Formation damage in underbalanced drilling operations

Abstract

Formation damage has long been recognized as a potential source of reduced productivity and injectivity in both horizontal and vertical wells. From the moment that the pay zone is being drilled until the well is put on production, a formation is exposed to a series of fluids and operations that can reduce its productive capacity. Any process that causes a loss in the productivity of an oil-, gas-, or water-saturated formation has a damaging effect on the reservoir. These damage mechanisms predominantly fall into three major classifications: mechanical, chemical, and biological. Underbalanced drilling operations involve drilling a portion of the wellbore at fluid pressures less than that of the target formation. This technology has been used to prevent or minimize problems associated with invasive formation damage, which often greatly reduces the productivity of oil and gas reservoirs, mainly in openhole horizontal-well applications. Underbalanced drilling is not a solution for all formation-damage problems. Damage caused by poorly designed and/or executed underbalanced drilling programs can equal or exceed that which may occur with a well-designed conventional overbalanced drilling program. Four techniques are currently available to achieve underbalanced conditions while drilling. These include using lightweight drilling fluids, injecting gas down the drillpipe, injecting gas into a parasite string, and using foam. This study provides an analysis of a number of potential damage mechanisms present when drilling underbalanced. It describes each one and its influence on the productivity of a well. Additionally it presents a general description of the different techniques that can be applied to carry out successful, cost-effective UBD operations, and discusses how these techniques may be used to reduce or eliminate formation damage.