A workover fluid is a type of special liquids used at the end of borehole drilling, i.e. during well operation or during reconstruction works. Such works, carried out at various stages of borehole operation, are aimed at maintaining or increasing the production of a specific well and at maintaining its proper technical condition. They may be carried out only after injecting the workover fluid into the borehole, which should generate counterpressure on the reservoir, preventing the inflow of reservoir media into the borehole, and should enable the maintaining of the hydraulic conductivity of the reservoir rock. To ensure that the basic requirements are satisfied by the workover fluid injected into the borehole, its physical and chemical properties must correspond to the geological and reservoir conditions of the specified level of reservoir rocks. Due to this, the composition of the workover fluid should be determined based on the reservoir pressure gradient, mineralogical composition of reservoir rocks and of their binder, as well as the chemical composition of reservoir waters. These are the basic criteria for selection of the composition and evaluation of the quality of the workover fluid, which enable control of the physicochemical processes occurring within the borehole zone, such as clogging of the porous space of rocks, hydration of clay minerals, capillary effects and changes in the surface tension at the interface, as well as the interaction of fluid with reservoir waters. Limitation of the intensity of occurrence of such processes, which affect the degree of damage to the permeability of the reservoir rocks in horizons featuring normal or reduced reservoir pressure, largely depends on the type of workover fluid used, i.e. brine without a solid phase and brine containing a solid phase or a liquid with density below 1.0 kg/dm3. The composition and technological properties of the workover fluid, properly selected to the specific geological and reservoir conditions, allow one to maintain the productivity of the well to a degree that does not require application of additional treatment, such as acid-treatment, fracturing and reperforations. The aim of the monograph is to show the role of a workover fluid in the conducted reconstruction treatments, as well as the importance of its technological properties in limiting damage to the permeability of reservoir rocks within the borehole zone. The presented issues comprise: • causes and threats to the deterioration of reservoir rock permeability resulting from the application of an improperly selected workover fluid; • tasks of the workover fluid and methods to improve its technological properties in terms of protecting the hydraulic conductivity of reservoir rocks; • types of workover fluids developed, the methodology for determination and assessment of their technological properties, as well as usability under reservoir conditions. The monograph also includes a short description of other special liquids used in the preparation of a well for exploitation. These are: washing and cleaning liquids, packer fluids and those used for perforation, as well as buffers for rope operations and pipe cleaning prior to packer fluid injection. The presented issue is a synthesis of a wide range of research and development works carried out at the INiG - PIB. It has been prepared based on the obtained results of laboratory tests carried out for geological and reservoir conditions existing in the productive horizons of the Carpathian Foredeep, as well as of the Carpathians and the Polish Lowlands. Keywords: borehole reconstruction, geological and reservoir conditions, workover fluid tasks, workover fluid properties, chemicals, blockers, permeability
Porous reservoir rocks with fluids: Reservoir transport properties and reservoir conditions