he art and science of drilling wells require the use of drilling fluids for several reasons including cuttings carrying and maintenance of wellbore stability. Drilling fluid selection is dependent on the behaviour of the formation to be drilled. Shale, the most abundant rock type in the earth interacts variably with the fluids used. Shales are low-permeability sedimentary rocks with small pore radii that characterized by low permeability, medium to high clay content, and medium porosity in addition to other minerals, such as quartz, feldspar, and calcite. Shale types range from soft Gumbo shale in offshore Louisiana, Gulf of Mexico to hard brittle shale in South Louisiana with each type presenting its own set of problems. They account for over 75% of formations drilled all over the world and cause over 90% of wellbore instability problems. The distinguishing features of shale are its clay content and low permeability, which results in poor connectivity through narrow pore throats. Shales are also fairly porous and are normally saturated with formation water, with several factors affecting their properties, such as burial depth, water activity, and the amount and type of minerals present (Joel, et al).

Interests in the design of water-based muds (WBM) have escalated due to wellbore instability issues that arise from the abundance of problematic shales encountered while drilling. Conventional water-based muds (WBMs) that are used to drill through water sensitive shale formations cause a high degree of wellbore instability. Consequently, oil based muds (OBMs) were adopted to solve the wellbore instability problems due to their superior shale stabilization properties. Unfortunately, high costs, environmental restrictions, cuttings and used mud disposal difficulties, and safety have largely limited the use of Oil base muds. Consequently, WBMs that have the ability to effectively reduce shale instability problems have once again come under the lime light to replace the OBMs. The limited availability of models to adequately describe shale fluid interaction has hindered the growth of inhibitive WBM development. Models based on chemical potential and hydraulic pressure had been developed by Osisanya (1991), and further work by V Osisanya, et al (1996) have indicated the complexity of theoretical analysis of driving forces and mechanisms that govern shale stability in the borehole.

The use of conventional WBMs in drilling shale formations results in the adsorption of water associated with the drilling mud onto the surface of shale (Chenevert 1970). Depending on the shale type, water adsorption may lead to various reactions such as swelling, cuttings dispersion, and increase in pore pressure ( Chenevert 1973) creating wellbore instability to varying degrees. Common failures that occur from shale instability using conventional WBMs include sloughing, caving, stuck pipe, bit balling and increased torque and drag. These failures can grow into massive expenses due to lost non-productive time. In general, drilling fluid weight and chemical compositions are the elements that are manipulated in order to control such instabilities. However, instabilities in shale may be caused by a complex mechanism of shale drilling fluid interaction ranging from mechanical to chemical reasons (Al-Bazali, 2005). Therefore proper selection of the drilling fluids to be used on a particular well site is an essential phase of any carefully planned drilling operation. When this drilling is expected to encounter shale zones, the selection of the fluid becomes even more important. To maintain a stable borehole through such zones, a carefully designed mud will be required. The design of successful fluids for this type of application depends largely on a knowledge of the physical and mineralogical characteristics of the shale and its behavior when in contact with drilling mud.

Potassium-based muds are used in areas where inhibition is required to limit chemical alteration of shales. Potassium performance is based on cationic exchange of potassium for sodium or calcium ions on smectites and interlayered clays. The potassium ion compared to calcium ion or other inhibitive ions, fits more closely into the clay lattice structure, thereby greatly reducing hydration of clays . Potassium-based muds perform best on shales containing large quantities of smectite or interlayered clays in the total clay fraction. Shallow shales, containing large amounts of montmorillonite, however, still swell in a potassiumbased system. In recent years, muds containing potassium chloride and a suitable polymer have been the subject of publications from several areas. Laboratory studies of the effects of several salt solutions on the hardness of cores from water-sensitive sands showed that 2% potassium chloride was a more effective stabilizing agent than was 2% calcium chloride or 10% sodium chloride.

In 1960, while drilling steeply dipping shales in the Cerro Pelado area of Venezuela, noted improved hole stability when mud containing potassium ion replaced the commonly used sodium or calcium ions to inhibit clay swelling. Hole enlargement in the shale section was significantly reduced a result attributed to the inhibitive properties potassium ion and cited in a patent application filed in September 1963. . The objective of this work, therefore, is to evaluate experimentally the degree of inhibition of different concentrations of KCl on Shale contaminated WBM.

341grams of water was measured and poured into the Hamilton mixing cup. 4.0grams of bentonite was added and prehydrated for 30 minutes under stirring condition. After 30 minutes, 0.2grams of xanthan gum, 0.4grams of Pac-R, 0.6grams Pac-L respectively were added to the mixing cup. These with prehydrated bentonite was stirred for 15 minutes before 0.25grams of Soda ash was added and stirred for another 10 minutes. Then 13.0 grams of barite was finally added and the mixture was stirred further for another 20 minutes for homogeneity before taking the rheological readings and(10 seconds/minutes) gel strength using VG meter.

The mixing procedure was repeated using the grounded sample of shale. Different weights of the shale (1%,2%,4%,7%,10%) respectively by weight of the formulated mud were added. Thereafter, the KCL(0.2%, 0.4%,1.0%, 2.0% and 4.0%) by weight of the formulated mud were added respectively. The rheological readings and(10 seconds/minutes) gel strength values were recorded as well. The plastic viscosity and Yield Point values were evaluated as applicable. III.

The results of the various tests are recorded in the tables below. 4.0%) by weight of the formulated mud sample respectively, there was progressive reduction in the rheological values with increase in KCl concentration, no increase for 0.2% KCl, ( 30cP to 22cP for 0.4%KCL), (32cP to 20cP for 1.0%KCl), (35cP to 18cP for 2% KCl) and (45cP to 16Cp for 4%KCL). Test results indicated that the KCl inhibited the swelling tendencies of the shale and the rheological values reduced drastically and considering the 600rpm reading, the percentage reductions were 0%, 36%, 60%, 94% and 181% respectively compared to results without KCL in the mud as indicated above. This agrees with previous studies that potassium chloride is very effective stabilizing agent in shale sensitive formation. Fig- 6 shows the Plastic Viscosity result of the mud with different concentrations of the shale. The test result indicated that as the concentration of shales increased, the plastic viscosity increases, however, there was a noticeable reduction in the plastic viscosity values with introduction of KCl.

Fig- 7 shows the yield point results with the different concentrations of shale. The highest shale concentration gave the least yield point value. This is an indication of dispersion and settling tendency of the solid particles in the mixture. Depending on the shale type, water adsorption may lead to various reactions such as swelling, cuttings dispersion, and increase in pore pressure (Chenevert 1973) creating wellbore instability to varying degrees. However, the introduction of KCL resulted to reduction in the yield point values.