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Quick 3-D Formation Model for Unconventional Carbonate Reservoirs by Resistivity Imaging-While-Drilling in Horizontal Wells


Resistivity imaging-while-drilling measurement in two horizontal wells was used to construct quick 3D formation model, which encapsulates both the structural information and the properties of horizontal sections through a thin (approximately 2 m thick) carbonate gas reservoir in a field in China. The true horizontal lengths were about 790 m and 840 m. The high-resolution resistivity images obtained while drilling revealed the geological structure's details as well as the variations of the formation properties in the reservoir, leading to optimal placement of horizontal wells in real time. The carbonate gas reservoir of the target formation consists of two types of sedimentary rocks with special resistivity-image textures and resistivity variations that correspond to the reservoir quality of porosity of 0.27∼18.7% and permeability of 0.01∼5.6 mD. Structural details become increasingly important in the thin, heterogeneous carbonate reservoir because the well borehole may exit the reservoir upon encountering sharply local changes in the formation's structural dip or azimuth caused by fault or by rapid vertical and lateral variability of depositional lithofacies that are below seismic resolution due to the limited frequency content of surface seismic data. Resistivity imaging-while-drilling is ideal for thin heterogeneous carbonate gas reservoirs; the method measures azimuthally focused laterolog resistivity at four depths of investigation: 15 cm, 13 cm, 8 cm, and 4 cm. This provides formation-resistivity data with minimal bed-boundary effects in high-angle and horizontal wells. With the use of 3D modeling techniques, integrating the structure details and formation property variations to a quick 3D formation model constrains the likely stratigraphic distribution of each layer and ensures that the well stays in the thin target layer. Upscaling the resistivity of the reservoir-rock properties to the formation model constrains the likely architecture of reservoirs in the target formation. The resistivity-property model further improves the understanding of thin unconventional carbonate gas reservoir. The quick 3D formation model from resistivity imaging-while-drilling can be used in drilling tendency adjustment to properly trace thin target layer and increase reservoir exposure and hence production. It makes real-time 3D reservoir modeling possible for subsequent reserves calculations, production simulation, and future well planning.