A new method of obtaining improved zonal isolation using drilling fluid solidification technology has been developed. A water-base drilling fluid is converted into a cement using a hydraulic blast furnace slag. Hydraulic blast furnace slag is a unique material which has low impact on rheologid and fluid loss properties of drilling fluids, can be activated to set in drilling fluids which are difficult to convert to cements with other solidifrcation technologies, is a more uniform and consistent quality product than Portland well cements, and is available in large quantity from multiple sources.Because of its low impact on drilling fluid properties, blast furnace slag may be added to a drilling fluid at low concentrations during drilling operations. The Glter cake and drilling fluid in washed out sections thereby contain a hydraulic materiaL ARer reaching casing point, a mixture of drilling fluid containing chemical activators and higher concentrations of slag may be usedto cement the casing string. Chemical activators from this mixture cause the slag in the filter cake and any bypassed drilling fluid to set. The result is a more complete seal for the annulus. Fluid and hardened solid properties of blast furnace slag and drilling fluid mixturm used for cementing operations are comparable to properties of conventional Portland cement compositions. The design, testing and field application for this technology are similar to conventional cementing methods. Fluids with densities between about 1198 kg/m3 (10 lblgal) and 2397 kg/m3 (20 lblgal) may be prepared. The mixtures have been applied in web where temperatures range from about 4' C (40°F) to 315°C (600°F).This new solidification method provides the proper combination of fluid and solid properties, simplicity of design and application, improved zonal isolation, and broad applicability to bring drilling fluid solidification technology into widespread use.A fundamental weakness of the conventional cementing process is the uncertainty of establishing atrue, reliable seal at the borehole w a l l and cement interface. In many cases, alayer References and tables at end of paper.

A universal fluid (UF) is typically a water-base fluid that has been treated with finely ground blast furnace slag and that still maintains the appropriate characteristics of a good drilling fluid. The slag becomes concentrated in the filter cake formed while drilling permeable formations and slowly sets to form a hard layer intimately bonded to the formation. True zonal isolation can be obtained by using a UF and subsequently cementing with slag-based mud solidification technology. Complete mud displacement and efforts to remove filter cake are not necessary prior to cementing since the solidified UF filter cake bonds strongly both to the formation and to the cement and since undisplaced portions of the UF set up as well. The universal fluid (UF) has been developed primarily out of the need to improve cementing in horizontal and extended-reach wells. To date UFs have been used to improve zonal isolation and to reduce or prevent lost circulation or cement fallback during drilling and cementing. Two Diatomite wells in the Belridge Field, California and the vertical portion of one horizontal well in the Midway Sunset Field, California were successfully drilled with a UF and cemented with slag mix cement slurries. Also, a well in the Midway Sunset Field, where losses are routinely experienced, was drilled with a UF specifically to control lost circulation, and no losses were experienced. Five wells in the Peace River area in Canada were successfully drilled with aUF to prevent cement fallback upon cementing.

Full-scale drilling simulations were conducted to determine effects of mud weight and static fluid loss on rate of penetration in medium-hard shale with water-base and oil-base muds. Two new 6 1/8 inch(156 mm) bits were tested: a natural diamond and a roller-cone insert type. Test data were analyzed using regression techniques. Results are presented in the form of four equations which predict rate of penetration – one for each combination of mud and bit type. Mud properties affected rate of penetration for the roller bit much more than for the diamond bit. Rate of penetration was usually greater for the roller bit except when high mud weight or overbalance conditions occurred.

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

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