The Research On Mini-frac Calibration Interpretation Of Coal-bed Methane Wells And Application

The mini-frac calibration is proceeded for evaluation and preparation before a main fracturing treatment,and can provide important reference for the design and operation of the main fracturing treatment.During the fracturing operation of normal oil and gas reservoirs,the mini-frac calibration has been used widely,but for the coal gas formations,its application and interpretation need to be improved.For the coal gas formations,the role of mini-frac calibration is obvious.Since there exist natural fissures in CBM(coal-bed methane)reservoirs,the natural fissures will open to form multi-fractures during the fracturing operation,and will lead to pressure increase abnormally.But the analysis of mini-frac calibration can determine early whether the natural fissures are opening or not,so some measures can be taken to avoid operation failures.In the thesis,considering the characters such as natural fissures and the pressure-dependent leak-off in CBM reservoirs,the proper interpretation methods are provided and used in the fracturing operations.The mini-frac calibration includes step-rate test,pump-in at constant rate andshut-down periods.The step-rate test also has two cases of step-up and step-down rate,and the shut-down period includes before-closure and after-closure.According to the relationship between pressure and rate during the step-up rate test,the fracture extension and closure pressure can be defined.The step-down rate test can be used to calculate the friction pressure of perforations and near wellbore,and to decide the conditions of fracture's re-orientations.The pressure change in the process of pump-in,can be used to estimate the fracture extension,and to calculate the fracture geometry.After the pump is shut down,the parameters such as leak-off coefficient,formation closure pressure and time,fracture geometry,can be determined through the before-closure analysis.The formation pressure and permeability also can be evaluated by the after-closure analysis.The above methods are combined to determine the parameters related to formation,fracture and fracturing fluid,so the fracturing design and operation can be improved.For the mini-frac calibration in CBM reservoirs,some new interpretation methods are proposed as follows:1.A new method to calculate fracture extension pressure is put forward during the step-rate test.Compared with the methods used before,this method is more simple and reliable.At the same time,the formation permeability is determined with the slope of the first straight line,and can be testified with other results.2.The coal permeability is sensitive to stress and the natural fractures are plenty,so these factors are considered.The relationship between fluid leak-off rate and pressure is formed by the before-closure analysis.The dynamic influencing model about the leak-off coefficient and the pressure is proposed,and the closing process is simulated.3.A new simulation method is raised for the pumping process,considering the pressure-dependent leak-off,so the results are more practical.Combining the dynamic influencing model,which relates the leak-off coefficient and the pressure,and the 2D PKN fracture model,the net pressure is simulated,so the fracture geometry can be calculated,and the results can be used for fracture design.4.The process after shut-down is devided into two periods:before-closure and after-closure,so the fracture and formation parameters can be confined by use of the proper models.The conventional well test theory is introduced,and the pressure after shut-down is analyzed according to single and double porosity media,and the results are compared with each other.In last,these methods are put use into the fracturing operations to verify applicability and accuracy.The application of mini-frac calibration will be prospective,because it not only proposes reference for the main fracturing treatment,but also evaluates the fracturing effect.Comparing with the methods such as temperature test,micro-earthquake and surface tilt-meter,the mini-frac calibration is simple and the cost is low.Furthermore,for the special characters of CBM reservoirs,the conventional pump-in/shut-down test will last for days,but the mini-frac calibration will wait justly for several hours until the fractures are closed,so the mini-frac calibration can replace the conventional well test.The mini-frac calibration has advantages not only in the determining of parameters such as formation,fracture and fracturing fluid,but also in fracture monitoring and well test,so the research on the mini-frac calibration is significant.