Wellbore stabilizers

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. 

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. 

The literature offers several papers that may serve as guidelines for issues such as selecting a proper clay stabilizing system or completing wellbore stability analyses of practical well designs [367,427-429,562,1565]. 

Horizontal completions in unconsolidated formations are being enhanced by a hydrochloric acid (HCl) breaker system for well clean up. Typically, the use of HCl in open-hole environments is avoided because of wellbore stability concerns. However, HCl successfully removes salt fluid loss control materials in wells without noticeable hole collapse [33]. 

M. Chen, Z. Chen, and R. Huang. Hydration stress on wellbore stability. In Proceedings Volume, pages 885-888. 35th US Rock Mech Symp (Reno, NV, 675-6/7), 1995. 

X. Chen, C. P. Tan, and C. M. Haberfield. Wellbore stability analysis guidelines for practical well design. In Proceedings Volume, pages 117-126. SPE Asia Pacific Oil Gas Conf (Adelaide, Australia, 10/28-10/31), 1996. 

L. Eoff, J. Chatterji, A. Badalamenti, and D. McMechan. Water-dispersible resin system for wellbore stabilization. In Proceedings Volume. SPE Oilfield Chem Int Symp (Houston, TX, 2/13-2/16), 2001. 

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. 

Invert emulsion drilling fluids are commonly selected for their temperature stability and their ability to prevent the wellbore stability problems associated with the hydration of clays in shale formations. The thermodynamic activity aw of the water in the aqueous (dispersed) phase is controlled by the addition of a salt (usually calcium chloride) to ensure that it is equal to or less than the activity of the water in the drilled shale formations. The emulsified layer around the water droplets is claimed to act as a semipermeable membrane that allows the transport of water into and out of the shale but not the transport of ions (61). When the activities (or, more strictly, the chemical potentials) of the water in the shale and invert emulsion are equal, then no net transport of water into or out of the shale occurs (i.e., the drilling fluid does not hydrate or dehydrate the shale). This equality of water activity has lead to the development of so-called balanced activity oil-based drilling fluids. 

Chemical Aspects of Wellbore Stability. A considerable (and continuing) effort has been made to develop water- and oil-based drilling fluids that will drill through the massive shale sections, which commonly... 

Choi, SK., CP Tan and R Freij-Ayoub. A coupled mechanical-thermal-physico-chemical model for the study of time-dependent wellbore stability in shales, this volume, 2004. 

Li J and Li Z. 1997. Rock elastic-plastic stresses around a wellbore and wellbore stability under permeation osmosis. Engng. Mech., 14(2), pp. 131-137. 

Liu Y. 1995. Mud density for wellbore stability when formation rock is damaged. Acta Petrolei Sinica, Vol. 3, pp. 123-128. 

A COUPLED MECHANICAL-THERMAL-PHYSICO-CHEMICAL MODEL FOR THE STUDY OF TIME-DEPENDENT WELLBORE STABILITY IN... 

Due to the relatively high porosity and low permeability, fluid saturated pores and the existence of bedding planes in most shales, it is necessary to take into account the effects of material anisotropy and poroelastic effects (Biot, 1956) on wellbore stability in such formations. The constitutive equation for a transversely isotropic poroelastic material can be found, for example, in Cheng (1997). 

Results of the modelling study conducted to date indicate that, in order to predict both short and long term stabilities of wellbore in shales, it is important to take into account the interaction of the processes and mechanisms described in this paper. The coupled model provides a tool for studying the relative importance of the various factors and processes on wellbore stability in shales, and how stability may change with time. 

Choi, S.K. and Tan, C.P. (1998). Modelling of effects of drilling fluid temperature on wellbore stability. SPE 47304, Proc. SPErtSRM EUROCK 98, Trondheim, Norway, 8-10 July 1998, Vol. I, pp. 47M77. 

Tan, C.P., Richards, C.G., Rahman, S.S. and Andika, R. (1997). Effects of swelling and hydrational stress in shales on wellbore stability. Asia Pacific Oil and Gas Conference and Exhibition, Kuala Lumpur, Malaysia, 345-349. 

Wellbore stabilizers are frequently included in drilling fluid formulations. These are used to control (limit) fluid interactions with the wellbore rock. One way to do this is to add particles whose sizes are chosen such that they can enter the wellface and near-wellbore region and plug the pores in the rock. Nanoparticles are being used for this, and this is currently an area of active research and development. 

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