Effects of Temperature and Pressure on Shale Cuttings Dispersion in Water Based Mud WBM Using NACL, CACL2, KCL Salts as Primary Inhibiting Agents and Polymer XCD Xanthan Gum as Secondary Inhibiting Agent

The primary cause of wellbore instability is the interaction of water based mud with shales which usually involves the movement of water and ions into or out of shale thereby causing alterations in mechanical property of the shale resulting in dispersion of shale particles into the mud. This work involves experiments to analyze the effects of chemical osmosis, diffusive flow prior to shale hydration and inhibition, determination of the effects of salt addition on pH and density of mud and also determination of the effect of Temperature and pressure other than just temperature on dispersion of shale cuttings into water based mud. It also shows the behaviour of shale in various inhibitive agents e.g NaCl, KCl, CaCl2 and Polymer XCD when being hot rolled. Results from experiment performed in this work confirmed that osmosis and ionic diffusion play a great role in shale hydration and dispersion. It also showed that subjecting the system to temperature and pressure conditions other than just temperature caused more dispersion of shale into mud. KCl was the best inhibitive agent as more shale cuttings were recovered from the mud while the least shale recovery was from CaCl2/salt formulation. A further experiment was performed by adding polymer XCD to the Salt/mud formulations so as to investigate its inhibitive capability; result showed that more shale cuttings were recovered as compared to when only Salt/mud was used. INTRODUCTION Wellbore instability encountered while drilling shale formations is a worldwide problem. Despite much experience and considerable research, drilling and completion operations continue to be plagued by various hole problems attributed directly to shale formations. Studies and solutions to shale problems are not a simple matter because of the wide variations in clay chemistry. Shales are sedimentary rocks which were generally deposited in marine basins. They are composed of compacted beds of muds, silts, and clays. In the soft or unconsolidated shale, mud or clay predominates, in the more consolidated formation it is shale or argillite, and in the metamorphic form, slate, phyllite, or mica schist. At increased depth the shales become denser due to the compaction caused by overburden weight. Shales may also be subjected to tectonic stresses, producing further alteration. When shales contain sand they are called arenaceous shales. If they contain calcium carbonate they are called calcareous shales. Those that contain iron are termed ferrungeneous shales and if large amounts of organic materials are present, they are called carbonaceous shales. The drilling fluid engineer is very much concerned with the degree of hydration of shales, and with the cementing materials binding shales together. Equally important are the inclination of the bedding planes and the stresses acting within or upon the shale formation. Shales contain various clay minerals which differ structurally. The more common of these are montmorillonite, illite, chlorite and kaolinite. Some of these clay minerals will hydrate while others will not. Shales containing montmorillonite will hydrate easily. The montmorillonite clay group has a high base-exchange capacity, where one cation will replace another and can increase or decrease the shales tendency to hydrate. The degree of hydration is also influenced by the type of cation involved and the pH of the fluid. Mass action by a high concentration of salts will suppress the hydration of clays. Illite is another mineral frequently found in shales. The basic structure of illite is similar to montmorillonite but it does not hydrate readily in fresh water. Both are composed of two silica tetrahedral sheets and a central octahedral alumina sheet. Illite, however, develops a charge deficiency (negative charge) from the replacement of silicon by aluminum on the surface of the silica sheet. Chlorite clay minerals are similar to illite and do not noticeably hydrate. They are composed of the same three layers. Kaolinite is somewhat different from montmorillonite, illite, or chlorite. The clay structure is composed of two layers instead of three: a single silica tetrahedral sheet and an alumina octahedral sheet. There is no charge deficiency and the particles are electrically neutral. Kaolinite does not swell but will readily disperse. The hydrating-type shales containing montmorillonite are found at shallow depths and are often referred to as gumbo shales while at greater depths their ability to hydrate decreases due to modification of the internal lattice structure. They tend to become a more illitic or chloritic type of clay. PROBLEMS CAUSED BY SHALE INSTABILITY Terms frequently used to describe troublesome shales are, sloughing shale, heaving shale, running shale, bentonitic shale, mud-making, plastic-flowing and pressured shale. Besides enlarged hole due to shale sloughing, other problems which occur include: ● Improper mud weight ● Hole cleaning difficulties ● Stuck pipe ● Bridges and fill on trips ● Increased mud volume and treating costs ● Poor cement jobs and increased cement requirements ● Well logging problems ● Limited sidewall core recovery ● Tight hole Factors Causing Shale Instability ● Tectonically stressed shales ● Abnormally pressured shales ● Erosion due to high annular velocities ● Drill string whipping, knocking shale off the wall of the hole ● While tripping, the drill string dislodges shale, either by direct contact or swabbing action. ● Dissolving salt within the formation 2 SPE-184310-MS