Wellbore stability

A variety of methods have been devised to stabilize shales. The most successful method uses an oil or synthetic mud that avoids direct contact between the shale and the emulsified water. However, preventing direct contact does not prevent water uptake by the shale, because the organic phase forms a semipermeable membrane on the surface of the wellbore between the emulsified water in the mud and the water in the shale. Depending on the activity of the water, it can be drawn into the shale (activity lower in the shale) or into the mud (activity higher in the shale) (95—97). This osmotic effect is favorable when water is drawn out of the shale thus the aqueous phase of the oil or synthetic mud is maintained at a low water activity by a dding a salt, either sodium chloride or more commonly, calcium chloride. The salt concentration is carried somewhat above the concentration required to balance the water activity in the shale to ensure water movement into the mud. 

Reamer A tool employed to smooth the wall of a wellbore, enlarge the hole, stabilize the bit and straighten the wellbore where kinks and abrupt doglegs are encountered. 

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]. 

Stresses caused by chemical forces, such as hydration stress, can have a considerable influence on the stability of a wellbore [364]. When the total pressure and the chemical potential of water increase, water is absorbed into the clay platelets, which results either in the platelets moving farther apart (swelling) if they are free to move or in generation of hydrational stress if swelling is constrained [1715]. Hydrational stress results in an increase in pore pressure and a subsequent reduction in effective mud support, which leads to a less stable wellbore condition. 

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. 

Geothermal cements are also employed to fix the steel wellbore casing in place and tie it to the surrounding rock (8). These are prepared as slurries of Portland cement (qv) in water and pumped into place. Additional components such as silica flour, perlite, and bentonite clay are often added to modify the flow properties and stability of the cement, and a retarder is usually added to the mixture to assure that the cement does not set up prematurely. Cements must bond well to both steel and rock, be noncorrosive, and water impermeable after setting. In hydrothermal applications, temperature stability is critical. Temperature cycling of wellbores as a result of an intermittent production schedule can cause rupture of the cement, leading to movement and, ultimately, failure of the wellbore casing. 

The nature of fines, together with the conditions under which they may be caused to migrate through the porous reservoir rock, is discussed in Chapter 7. Fines may be produced into the wellbore with the fluid. Being small particles of relatively low mass, they can be carried by produced fluids up the wellbore and to the surface. Because of their charged nature, fines can stabilize unwanted emulsions in surface equipment (21). Fines also can cause plugging of the reservoir pores, reducing fluid flow. 

Early modeling of wellbore stresses assumed that the rock behaved elastically (52). Bratli and Risnes (27) and Risnes et al. (53) included a plastic zone around the wellbore with variable permeability (Figure 11). In unconsolidated sands, the plastic zone is of the order of about 1 m radius consolidated formations have a smaller plastic zone. For stress solutions with fluid flowing into an uncased wellbore (cylindrical geometry around the wellbore), a stability criterion that equates fluid flow parameters to rock strength parameters is found ... 

In a cased and perforated well, only a slightly smaller plastic zone exists around the cased hole. In a perforated well in a poorly consolidated sand, the zone just around the wellbore is in a state of plastic stress, and greater flow rates can be attained without failure than will be possible in open hole situations. The production of solids will be governed by the stability of the sand arches behind the perforations. One of the important considerations in reducing solids production is to identify the poorly consolidated layers and avoid perforating them. 

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. 

Han, G. and MB Dusseault, Coupled analysis of sand stability in petroleum wellbores, 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. 

Han, G, and Dusseault, M.B. 2003. Coupled Analysis of Sand Stability in Petroleum Wellbores. Proc. GeoProc 2003 Conference, Stockholm, Sweden, 6 pages. 

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). 

---