Pak, M. (a Schlumberger Company) | Gumich, D. (a Schlumberger Company) | Zinnatullin, I. (a Schlumberger Company) | Mukkisa, S. (a Schlumberger Company) | Bits, Smith (a Schlumberger Company) | Gaynullin, I. (Lukoil)
Developing reserves in the North Russia, Usinsk requires the operator to drill extremely abrasive sandstone formation at bottom of 8.625-in. reservoir section. Drilling with PDC bits has been challenging due to severe cutter wear. Heavy-set standard PDC cutter bit would last on average from 45 to 100 meters depending on field. With the goal of increasing production and reducing costs, reservoir section can be from 170 up to 350 meters long.
With standard PDC bits, it would take from three to four bits to finish the interval. The operator required new type of PDC bit that could efficiently drill abrasive sandstone at high ROP and cutting structure that would remain sharp over longer interval.
To develop a durable bit solution, engineers used an FEA-based modeling system to implement an innovative bit by positioning new shearing elements (rolling cutters) to address cutter wear. Several designs were implemented that hold the cutter securely in place and allow full cutter rotation. Rolling cutter PDC shearing element utilizes the entire 360 degree of diamond edge to shear the formation while reducing frictional heat and, by strategic positioning of these rolling cutters in the high wear areas of the bit, provides longer footage and higher ROP.
New design 8-bladed PDC bit with rolling cutters was tested and compared against direct offsets in two different fields in Usinsk, Russia. In first field, rolling cutter bit drilled entire reservoir section 335 meters with 4.6 m/hr ROP improving 140% meterage and 44% ROP. In second field, two rolling cutter bits drilled reservoir section and replaced four standard PDC bits saving three days.
New concept of PDC bit utilizing rolling cutter expands the limit of PDC bit durability drilling extremely abrasive sandstone formations. New type of bit can efficiently drill in those challenging geological conditions improving the drilling time and cost.
Depending on rock excavatability and rock abrasivity disc cutters can exhibit significant wear. Detailed data on 7 tunneling projects comprising a total length of more than 127 km and more than 12,000 replaced disc cutters have been collected in a custom-made data base. A detailed analysis has been carried out to characterize failure modes and wear patterns of disc cutters. A prediction model has been established that includes cutter life and cutter costs.
Tunnel boring machines for hard rock use disc cutters as the primary excavation tool. Common sizes are 17 or 19 inch (432 and 483 mm) in diameter, while bigger and smaller sizes are used on some projects. Historically, a large range of types regarding geometry, number of cutter rings, application range, and cutter housing styles were developed. A typical 17 inch (432 mm) single disc cutter is shown in Figure 1. The cutter ring (no. 12) is performing the actual cutting of the rock. Due to the rotation of the cutterhead the cutter ring is rotating around the shaft (no. 1) connected by a set of tapered roller bearings (no. 5 and 6). To keep the lubricant for the bearings inside the disc cutter and dirt out of the interior a pair of face seals (no. 4) is used. The hub (no. 7) forms with the seal retainers (no. 2 and 9) the envelope of the disc cutter. Worn disc cutters can be refurbished. Upon the result of an entry check either a simple re-ring can be performed a complete rebuilt. Re-ring includes replacement of the worn cutter ring by a new cutter ring and an oil change. A complete rebuilt includes the disassembly of the entire disc cutter, replacement of all worn parts, and re-assembly.
2. CUTTING PROCESS
Disc cutting utilizes the fact that rock has typically a much lower tensile strength compared to its compressive strength. Schematically, the cutting process is shown in figure 2. A high point load is applied which leads to compressive failure in the contact area. A crushed zone is created. In addition, cracks caused by tensile failure are induced into the zone surrounding the crushed zone. Once those cracks start to intersect between two disc cutter positions, a rock chip is formed and released. The cutting process is influenced by a range of parameters. An overview is provided in figure 3. A number of models have been developed to quantify the cutting process. One of the most common is the model developed at the Colorado School of Mines as published by . During operation each disc cutter on the cutterhead performs a movement as illustrated in Figure 4.
3. TOOL WEAR
Wear of materials is characterized by the fact that each tribo-system is unique. A tribo-system consists of the solids whose surfaces are subject to a loss of material (wear) and a medium that in between. For the specific problem of disc cutter wear a model can be created. For convenience it is prudent to distinguish to general types of wear occurring at disc cutters 
Abtahi, A. (Memorial University of Newfoundland) | Butt, S. (Memorial University of Newfoundland) | Molgaard, J. (Memorial University of Newfoundland) | Arvani, F. (Memorial University of Newfoundland)
X-ray computed tomography (CT scanning) was used to determine the vesicular porosity, average vesicle size, and average vesicle perimeter of fourteen specimens taken from five boulders. Band Pass Filtering was determined to be the best image processing technique to determine vesicular porosity. Porosity values determined using density values were 1.2 to 2.0 times greater than those determined using CT scanning. Porosity values and average vesicle size were very consistent between boulders but vesicle perimeter varied between specimens from the same boulder. The difference is attributed to the shape of the vesicles. Unconfined compressive strength of the specimens was also determined and plotted as a function of porosity as determined using Band Pass Filtering. One anomalous result was attributed to a difference in vesicle shape. CT scanning is an appropriate technique for measuring porosity and vesicle characteristics in a repeatable, non-destructive fashion. Additional work is being conducted for determining vesicle shape parameters to further develop a predicative relationship between strength and porosity.
It is well established that porosity, both microporosity and macroporosity, has a detrimental effect on the strength and stiffness of rock. In order to investigate the effects of porosity on strength and stiffness it is imperative to have accurate measures of total porosity. Other porosity parameters such as pore size, pore shape, and relative location of pores within a specimen would also be valuable parameters. Vesicular basalt specimens obtained from surface boulders from southern Nevada were used in this study. In terms of mechanical strength, basalt is one of the most competent of all rocks. Formed by extrusive volcanic action, it commonly has a micro-fine texture and consists of micro-crystals of augite and plagioclase held together by strong mechanical bonding . A characteristic of basalt that reduces its intact strength and stiffness is the presence of voids (vesicles) formed by trapped gases unable to escape during its rapid cooling process. Vesicular porosity within basalt is typically comprised of isolated pores that may or may not have coalesced with other pores to form larger pores . This paper presents the results of an investigation of the vesicular porosity of fourteen vesicular basalt specimens cored from boulders from southern Nevada. The vesicular porosity was determined using x-ray computed tomography (CT) images from the Montana State University CT scanner. The CT images were analyzed using three different image processing techniques. Vesicular porosity was also determined using specific gravity values of basalt. The porosity values from the four techniques are compared and relationships between vesicular porosity and unconfined compressive strength are presented.
2. PORES AND POROSITY
Pores are open spaces between mineral grains. All pores, regardless of size, shape, location, etc., influence macroscopic physical and engineering properties. There are a number of methods of classifying pores but perhaps the most convenient is accessibility to external fluids. Figure 1 is an exaggerated core slice showing different types of pores. The heavy black line represents the hypothetical edge of the core. The actual edge of the core is gray.
Smith, Christopher B. (Friction Stir Link, Inc) | Krol, Slava J. (Friction Stir Link, Inc) | Ajayi, Oyelayo (Argonne National Laboratory: Energy Systems Division 212) | Lorenzo-Martin, Maria (Argonne National Laboratory: Energy Systems Division 212)
As part of a larger project to build a new casing wear test machine, an analysis of wear mechanisms on several down-hole hardbanding/casing components has been conducted. Specimens included casing and hardbandings from a DEA-42 casing wear tester, casing and hardbandings from a new small-scale casing wear tester and a tool joint from field service.
Similar wear mechanisms including adhesion, abrasion, checking and spalling were observed on specimens from all three sources. Additionally, significant hardworked layers developed on all wear surfaces and adhesive wear was typically dominant. The study indicates that the new small-scale casing wear tester is capable of producing wear mechanisms comparable to both historical data generated by the DEA-42 test and from commercial down-hole service.
A laboratory test that attempts to simulate a revenue service application must produce comparable information to the service situation. As strict as possible adherence to operating conditions is required. A second goal of the laboratory test is the simplification of operating parameters to minimize cost and time but more importantly to minimize the variability in results that are caused by random operating conditions. The same strategy is necessary when designing test equipment intended to produce similar data as an existing laboratory test. Any attempt to simulate a particular wear situation requires duplicating both wear mechanisms and relative wear rates. The wear mechanism is especially crucial because it is of utmost importance in determining the wear rates. Additionally, unless a completely independent wear database is generated with a new machine, the relative wear rates of the old and new must have some predictable correlation.
Attempts have been made in the past to understand wear on both tool joint (hardbanded and not) and casing in downhole operations. Best described several types of wear mechanisms produced in a laboratory rig that included adhesive, abrasive, ploughing and fatigue. (1, 2) The testing was conducted in a rig very similar to the current DEA-42 standard. Williamson used a different test rig that also used full sized tooljoints and casing segments to test the effects of contact pressure and found an experimental relationship between contact pressure and casing wear and used that data to develop a predictive casing wear rate model. (4) White et al also used a test rig with full size tool joints and three casing grades (K55, N-80, P110) and related casing wear to energy dissipation as a result of friction. (3)
von der Ohe, Christian B. (National Oilwell Varco Norway AS Norwegian University of Science and Technology) | Johnsen, Roy (Norwegian University of Science and Technology) | Espallargas, Nuria (Norwegian University of Science and Technology)
McLeod, Alan J. (NCAST Cooperative Research Centre Process Engineering and Light Metals Centre Central Queensland University) | Clegg, Richard E. (NCAST Cooperative Research Centre Process Engineering and Light Metals Centre Central Queensland University)
Challenging wells have been drilled recently utilizing advanced real time tools and techniques to optimize drilling operation while reducing risk and increasing safety. Moreover, the real time tools and techniques help identify upcoming drilling problems using real time data before they occur. Real time drilling analysis begins when real time drilling data are available and transmitted to the office locations via a remote server. The data can then be interpreted and analyzed by the engineers implementing various models for appropriate decision making. Lately, real time bit wear estimation has been a challenge in drilling a well to reach to the highest drilling performance and avoid bringing serious problems to the bit.
It has been shown that the combination of Mechanical Specific Energy (MSE) and drilling rate models can be used for real time bit wear estimation while drilling. As MSE does not take the bit wear effect into account while drilling rate or rate of penetration (ROP) models do, their difference can be used to monitor and identify bit wear status while bit is in the hole. This paper demonstrates a new form of a developed model to predict bit wear status while drilling which is built by combining rock energy (MSE) model with a newly developed drilling rate model for roller cone bits as well as a previously developed model for PDC bits. Rock confined compressive strength (CCS) is obtained from ROP models and used in conjunction with MSE values to predict bit wear trend. Several bit run sections from offset wells in Alberta, Canada were tested utilizing the model and final results are compared with the reported bit wear outs from the field. Encouraging results show that this methodology can be applied to detect changes in drilling efficiency by monitoring bit wear trend in real time while drilling.