Reservoir Evaluation Technology During Underbalanced Drilling of Horizontal Wells in Gas Reservoirs

This paper describes a method for real-time reservoir characterization during underbalanced drilling. Therefore, this study considers wellbore and formation as a system and establishes wellbore multiphase flow calculation and formation fluid-flow models. To acquire the basic data for the characterization in a timely way, the study develops and adopts the system of ground-monitoring devices to measure the formation fluid flow into the wellbore and to the surface in real time. The proposed mathematical models, construction parameters, and wellhole structure were used to calculate the downhole underbalanced pressure. Using the measured gas production and formation fluid-flow model, the permeability can be calculated. The mathematical models also reveal the actual formation fluid flow during the drilling phase, and thereby we could identify fracture characteristics (such as fracture development and heterogeneity) using a plot of permeability versus well depth. The proposed method was adopted to evaluate a depth range of 4,092 to 4,285 m in the Yingcheng formation of well X in the Jilin oil field and acquired the permeability according to the depth of the horizontal well in the gas reservoir (the average permeability was 0.059 md and microcracks were well developed). The proposed technique, which combines the recognition of formation parameters and monitoring of wellbore pressure and fluid flow, can improve the precision of underbalanced drilling control, and the calculated permeability truly reflects the fluid-flow situation during the drilling processes.