Swelling shales

Addition of a salt can transform the shale by cation exchange to a less sensitive form of clay, or reduce the osmotic swelling effect by reducing the water activity in the mud below that which occurs in the shale. These effects depend on the salt concentration and the nature of the cation. Salts containing sodium, potassium, calcium, magnesium, and ammonium ions ate used to varying degrees. 

The method of action of the polymers is thought to be encapsulation of drill cuttings and exposed shales on the borehole wall by the nonionic materials, and selective adsorption of anionic polymers on positively charged sites of exposed clays which limits the extent of possible swelling. The latter method appears to be tme particularly for certain anionic polymers because of the low concentrations that can be used to achieve shale protection (8). 

Of particular interest in Figure 4-292 is the shaly sandstone in the 690-720-ft interval. In this zone, the LWD sonic measurements are consistently faster than the wireline measurements. Since the wireline logs were acquired 10 days after drilling, it is likely that shale swelling in the shaly sandstone has taken place. This phenomenon, known as formation alteration, causes the wireline sonic measurements to be slower. In this type of zone, LWD sonic yields a more correct At, which will better match surface seismic sections. 

The formation may be reactive and swell. It may be unconsolidated and collapse on the tool joints or drill collars. We may have mobile formations such as gumbo shales or salt beds in a plastic condition. 

Polyethercyclicpolyols possess enhanced molecular properties and characteristics and permit the preparation of enhanced drilling fluids that inhibit the formation of gas hydrates prevent shale dispersion and reduce the swelling of the formation to enhance wellbore stability, reduce fluid loss, and reduce filter-cake thickness. Drilling muds incorporating the polyethercyclicpolyols are substitutes for oil-based muds in many applications [195-197,1906,1907]. Polyethercyclicpolyols are prepared by thermally condensing a polyol, for example, glycerol to oligomers and cyclic ethers. 

Swelling due to shale hydration is one of the most important causes for borehole instability. Three processes contributing to shale instability are considered [127] ... 

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. 

Shale stability is an important problem faced during drilling. Stability problems are attributed most often to the swelling of shales. It has been shown that several mechanisms can be involved [680,681]. These can be pore pressure diffusion, plasticity, anisotropy, capillary effects, osmosis, and physicochemical alterations. Three processes contributing to the instability of shales have to be considered [127] ... 

The stability of shales is governed by a complicated relationship between transport processes in shales (e.g., hydraulic flow, osmosis, diffusion of ions, pressure) and chemical changes (e.g., ion exchange, alteration of water content, swelling pressure). 

Clays or shales have the ability to absorb water, thus causing the instability of wells either because of the swelling of some mineral species or because the supporting pressure is suppressed by modification of the pore pressure. The response of a shale to a water-based fluid depends on its initial water activity and on the composition of the fluid. The behavior of shales can be classified into either deformation mechanisms or transport mechanisms [1765]. Optimization of mud salinity, density, and filter-cake properties is important in achieving optimal shale stability and drilling efficiency with water-based mud. 

Further stability models based on surface area, equilibrium water-content-pressure relationships, and electric double-layer theory can successfully characterize borehole stability problems [1842]. The application of surface area, swelling pressure, and water requirements of solids can be integrated into swelling models and mud process control approaches to improve the design of water-based mud in active or older shales. 

L. Bailey, P. I. Reid, and J. D. Sherwood. Mechanisms and solutions for chemical inhibition of shale swelling and failure. In Proceedings Volume, pages 13-27. Recent Advances in Oilfield Chemistry, 5th Royal Soc Chem Int Symp (Ambleside, England, 4/13-4/15), 1994. 

C. P. Tan, B. G. Richards, S. S. Rahman, and R. Andika. Effects of swelling and hydrational stress in shales on wellbore stability. In Proceedings Volume, pages 345-349. SPE Asia Pacific Oil Gas Conf (Kuala Lumpur, Malaysia, 4/14—4/16), 1997. 

Acids were an early exception to the no water rule. It was recognized that aqueous solutions of acids would inhibit swelling of clays and shales as well as dissolve any acid-soluble minerals contained in a formation. By 1933 commercial well stimulation with hydrochloric acid was of great interest. A whole separate methodology and treatment chemistry has since evolved around acidizing and fracture acidizing(54). Water emulsions, mainly emulsified acids, and gelled acids thickened with polymeric additives were applied early in the history of well treatment. 

The effect of pH on both clay swelling and fines production has been widely discussed(89-95). Little consensus is found in this literature. Suggested treatments range from application of fluoboric acid(96) to 15% KOH(92) solutions — both treatments are believed to create a protective silicate film that inhibits release of fines. Polyacrylate polymers can provide protection against swelling of smectite clays and shales(97-100). 

Morgenstem, N. R., and Balasubramonian, B. I. (1980). Effects of pore fluid on the swelling of clay shale. In 4th International Conference on Expansive Soils, pp. 190-205. Denver. 

Shales. Shales are derivatives of clays, formed in slow diagenetic processes. Shales have little or negligible swelling capacity and have medium plasticity. They form effective aquicludes at thicknesses of at least a few meters to tens of meters. 

There are historic reports of floods in the area (32 J. Rutherford, personal communication) and of an earthquake in 18 B.C. (33). The movement of the expansive Esna shale bed formation that underlies the whole area is also a source of disruption (32). The Esna shale bed formation, which contains montmorillonite clay, swells upon hydration by about 12.5% (32). Therefore, if an adequate amount of water is available, for example from increased irrigation, humidification, or flooding, the movement of this shale formation accelerates. Collectively, these dramatic natural events are the most probable causes of the loss of plaster and painted murals in the lower chamber of the tomb prior to its discovery, and they increasingly (32) threaten tombs in the Thebes area. 

A more pervasive problem is the maintenance of wellbore stability in shale formations [i.e., formations that have a high clay content, typically in excess of 50 wt% (10)]. In the presence of water, shales can take up water and swell and disperse or they can fracture. Problems associated with wellbore stability in shale sections are sticking of the drill pipe (usually termed stuck pipe), hole enlargement, and excessive generation of drilled solids. 

The imbalance in the chemical potential of the water in the shale and drilling fluid results in a tendency for water to enter the shale. Equation 110 is applicable to both water- and oil-based drilling fluids. When fij = fjL sh equation 110 gives the well-known expression for the swelling pressure (Psh — P) between the shale and the drilling fluid. The permeability of shales is very low and the rate of filtration into the shale will be below the critical filtration rate (140) and no filter cake will form on the surface of the shale. 

The other extreme of behavior is represented by shales that are highly cemented and unable to swell (awsh constant) and the response to the imbalance of chemical potential is an increase in Psh. The shales will tend to exhibit tensile failure by fracturing and large angular fragments may detach from the borehole wall. These shales, which are usually geologically older, contain mostly illite and kaolinite clays. Intermediate behavior of shale hydration has been observed (157). 

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). 

Abstract Shales present particular problems to drilling activities. Swelling, shrinkage and strength degradation may occur from pressure, chemical and thermal diffusion processes, leading to hole control problems that may lead to excessive times, hole loss, even blowouts. The article will describe qualitatively the major physical processes, field conditions, and explain how various mud systems address these issues. 

The processes can be considered as three initially independent processes. These include the undrained load-deformaiion-failure process and the fluid and heat flow processes. The fluid flow process may include solute transport. Other mechanisms include swelling of shale caused by change in water potential resulting from the other processes. The main coupling parameters are stress, pore pressure and temperature. 

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