Borehole instabilities in shales are regarded as the prime technical problem area in oil and gas well drilling. One of the most important problems is the interaction rock-fluid that can affect strongly the drilling operations, leading a variety of wellbore instability problems. The purpose of this paper is to describe an experimental technique using developed electro-chemical immersion equipment, evaluating the impact of calcium, potassium and sodium chlorites on the electrochemical and physicochemical rock samples properties from a shale formation located in Western Venezuela. Solutions with high concentration of calcium and potassium chlorides generate the highest efficient inhibitor impact in the rock composition to reduce variations on water flux from the rock matrix to the solutions. The results indicated the high efficiency of the developed equipment, used methodology to deal with potential reactivity formations and that inorganic solutions reduce the potential reactivity of the shale. Consequently, recommendations to mitigate the effects of this reactivity are provided.
During the drilling of oil wells, the drilling fluid interacts with the shale layers at the wellbore, with the cuttings and at the drill bit-rock contact. Chemical instability can be prevented through selection of proper drilling fluid, suitable mud additives to minimize the fluid/shale interaction, and by reducing shale exposure time. Besides having to cut costs to main profitability, the industry is currently facing new environmental and technical challengers as drilling in deep waters, HPHT formations and fractured reservoirs. The international and some national laws are restringing the use of oil base fluids to avoid environmental contaminations. According to van Oorth and Hale [1], water- based muds are attractive replacements from a direct point of view, but conventional systems have shown poor shale drilling performance and can create much wellbore instability during drilling, especially in over pressure and clay rich formations. The interaction rock-fluid may also have a strong effect on the rock properties; typically a reduction in strength of about 30–100% is seen in many rocks [2]. Cation exchanges variations during the rock-fluid interaction can affect the rock strength due to variation of water adsorbed from the drilling fluid. To avoid or to reduce this interaction, new fluids have been developed, mainly due to environmental legislations that demand that the drilling fluids to be biodegradable, and with low content of oil [3]. Shale inhibitors as chlorites have been used as a good shale inhibitor. Recently its solutions still have been used, but associated with polymers [4] to reduce reactivity and due to its high solubility and miscibility.
Traditional immersion tests are very useful test to study the interaction shale-fluid. This test present low cost and generally consist in immerge some rock samples (well preserved) in a glass vases and monitor visually the rock-fluid interaction for about 72 hours. In this study, a new apparatus was developed in which shale sample is put in contact with fluid and special sensors measure the electro-chemical properties of the fluid throughout the test.
