by Dennis Nagy, Vice President, Business Development, CD-adapco
The ongoing push for more producible oil and gas reservoirs is leading the industry into greater offshore depths and harsher environments. The challenge of developing and producing from fields at depths up to 25,000 feet requires workers to operate unproven technology in harsher environments, including greater ambient pressure, lower temperature and different well fluid temperatures, pressures and compositions. Traditional design approaches to developing equipment and systems for shallower offshore wells often don’t work effectively in these deeper, harsher environments. Further, the nature of drilling makes it very difficult to diagnose the performance of down hole (in the wellbore) equipment in the field. How can petroleum and marine mechanical engineers move outside their traditional realm of experience to develop new designs that will improve drilling performance in these new challenging environments?
To address these challenges, oil and gas companies and their suppliers are increasingly using computational fluid dynamics (CFD) simulation to optimize the performance of drilling equipment. CFD simulation offers the advantages of providing extensive diagnostic information such as the ability to visualize flows and pressures around the bit under actual drilling conditions. Simulation also makes it possible to quickly and easily evaluate the relative performance of different designs. Dr. Michael Wells, Director of Research for the Hughes Christensen division of Baker Hughes is one of the industry’s foremost experts on simulation in oil and gas applications. Dr. Wells recently sat down with the author and provided some examples of how simulation is helping to address a variety of drilling challenges.
Optimizing PCD drill bits
Dr. Wells has spent a considerable amount of effort in hydraulic optimization of polycrystalline diamond compact (PDC) drill bits. PDC bits typically have from 3 to 12 blades emanating from the center of the bit outwards. Embedded in these blades are polycrystalline diamond cutters (PDC) which consist of thin diamond wafers attached to carbide backings. The cutters are laid out to ensure that the entire bottom of the hole is cut while promoting bit stability. The region between the blades, called the junk slots, provides a path for the removal of rock cuttings. Commonly, a single nozzle is positioned at the top of each blade that feeds high velocity liquid (drilling mud) through the junk slot to clear loose rock debris from beneath the bit. Various nozzle configurations are possible. Bits are designed with multiple nozzles in a junk slot, a single nozzle feeding two junk slots and with no nozzle at all in a junk slot.
Dr. Wells indicated that the first step in constructing simulations of down hole drilling involves the creation of a full, detailed solid model of the bit and the sides and bottom of the hole. The region modeled is a sealed volume element comprising the space occupied by the drilling fluid between the bit and the hole walls and bottom. A realistic bottom hole pattern, created as though generated by the cutters on the bit, is incorporated into the model and the bit is displaced into the rock (hole bottom) by an amount typical for the geological region under consideration. The solid modeling environment is also used to generate text information, blade locations, cutter face centers, nozzle centers and other information that is later used by the CFD model. Realistic operational parameters such as flow rate, nozzle size, rpm and fluid density (mud weight) representative of the region or application being studied are also incorporated into the model. The goal of the simulation is typically to size, locate and orient the bit nozzles to maximize cuttings transport and minimize erosion. Dr. Wells has developed an optimization process that involves configuring the hydraulics across the bit so that the percent of the total flow rate that passes through a particular junk slot is roughly equal to the percent of the total cuttings volume generated by the adjacent blade.
“The key to successful implementation of CFD in drill bit hydraulics is the ability to correlate the CFD results to actual down hole performance of the drill bits,” Dr. Wells said. Hughes Christensen uses a drilling simulator to tie computed results to actual drilling performance in a controlled environment under representative drilling conditions. The high pressure drilling simulator employs actual drill bits (up to 12-1/4 inch in diameter), under realistic pressures (up to 15,000 psi) drilling actual rock cores to evaluate drilling efficiency and the transport of rock cuttings. Rock cores commonly used include Mancos Shale, Berea Sandstone, Wellington Shale, Crab Orchard, Catoosa Shale, Indiana Limestone, Pierre Shale, Carthage Marble, among others. In some cases, special rock cores are obtained from outcroppings of the formations of interest to particular customers. Historically, a variety of oil and water based drilling muds have been analyzed with having mud weights as high as 16 ppg.
Erosion along the face of the bit is also a major concern in a number of fields worldwide. To evaluate erosion rates in drill bits and other down hole tools, Dr. Wells has developed a particle erosion model and incorporated it into CFD. The process began by performing experiments using a single fluid jet to impinge particles on mild steel and carbide matrix (bit materials) specimens. The tests were designed to measure the erosion coefficient, defined as the ratio of the grams of material eroded from the specimen surface per grams of erodent material impinged on the surface. Dr. Wells has successfully applied this model in wide variety applications. Typically the designer used these predictions to move and orient the bit nozzles to minimize the rate of erosion on the bit surface. Figure 2 shows a successful application of the erosion model where the overall rate of erosion on the surface was reduced by roughly 67%. While erosion can not be entirely removed it can be greatly reduced and often directed to less critical features of the bit.
Optimizing nozzle exit geometry
Dr. Wells has also used CFD to investigate the effects of the nozzle profile and exit geometry on the efficiency of the drilling process. He numerically analyzed the flow produced by several unique, commercially available nozzle designs to identify flow features that might lead to improved bit and bottom hole cleaning. Drilling simulator and field tests were conducted to correlate rate of penetration (ROP) improvements with identifiable flow enhancements brought about by nozzle design. Four of the several nozzle designs examined in this study are shown in Figure 3.
The study concluded that small features built into the exit of a bit nozzle have little effect on the resulting jet. The size of the exit feature is limited by the small diameter of the nozzle body. The smallest features, as used by the Y, star and cross nozzles (Figure 3b, 3c and 3d), tend to disappear in the flow at a distance of roughly one to two nozzle diameters from the nozzle exit. Larger features, as seen with the slot nozzle (Figure 3a) may persist for longer distances from the exit but the flow tends to scale with the smaller dimension (width) and thus the jet decays more rapidly with distance from the nozzle exit. These results suggest that features in the nozzle exit must be relatively large to affect the structure of the jet and thereby the performance of the bit hydraulics. Other nozzles designs were evaluated that either force the jet to swirl about its axis or redirect the jet to some angle with respect to the nozzle axis. The simulations showed the turbulent jets generated by the test nozzles differed only slightly from the standard nozzle. The laboratory drill tests and field results correlated with the simulation by showing no change in bit performance.
Using CFD a new Figure-8 nozzle was designed then built to address specific applications where a single bit nozzle was required to provide flow to two junk slots, a scenario typically referred to as split flow (Figure 4). In this environment a single nozzle is directed toward the end of the blade separating the two junk slots, Figure 4b. A drilling simulator test was conducted to evaluate the benefit of the new Figure-8 nozzle design. The drill tests were conducted in Catoosa Shale at 120 rpm and 290 gallons per minute (gpm) using 11/32 nozzles. When conventional nozzles were used the drill bit starting balling (clogging up with drill cuttings which dramatically reduces performance) at a rate of penetration (ROP) of 52 feet per hour. When the Figure-8 nozzles were used, the bit did not start balling until it reached a rate of penetration of over 190 feet per hour. Balling is the situation where the reground cuttings and solid particles remaining on the hole bottom tend to adhere to the bit body, particularly in sticky formations such as shales, limestones, and chalks. The configuration using a roller cone drill bit, shown in Figure 5, resulted in a new field record at Saudi Aramco.
Dr. Wells concluded:
“These applications demonstrate that CFD offers the potential for huge advancements in drilling, especially under more challenging conditions. CFD gives design engineers the ability to easily and accurately analyze fluid flow, under harsh realistic drilling conditions making it possible to rapidly evaluate alternatives and provide comprehensive diagnostic information. The method also allows design engineers to optimize the fluid flow around the drill bit during the design phase, rather than after the product has been manufactured.”
Dr. Nagy is a thirty-seven-year veteran of all aspects of CAE, from R&D through executive leadership. Prior to joining CD-adapco, he held various executive positions in the CAE world including CEO of Engineous, Senior VP of Worldwide Sales and Support at MSC.Software, VP of Marketing at Fluent, and most recently VP of International Business Development at Blue Ridge Numerics.
Dr. Nagy is an engineering graduate of MIT (BS, MS) and the University of California, Berkeley (Ph.D.).
CD-adapco is a US$90M+ global leader in Computer-Aided Engineering solutions providing flow, thermal, and structural simulation software and services for over 29 years.