The documented use of drilling fluids dates back to third century Egypt where water was used to assist in bailing cuttings from shallow quarry boreholes. Since then, the only significant improvements made in the drilling process and drilling fluids have occurred in the last century and a half.
Despite these advancements, water remains the primary constituent of water well drilling fluids, and cuttings transport is still a primary function of that fluid. However, we now incorporate a host of additives in the water to help improve its performance. Today, the most commonly used additives are sodium bentonite, polyanionic cellulose (PAC), partially hydrolyzed polyacrylamide (PHPA) and wetting agents. The geology and the end goal of water production dictate the additives used in well construction. Fluids are optimized for hole cleaning and borehole stability, but also should minimize formation damage to help ensure maximum productivity.
Hole CleaningIn most fluids, bentonite, specifically high-yield sodium bentonite, provides the majority of viscosity and gel strength. Viscosity, in conjunction with adequate annular velocity, transports the cuttings from the bit to surface while pumping. The fluid's gel strength can suspend the cuttings during periods of stasis and low annular velocity. Both PAC and PHPA can contribute to viscosity while having little contribution to gel strength.
Borehole Stabilization and Formation Damage MinimizationBorehole stability can be achieved by maintaining the formation in as near "native state" as possible. Fluids are engineered differently depending on the geology of the formation. The reason for choosing high-yield bentonite in water wells is for viscosity development with minimum solids. This helps minimize the fluid's density, thereby exerting less pressure on the borehole and minimizing infiltration of the fluid into the surrounding formation. Typically, the density of freshly mixed drilling fluid will be less than 1.02 g/cm3 (8.5 lb./gal.). This can allow the formation to remain stable. If the formation is allowed to become water-wet, instability may ensue. The bentonite helps provide viscosity and a physical barrier for the fluid because of its platelet structure. It deposits a thin, semi-permeable filter cake on the borehole wall in permeable formations.
When highly permeable zones are encountered, PAC is used to help reduce fluid filtration further than can be accomplished by bentonite alone. Limiting filtration into the surrounding formation can stabilize the formation and aid in development by reducing the distance that particles incorporated in the drilling fluid are allowed to migrate. Reducing this distance means that these particles may be reached through mechanical means and removed so as not to deter production.
Clay and shale formations tend to disperse or swell upon contact with water. Swelling formations cause a restriction in the borehole. If the formation readily disperses, the resulting hole enlargement may lead to hole cleaning problems. PHPA can be added to the bentonite slurry to encapsulate clay and shale, thereby limiting swelling and dispersion. This can aid in maintaining the dimension of the borehole and retard the tendency of the formation to adhere to the bit and drillstring. Wetting agents also can be used to assist in counteracting the sticking tendencies of clay.
An integral part of stabilizing and cleaning the borehole is the employment of some type of solids control method so that a low-density fluid is maintained. The solids contributing to the fluidOs density need be of beneficial nature, i.e. solids added purposefully to achieve specific properties. If the drilling slurry becomes predominantly composed of non-beneficial or drilled solids, the filter cake deposited on the bore wall can allow invasion and become thicker. The invading fluid can carry solids with it and may become difficult to develop.
A well-designed drilling fluid system combined with efficient solids control equipment can lead to efficient well construction and more productive water wells. Despite the vast improvements in drilling technology, a fundamental knowledge in the use of drilling fluids can provide efficient water wells for this century and beyond.