Drilling Fluid: Stabilising the hole  Surface and near surface sand and gravel often  lack cementation material and are referred to as an unconsolidated formation.  The problem with unconsolidated formations is the fact that they cannot be supported by hydrostatic overbalance alone. For example, unconsolidated sand and pea gravel often fall into the hole and pack off around the drill string. Problems also occur if insufficient filter cake is deposited on loose, unconsolidated sand to prevent it from "flowing" into the well bore and packing off the hole.  Torque, drag and fill are common indicators of such problems. Solids-control equipment will be overloaded with quantities of solids that do not correspond to the rate of penetration. These formations are highly permeable and any mud or water that invades the pore spaces also destabilizes the hole. To drill these formations, the mud should provide a good-quality filter cake to help consolidate the formation so that hydrostatic pressure can "push against" and stabilize the formation. Since hydrostatic pressure is helping to stabilize the formation, consideration should be given to keeping pressure in the hole constant.  Sudden pressure changes should be avoided. In gravel formations treatments with seepage-loss material, will help seal these formations and provide a base for the filter cake.  The bentonite particles should be kept in the dispersed or deflocculated state and not allowed to flocculate. Bentonite in the flocculated state will increase the water loss and have a thick, soft filter cake. It is important to control the filtrate loss of the mud so that the water will not destabilize the formation.  A drilling mud with a low filtrate water loss will form a thin, tough filter cake. Specially formulated, bentonite product can provides filtrate control and a thin, tough filter cake.  Any contaminates that flocculate the bentonite should be treated out immediately. To minimize erosion, avoid any unnecessary reaming or circulating opposite unconsolidated formations. Clay minerals originate from the degradation of igneous rocks in situ. The parent minerals are the micas, the feldspars and ferromagnesium minerals, such as horneblende. Bentonite is formed by the weathering of volcanic ash.  Clays formed in situ are termed primary clays. Secondary clays are formed from primary clays carried down by streams and rivers, and deposited as sediments in fresh water or marine environments. When first deposited, the sediments are soft and have a high water content. A good example would be the gumbo clay that occurs along the Gulf Coast of the United States and in the North Sea. One characteristic of these types of deposits is that they are sticky and form balls on the bit and drill string. As these sediments are buried by succeeding sediments, the water is squeezed out by compaction.  At shallow depths the effects of compaction are fully reversible. The drill cuttings recovered at the surface can be readily dispersed to individual grains. At greater depths the grains are eventually cemented together forming indurate shale and clay stone. When thus fully consolidated the rocks cannot be dispersed except by direct mechanical action such as grinding. Hole instability due to clay takes several forms. With the recently deposited clays the primary problem will be that of bit and drill string balling. The surface indication of this problem is decrease in rate of penetration, higher pump pressures, high torque and loss of circulation due to hydraulic fracturing.  In the older consolidated clay that are hard and brittle the problem will be that they splinter under stress and cave or slump into the hole. This leads to hole enlargement and bridging caused by inadequate hole cleaning.  These problems add greatly to drilling time and costs, and may result in stuck pipe or re-drills. Hydration by water is one of the most significant causes of well bore instability.  Hydration takes two forms in water-sensitive clays: surface and osmotic adsorption. Surface hydration occurs when a small volume of water is strongly adsorbed onto the clay surfaces that cause little softening or swelling, but can lead to excessive stresses if the swelling is confined.  Osmotic swelling occurs when a large volume of water is attracted to the clay surfaces by electrostatic forces. Osmotic swelling causes softening and significant swelling as the adjacent clay layers hydrate and expand. It is important to prevent the transfer of water between the drilling fluid and the clay. An ideal drilling fluid would be one that does not alter clay in any manner. Many different types of fluids have been tried and no one fluid has been completely satisfactory in all cases. From an overall success standpoint, the potassium-based polymer system has been the most successful in problem clays.  The potassium polymer systems work so well because they attack the clay-hydration mechanism in several ways. The potassium ions converts the clay to a less reactive state. In a sufficient concentration, polymers work to coat exposed clay and cuttings, "encapsulating" them with a bound layer of polymer.  Polymers also increase the filtrate viscosity of the fluid so the transfer of water will be slower. In addition, water insoluble materials and plugging agents  can substantially improve well-bore stability. They further reduce water invasion by plugging the micropores.