic fracture is limited,the probability of screening out sharply increases and the difficulty in raising up proppant concentration intensifies,which ultimately leads to low conductivity of effectively propped fractures and poor performance of hydraulic fracturing.As the stress state changes,the internal natural cleat cracks open and consequently the porosity and permeability increase.The filtrate volume of fracturing fluids increases as a result of increasing permeability,which decreases the efficiency of fracturing fluids,intensifies the difficulty of fracture propagation and narrow the communication range between the hydraulic fractures and natural fractures.This study focused on the complex geometry characteristic of hydraulic fracturing in coalbed methane reservoir.With the applications of rock mechanics,elastic mechanics,fluid mechanics,computational mathematics and so on,considering the complex induced fractures in the field practice,this study established the propagation theory of T-shaped fractures and multiple fractures,conducted fracturing designs directed at these two fracture systems,aiming at offering technical guidance for optimizing the hydraulic fracturing performance in coalbed methane reservoir.This paper,based on the previous research findings,conducted a systematic research on the design of complex fractures in coalbed methane reservoir fracturing mainly through the following ways:(1)The coalbed fractal and structural characteristics were concluded.A series of fractal dimension methods were concluded and the software to acquire fractal dimensions was programmed on the Matlab platform.The images of fracture distribution of coal samples were obtained after cutting and washing.The fractal dimensions of six coal samples were acquired after gaining and noise deduction,ranging from 1.3 to 1.5.(2)The fracture initiation and propagation during hydraulic fracturing treatment in CBM reservoir were investigated in this paper.The complex fracture geometry includes T shaped fracture and multiple fracture system.The T shaped fracture is composed of a vertical fracture that propagates only in pay zone and a horizontal fracture grows along the bedding interfaces due to fracture termination and fluid distribution.The fracture mechanisms responsible for stress filed analysis at fracture tip was revealed,moreover,opening and sliding crack types were combined into T shaped fracture creation law.The penny model was applied to simulate horizontal fracture with extend ratio and the classical KGD model can be used for the simulation of vertical fracture.Furthermore,the flow distribution on the interface is assumed to obey pressure and flow balance.Thus the established novel model can describe ‘T’ shaped fracture propagation in CBM reservoir.(3)The multiple fracture system refers to the complex and irregular fractures.This paper employs fractal dimension and volume density balance theory to describe disordered natural fractures.And the regular parallel fracture clusters that intersect with hydraulic fracture can be replaced for the actual multiple fracture system.Through stress filed analysis at fracture tip and opening and sliding fracture creation mechanisms,complex fracture behaviors includes fracture step-over,fracture deviation and natural fracture open.The integrated pseudo-three fracturing model with pre-existing fracture array were built for simulate multiple fracture system propagation.It’s noted that the ratio of natural fracture opening length and main hydraulic fracture length was controlled by stress interference and fluid energy transport,so natural fracture can only extend to limited area.(4)The hydraulic fracturing design software for CBM reservoir was programmed.Parametric studies are carried out for different natural fracture density,pump rates,Young’s modulus contrasts,in situ stresses,and fluid viscosity etc.According to numerical results,the critical factors are in situ stresses and pump rate.Meanwhile,the results show the influences of natural fracture density on fluid leak-off rate,hydraulic fracture length and fracture conductivity were obvious.To validate the reliability of this model,the programmed fracturing simulator based on the mathematical model further applied to two CBM gas wells in Qinshui Basin,China.The model accuracy is verified in terms of comparisons with existing in-situ results,thereby providing a good tool for hydraulic fracturing design in CBM reservoirs.