Evaluation of formation damage from a constant-head borehole test

Abstract During geotechnical and geomechanical activities, boreholes are often drilled to investigate the detailed properties of underground formations. Some of the formation parameters, particularly transport properties such as hydraulic conductivity and storativity, are then determined by carrying out hydraulic testing in the boreholes. Conventional methods for interpretation of such borehole tests usually assume uniform formation properties, i.e. the formation is homogeneous and parameters do not change with space and time. This is often not appropriate: the formation may be damaged because of stress-induced compression, drilling breakage, drilling solids invasion, changes in saturation or wettability, fracture filling wash-out, and so on. Such processes usually result in alteration of parameters, therefore borehole test analysis results will not reflect true (far-field) formation characterization. Spurious results can affect other hydrogeological and geomechanical analyses such as underground water flow prediction, pore pressure propagation, borehole stability analysis, and slope stability. A new method is presented for damaged formation characterization based on a new constant-head hydraulic solution which considers formation alteration effects. The new solution simulates alteration effects by considering a gradual variation of permeability with radius in the form of a power relationship which contains two parameters, one to describe the alteration degree, the other to describe alteration zone size. Procedures for estimation of formation alteration characteristics by evaluating these two parameters quantitatively from constant-head test data are presented.