The majority of wells drilled with water-based fluid systems in pressure-depleted reservoirs are subjected to high overbalance pressures due to drilling fluid densities greater than the density of water. These overbalanced conditions are directly related to loss of circulation, stuck pipe and especially, formation damage due to solids and fluid invasion. Specialized technologies, such as under-balanced drilling with ultra-low density foams or aerated fluids, have been used to resolve most of the aforementioned problems1, 2 High application costs, however, have made these alternative systems almost prohibitive in many mature fields.
With appropriate surfactant and polymer selection and the implementation of unique bridging methods, drill-in fluids may be designed with highly thixotropic behavior, minimal filtration properties, excellent crude-oil compatibilities and high return permeability. This paper presents the development of a low-density water-based drill-in fluid stabilized by a specially selected polymer-surfactant complex and bridging materials designed specifically for low pressured sandstone and carbonate reservoirs.