Bit Balling in Water-Reactive Shale During Full-Scale Drilling Rate Tests

Abstract

SPE Members Abstract This paper demonstrates a useful means of studying bit balling in laboratory drilling tests. Full-scale tests were conducted using a new 6-1/8 in. (156 am) tricone insert-type bit, realistic muds, and Pierre shale, which is similar to water-reactive shales which are apt to cause balling in the field. Balling ranged from slight to severe with an uninhibited water-base mud, and did not occur with an oil-base mud. The ratio of bit torque to weight-on-bit was a reliable indicator for detecting the degree of balling. The results show that proper modeling of bit balling in the laboratory must address the degree of overbalance which results from insufficient mud weight in the field. They also indicate that increasing mud weight in the field is an effective means to reduce or prevent bit balling if, and only if, balling is caused by mechanical (not chemical) wellbore instability. Although bit balling would become (irreversibly) severe within seconds, it could be prevented by significantly reducing weight-on-bit prevented by significantly reducing weight-on-bit quickly enough after incipient balling. Introduction Slow rate of penetration (ROP) in shale intervals has been identified as a majo drilling problem. Overall drilling costs can be reduced in such slow drilling formations by increasing ROP. An important factor related to slow ROP is bit balling in water-reactive, or "sticky," shale as found, for example, in young sediments in the Gulf of Mexico. Balling occurs when rock-cuttings adhere to bit cutters, which results in a lower ROP. Based on a successful pilot study, we decided to use the full-scale pilot study, we decided to use the full-scale drilling simulator at th Drilling Research Laboratory (DRL) to develop a comprehensive methodology to reduce shale drilling costs. The first step in that effort used Mancos shale, medium-hard shale obtained from surface outcrops. Although some bit balling was observed in the Mancos shale tests, the degree of balling was insufficient to model the water-reactive shales which are apt to cause extensive balling in the field. Large out crops of Pierre shale were located which, from a mineralogical standpoint, seemed to be a likely candidate for a model study of bit balling. As discussed in the next section, significant differences between Mancos and Pierre shales include the degree of water saturation and compressive strength. SHALE PROPERTIES Small plugs (2 inches (51 mm] long by 1 inch (25 mm] diameter) were taken from the large (36 inches (914 am] long by 15-1/2 inches (394 am] diameter) Pierre shale cores which were used for the drilling Pierre shale cores which were used for the drilling tests. Samples were submitted for X-ray diffraction analysis to determine the mineralogy. The results (Table 1) showed that Pierre shale contains 30 to 35 percent clay minerals by weight. The predominant percent clay minerals by weight. The predominant clay is montmorillonite, which exhibits relatively high swelling strain when exposed to water. A petrophysical core analysis (Table 2) showed that Pierre shale has approximately 23 percent porosity and is 10 percen water-saturated. Attempts porosity and is 10 percen water-saturated. Attempts to measur liquid permeability revealed only that it is essentially impermeable. Conventional triaxial compression tests were performed on the small plugs. At 2,000 psi (13.8 MPa) performed on the small plugs. At 2,000 psi (13.8 MPa) confining pressure, compressive strength was low (1,200 psi (8.3 Mpa]) and increased only slightly (1,300 psi (9.0 MPa]) at 8,000 psi (55.2 MPa) confining pressure (Table 3). As shown in Table 3, the compressive strengt of Pierre shale is significantly lower than that of Mancos. P. 169

Raiders of the Lost Mud: the geology behind drilling incidents within the Balder Formation around the Corona Ridge, West of Shetland

“…The initial drilling issue to occur in this narrow, 20 m depth range through the Balder Formation is bit balling, which is when the formation interacts with the drilling fluid, and then proceeds to swell and stick to the drill bit (Fig. 9) (Cheatham & Nahm 1990). Bit balling can significantly reduce rates of penetration (ROP), preventing the drill bit from contacting the formation, and the built up mass of clay can make the bottom hole assembly behave like a piston in a cylinder, producing additional surge and swab pressures (Hariharan & Azar 1996).…”

“…The swelling aspect can clearly be linked back to the lithological character of the Balder Formation, particularly the volcanic tuffs which are now largely altered to smectite (Knox & Morton 1983), a clay type particularly prone to swelling (Norrish 1954;Cheatham & Nahm 1990). An important aspect of the Balder Formation tuffs is that their distribution is regionally variable, particularly when associated with marginal to non-marine deposition compared to marine settings (Watson et al 2017).…”

One-dimensional Swelling Behavior of Clay and Shale Under Electrical Potential Gradient

“…Instability in slopes and embankments are commonly encountered (Brooker and Peck 1993). Water-sensitive shale formations cause costly and time-consuming problems in drilling and completion of wells in petroleum industry (O'Brien and Chenevert 1973;Cheatham and Nahm 1990;Ipek et al 2006). In the USA, the estimated damage costs for buildings and infrastructure in swelling soils and rocks exceed $15 billion each year (Nelson and Miller 1992).…”