The traditional casing design method for Japan Petroleum Exploration Co., Ltd. (JAPEX) was based on specific margins (either ratios or differences) between the anticipated load and the strength of the casing. The specific margins, however, had never been explained as to the downhole conditions on which the margins are based. The size of the margins had been accepted by the company engineers without reasonable explanations. In the mid-1980's, when JAPEX drilled a 6, 000-m Japanese record depth well, the triaxial stress design method was studied. However, due to its relative complexity, the triaxial stress design method was utilized only for some special cases to confirm the soundness of the traditional design. In 1989, JAPEX decided to disuse the traditional design method and adopted a new design method. The new design method assumes specific downhole conditions for each part of the design, namely collapse, burst and tension. Design factors were adopted from a combination of the JAPEX traditional design method and the Maximum Load Casing Design method presented by Mr. C. Prentice. Through the repeated field use since then, the new JAPEX method has been modified to better reflect the downhole conditions and to reduce casing cost. Application of different sets of design scenarios to exploratory wells and development wells is an example of such modifications. In the future, the concept of probabilistic distributions of the strength and the load of the casing shall be introduced in casing design. This approach will quantify the risk of casing failures and will provide a new methodology to preserve an acceptable level of safety without sacrificing economic considerations.
A 540°C, 3, 729m geothermal well, WD-1A, was drilled in Kakkonda, Japan. One major issue for drilling high temperature wells was how to protect temperature sensitive downhole tools from high temperature stagnant mud while RIH (Running-in-the-hole). To solve this problem, a TDS (Top-drive-drilling-system)-Cooling-Method was developed to drill the WD-1 A well. With this method, mud was pumped continuously while running a BHA (Bottom-hole-assembly) into the hole using a TDS. To evaluate this TDS-Cooling-Method, an experiment was conducted and discovered to be very effective for cooling BHAs while RIH. Bit performance for two geothermal wells, drilled with and without using the TDS-Cooling-Method, was evaluated and revealed that the TDS-Cooling-Method could prolong three-cone bit life 5times. Also, economic evaluation of using the TDS-Cooling-Method was made based on the field data. More than a 40% cost reduction was simulated by using the TDS-Cooling-Method when drilling the bottom 1, 000m section of a 3, 500m well. Several deep, high-temperature oil wells, with bottom hole static temperatures ranging from 180 to 230°C, have been drilled since 1990 in Japan by MITI. Reports are that bit life decreased to 15 to 30 hours, when the deeper sections of these wells were drilled. Bit performance of the MITI-Nishikubiki well was surveyed and judged that o-rings were damaged while RIH due to high temperatures. Judging from the WD-1A case, the TDS-Cooling-Method was considered to be a possible solution method to prolong the bit life. Economic evaluation of the use of TDS-Cooling-Method was made for the MITI-Nishikubiki well and determined that a 56 million yen cost reduction could be achieved by using the TDS-Cooling-Method if applied for the last 600m section. TDS-Cooling-Method can be expected to extend the use of temperature sensitive tools and prolong their life for both geothermal and oil wells. Therefore, even greater cost savings can be expected in addition to those simulated.
The importance of borehole instability has been acknowledged as one of the main causes of unproductive time and reduction of drilling performance in the oil industry. Recent intensive studies on borehole instability in some regions have shown the successful applications of geomechanics to borehole instability in the overthrust region. TRC/JNOC has started a research program to establish a comprehensive scheme to tackle important but scientifically complex phenomenon. Many physical and chemical processes are related to the mechanical behavior of the formation surrounding wells during and after drilling. Also, the establishment of a geomechanics model for deep underground is an important and difficult subject of this study. In this paper, the author presents the basic concept of the borehole failure mechanism, recent advances of such studies including realistic constitutive laws, failure criteria, poroelastic treatment for low permeable rocks and a discrete model of rock mass failure. The importance of the geomechanical model for this problem is discussed, and the results of an investigation results of drilling troubles caused by borehole instability in Japanese fields are presented.
Iwaki Platform was built in 1984, which is located about 40 km off the east coast of Fukushima prefecture, and natural gas has been produced from C-Structure for 15 years. There are a few Satellite fields toward the south-south-west from the platform, which is called C-Satellite North, C-Satellite Central and C-Satellite South and those approximate distances from the platform is 4km, 6km and 7.5km each. Johban Oki-2 well was drilled to C-Satellite North structure as the first extended reach well in Japan and completed successfully in 1994. After the success of the well, some engineering study mainly regarding rig modification was done for the next ERD to C-Satellite Central structure, however the detail engineering and the execution had been temporarily given up for the various reasons. From the end of 1998, feasibility study of the ERWs to both C-Satellite Central structure and C-Satellite South structure was conducted as the alternative development instead of subsea completion. The FS covered preliminary drilling engineering, structural analysis of drilling structures and rig modification, and the FS took almost nine months. After all, the execution was totally given up mainly due to the difficulties of the rig modification and reinforcement. This paper reports the critical issues of drilling engineering in this study, 1) well path design, 2) hole cleaning, 3) torque & drag and mud friction, and 4) borehole stability, which will match the subject “The Integration of Technology and Engineering” of the 2000 JAPT Drilling Symposium.
A downhole temperature, and pressure simulation program for drilling, especially for safe methane hydrate drilling, has been developed under a joint R & D project led by Japan National Oil Corporation. The simulation program integrates a database management, a numerical computation core, and a graphical user interface (GUI) in a Windows-based application. It aims to support the control of methane hydrate drilling by simulating wellbore temperature and pressure simultaneously. Three key models are employed in the computation core, 1) a one-dimensional flow dynamics model, 2) a two-dimensional heterogeneous heat conduction model, and 3) a wellbore-formation interface model. New features in the development include, 1) computation of circulating temperature profiles of not only the wellbore but also radially in the formation, 2) the ability to show the in-situ P-T conditions and hydrate equilibrium curve simultaneously, 3) use of multiple heat transfer correlation and rheology model options, 4) ability to simulate heat transfer in a marine riser including buoyancy modules, 5) capability to simulate environmental effects, such as current, sea temperature, etc., and 6) friendly GUI. The validation and verification (V & V) have been performed using field data. The comparison showed a good agreement based on the return and downhole circulating temperature and standpipe pressure. After the work of V & V, the simulation program has been successfully applied in the planning of, and during the drilling of the Japan MITI methane hydrate exploratory well“NANKAI TROUGH”.