Drilling Fluids-lec 7 PDF

Clay Chemistry A good understanding of clays can be the mud engineer's most valuable tool. Clay may be added specifically., such as Bentonite, or it may enter the mud as a major contaminant through dispersion of drill solids. it is necessary to understand basic clay chemistry in order to p...

Clay Chemistry A good understanding of clays can be the mud engineer's most valuable tool. Clay may be added specifically., such as Bentonite, or it may enter the mud as a major contaminant through dispersion of drill solids. it is necessary to understand basic clay chemistry in order to properly control water-base muds. Clay chemistry is also important with regard to interactions between water-base muds and shales which affect wellbore stability. Clay is a broad term commonly used to describe sediments,soils or rocks consisting of extremely fine-grained mineral particles and organic matter. Clay is also used as a group term for particles with a size less than 2 microns in diameter, which includes most of the clay minerals.  Clay Chemistry Clay minerals are fine-grained aluminum silicate minerals having well- defined microstructures. In mineralogical classification, clay minerals are classified as layered silicates because the dominant structure consists of layers formed by sheets of Silica and Alumina.  Clay Chemistry Most clay minerals are platy in morphology. Depending on the repeating units of the structure, clay minerals can be further classified as to the ratio of silica to alumina layers such as 1:1, 2:1and 2:1:1, as well as according to whether they are layered or needle-shaped clay minerals. In the drilling fluids industry, certain clay minerals such as smectite, a major component of bentonite, are used to provide viscosity, gel structure and fluid-loss control.  Clay Chemistry Phyllosilicates are also structurally separated based on the stacking of the tetrahedral (T) and octahedral (O) layers.The stacking of T and O sheets are typically written as 1:1 (repeating TO layers), 2:1 (repeating TOT layers), and 2:1:1 (repeating TOT-O layers), where the first and the second index indicate the number of the T and the O sheets and the third number indicates that there is a separated O sheet between the TOT sheets. Phyllosilicate sheet structures showing the a) 1:1 or TO, b) 2:1 or TOT, and c) 2:1:1 or TOT-O sheet structures.  Clays Clays exist in nature with a stacked or layered structure, with each unit layer roughly 10 Angstroms (Å) thick. So there are about a million layers of clay per millimeter of thickness. Each clay layer is highly flexible, very thin and has a huge surface area. One gram of sodium montmorillonite has a total layer surface area of 8,073 ft2(750m2)! 1 mm= 10000000 Å  Clays Clays are usually either of the: Two layer type like kaolin Three-laver type such as montmorillonite, chlorite or illite. Each plate-like clay particle consists of a stack of parallel unit layers. Each unit layer is a combination of: Tetrahedral(pyramid) arranged silica sheets and Octahedral (eight faced) arranged alumina or magnesia sheets. Two-layer clays are built of unit layers consisting of only one tetrahedral and one octahedral sheet. Three-layer clays are built of unit layers composed of two tetrahedral sheets on either side of one octahedral sheet, somewhat like a sandwich Clays can either be electrically neutral or negatively charged. For Example, pyrophyllite [AI2Si4010-(OH)2], a neutral clay, is similar to the negatively charged montmorillonite. Clay structure comparison Types of Clays There are a large number of clay minerals, but those with which we are concerned in drilling fluids can be categorized into three types: 1-Attapulgite and Sepiolite (needle-shaped, non-swelling clays). 2-Illite,Chlorite and Kaolinite (plate-like, non-swelling (or slightly swelling)clays). 3-Montmorillonites (plate-like, highly swelling) The second and third types of clay minerals are found in formation shales in the following order in decreasing amounts: (1) illite, (2)chlorite,(3) montmorillonite and (4) kaolinite. Because these clays are present in drilled formations, they become dispersed in the drilling fluid system in varying amounts. Smectite Group A category of clay minerals that have a three-layer crystalline structure. The ability of smectite to adsorb water is much greater than other clay minerals:  Adsorbing water and cations on external surfaces  Absorbs water and cations to surfaces between layers in its crystalline structure. The ability to adsorb water, the quantity of exchangeable cations (CEC, cation exchange capacity) and the surface area are closely related phenomena that are sometimes termed colligative properties of clay These colligative properties are basically measurements of the reactivity of the clay. Because CEC is easy to measure, it is a practical method to assess clay or shale reactivity. Smectite Group Smectite is clearly much more reactive than other clay mineral materials. Shales containing smectite are the most water sensitive and hydrate the most. Shales containing other clay minerals have less ability to hydrate but still may be water- sensitive. The reactivity of a shale depends on: the types and amounts of clay minerals present in the shale. Often the CEC is a better measure of clay reactivity than the mineralogical analysis inferred from X-ray diffraction analysis. Montmorillonite A type of Smectite clay mineral that tends to swell when exposed to water. The montmorillonite in shales is usually Calcium Montmorillonite since it is in equilibrium with the formation water,which is normally rich in calcium. Sodium montmorillonite (Wyoming bentonite, M-I GEL and GEL SUPREME)is also normally added to a mud to increase viscosity and reduce fluid loss. The filtration and rheological properties of the mud become a function of the amounts of various clays contained in the mud. Since the montmorillonite is intentionally added to a mud to control these properties, the other clay types may be considered contaminants, as they are not as effective as a commercial clay. Clays Purpose: The clays serve a dual purpose: To give viscosity to the drilling fluid. To deposit a filter cake that will seal permeable formations in order to limit filtration losses and prevent stuck pipe. Idealized montmorillonite particle Three-layer clays are built of unit layers composed of two tetrahedral sheets on either side of one octahedral sheet, somewhat like a sandwich Hydration of Clays The bentonite crystal consists of three layers: An alumina layer with a silica layer above and below it. The clay platelet is negatively charged and has a cloud of cations associated with it. If a significant amount of these cations are sodium, the clay is often called Sodium Montmorillonite. If they are primarily calcium,then the clay is called Calcium Montmorillonite. Hydration of Clays Depending on the cations present, the interlayer spacing of dry montmorillonite will be between 9.8 (sodium) and 12.1 Å (calcium) and filled with tightly bound water. When dry clay contacts freshwater,the interlayer spacing expands, and the clay adsorbs a large "envelope" of water. These two phenomena allow clays to generate viscosity. As shown in the Figure,calcium-base bentonites only expands to 17 Å while sodium bentonite expands to 40 Å. Hydration of Clays The thickness of the adsorbed-water film is controlled by:(the type and amount of cations associated with the clay). Divalent cations such as Ca2+ and Mg2+ increase the attractive force between platelets,thus decreasing the amount of water that can be adsorbed. Monovalent cations such as Na+ give rise to a lesser attractive force and allow more water to penetrate between the platelets. Because sodium bentonite swells four times as much as calcium bentonite, sodium bentonite will generate four times the viscosity. Smectite, in addition to adsorbing water and cations on external surfaces, absorbs water and cations to surfaces between layers in its crystalline structure.