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

This paper discusses the planning, results, problems, and lessons learned during the first use of the Extended-Range Electromagnetic (EM) MWD system for underbalanced drilling in an offshore well. The use of EM MWD in offshore applications has historically been limited due primarily to the difficulty of seabed deployment of the receiving antenna. In underbalanced drilling, however, EM is the MWD system of choice because mud-pulse telemetry has great difficulty operating in two-phase flow conditions commonly used. If underbalanced drilling is to become more frequently used offshore, then one limitation that must be overcome is the difficulty for MWD systems to provide real-time annular pressure, directional, and formation evaluation information. Repsol-YPF-Maxus successfully used underbalanced drilling on Krisna Well D12 in offshore Indonesia to prevent massive lost circulation and stuck pipe in the reservoir. Although the total vertical depth of the well was relatively shallow (approximately 4,200 ft subsea), the very low resistivity of the formations above the reservoir precluded use of standard EM MWD services. The Extended-Range EM MWD system was used to extend the depth capability. Background SPE/IADC 67689 described the overall planning and drilling of Well D12 in Repsol-YPF-Maxus' Krisna Field in offshore Indonesia, using underbalanced drilling. 1 The primary goal of underbalanced drilling on Well D12 was to eliminate the significant drilling problems encountered in previous wells using conventional overbalanced drilling methods. Low pore pressure (2. 98 ppg) in the reservoir, the Upper Rata Baja, caused costly and time-consuming drilling problems in these earlier wells. The most significant drilling problems were massive lost circulation (in excess of 50,000 bbls of fluid in one well) and stuck pipe. Reducing formation damage and identifying fractures while drilling were secondary goals of the project. Both secondary goals relied on the ability to maintain an underbalanced condition. The Reading and Bates Falcon jackup rig Roger W. Mowell was positioned in 77 feet of water over Well D12. Seven-in. casing was set at 5,029 ft measured depth (MD) at an 80 deg. angle of hole inclination. After drilling out the casing shoe, 1,159 feet of formation were drilled underbalanced using 6-1/8 in. bits and 4-3/4 in. EM MWD tools with hole inclination ranging from approximately 73 to 83 degrees. After experiencing some minor startup problems using the underbalanced drilling system, all the operational and technical problems were solved. One major change was swapping fluid systems from a KCl mud gasified with nitrogen to a diesel mist. By the end of the well, the underbalanced drilling system was working well. All objectives of underbalanced drilling were achieved and the well was drilled without any major drilling problems. Planning of Krisna Well D12 This paper describes the use of the Extended-Range EM MWD system on Well D12. The main reason for using EM MWD was to provide real-time data for annular pressure and directional information.

The development and field testing of a new three-axis shock and vibration MWD sensor are presented. The sensor is shown to have the capability to detect harmful BHA dynamic conditions such as whirling. lateral BHA shocks. stick-slip. and bit bounce by measuring the changes in acceleration in the axial. tangential. and radial directions. Interpretations of these conditions have improved insight into the drilling process and are expected to result in improved drilling efficiency.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractThis paper discusses the planning and drilling of Big Prize #2-20 well, drilled by BP America in South Panola Field of the Arkoma Basin in Oklahoma. Wells previously drilled in the field have been prone to problems with lost circulation and stuck pipe, high drillstring torque and drag, low ROP, and directional control. The primary causes of these problems are the hard and fractured formations, steeply dipping formation beds, and small geologic targets. To reduce drilling time and cost, major changes were made to standard drilling practices in the area. The major changes were contracting a state-ofthe-art land rig, modifying the drilling fluids program and surface treatment, using steerable motors from surface to total depth (TD), replacing mud-pulse MWD with electromagnetic (EM) MWD for the first time in the field, and employing premium drill bits. The result was substantial reduction of drilling time and cost. The Big Prize #2-20 well was drilled to a TD of 15,850 ft measured depth (MD) in 70 days compared to 96 days for the offset well drilled on the same pad to a similar depth, which represented an improvement of 26 %.