Shale drilling fluid problems

High initial cost and environmental restrictions prevent use of oil and synthetic muds in many cases where shale problems are expected. It is necessary then to treat a water-base mud to minimize the destabilizing effect of the drilling fluid. Salts, polymers, and other organic materials are added to the mud to reduce the water sensitivity of the shale, shale sweUing, and weakening arising from mud contact, or the rate of water uptake by the shale. 

In onshore drilling there is no need for chlorides above these background levels. Potassium chloride has been added to some drilling fluids as an aid to controlling problem shale formations drilled. Potassium acetate or potassium carbonate are acceptable substitutes in most of these situations. 

Synthetic cationic polymers have been used in water-based drilling fluids to control shale problems. Recent applications include the use of low molecular weight poly amines (48, 49) some of these cationic polymers are also used for water treatment (48). The use of low molecular weight polyols and glycols has recently been advocated with claims that they act to control both fluid loss and shale hydration (50). 

Invert emulsion drilling fluids are commonly selected for their temperature stability and their ability to prevent the wellbore stability problems associated with the hydration of clays in shale formations. The thermodynamic activity aw of the water in the aqueous (dispersed) phase is controlled by the addition of a salt (usually calcium chloride) to ensure that it is equal to or less than the activity of the water in the drilled shale formations. The emulsified layer around the water droplets is claimed to act as a semipermeable membrane that allows the transport of water into and out of the shale but not the transport of ions (61). When the activities (or, more strictly, the chemical potentials) of the water in the shale and invert emulsion are equal, then no net transport of water into or out of the shale occurs (i.e., the drilling fluid does not hydrate or dehydrate the shale). This equality of water activity has lead to the development of so-called balanced activity oil-based drilling fluids. 

The basic problem of the interaction of drilling fluids with shale formations is an imbalance in the chemical potential of the water in the drilling fluid and in the shale. During the compaction of the shale, water is expelled and the clay-water ratio increases (see eq 96, for example). The presence of the exchange cations associated with the surface of the clay causes the water activity in the shale awsh to decrease as the water content decreases. The chemical potential / wsh of the water in the shale is given by... 

Damage mechanics principles have been applied extensively for well stability problems, especially for hard, brittle and fractured shale rocks. Constitutive models using damage concept, coupled with hydro-chemical swelling models, have been developed based on experimental results, and applied for design of drilling fluid density, combined with FEM methods (Liu, 1995). 

Flexibility of Foam Drilling. Foam is the most versatile of all the low density drilling fluids. The water (LVF) and percentage of gas (GVF) can be easily controlled at the surface to achieve bottom hole pressures sufficient to drill the well. Various surfactant chemicals can be added to the water phase to address such problems as influxes, shale... 

The problems caused by shales in petroleum activities are not new. At the beginning of the 1950s, many soil mechanics experts were interested in the swelling of clays. It is important to maintain wellbore stability dining drilling, especially in water-sensitive shale and clay formations. The rocks within these types of formations absorb the fluid used in drilling this absorption causes the rock to swell and may lead to a wellbore collapse. The swelling of clays and the problems that may arise from these phenomena are reviewed in the literature [528,529,1788,1900]. Various additives for clay stabilization are shown in Table 3-1. 

---