SPE Paper: New Nozzle Hydraulics Increase ROP for PDC and Rock Bits

J.E. Akin, Ph.D., PE., SPE, Rice University, N.R. Dove and S.K. Smith, Vortexx Group Inc., and V.P. Perrin, SPE, Chevron Petroleum Technology Co.

This paper is a draft of that prepared for presentation at the 1997 SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 4-6 March 1997. Copyright J.E. Akin. For final version, see SPE.org or write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract

The use in drill bits of individual asymmetric nozzles, with special interior transitional surfaces, significantly improves the rate of penetration (ROP) compared to that achieved with conventional circular nozzles. The new fluted nozzles change the pressure distribution and turbulence of the flow. The studies of these new nozzle designs have included controlled drilling laboratory tests, actual horizontal and vertical field wells, computational fluid dynamics models, and flow visualization tests. Drilling results for both polycrystalline diamond compact (PDC) bits and rock bits are given to verify a significant increase in ROP. The fluid mechanics concepts employed are discussed and other validation efforts are outlined.

Introduction

A new technology for improving drill bit nozzle hydraulics has been developed and is being successfully applied to both polycrystalline diamond compact (PDC) bits and rock bits. The goal of that technology is to use individual asymmetric nozzles in drill bits so as to improve penetration rates in oil and gas drilling by at least 20% over the rates achieved with conventional circular nozzles. Several researcher groups had concentrated on trying to improve the hydraulics of drill bit nozzles, or jets. Some early enhanced nozzle designs were based on using cavitating or resonating fluids but they required conditions that could not be maintained in an actual field environment. Some recent nozzle designs tried to produce swirling flows with several converging small diameter tubes inside the nozzle, or by placing small guide vanes inside the nozzle. But these, and other, methods have not been consistently successful in actually drilling wells. The Vortexx Group Incorporated (VGI) chose a different approach and employed a unique interior nozzle geometric transition shape, and basic fluid mechanics concepts to develop a negative impingement pressure nozzle that also increases turbulence. This behavior is quite different from the standard circular axisymmetric nozzle which is well known to produce only regions of positive impingement pressure on the surface [1]. In some applications the nozzle actually causes a suction (negative pressure) at the impingement surface. For subterranean drilling it decreases the impingement surface pressure to less than hydrostatic. Verification of the negative impingement pressure phenomenon was first obtained, with water, in controlled laboratory pressure measurement and flow visualization tests. Those data were then confirmed by detailed computational fluid dynamics (CFD) studies. The "fluted nozzles" have been shown to have the additional benefit of causing a factor of four or five increase in the volume of fluid that is entrained or drawn into the jet. That causes much more fluid recirculation around the face of the bit and the opposing rock face. Also, the peak turbulence levels increase significantly around the jet.

Drilling Laboratory Tests

PDC and rock bit rate of penetration tests were conducted with existing bit designs at the Amoco Drilling Research Laboratory (ADRL), Tulsa, Oklahoma in February 1996. The intent of those tests was to vary only the weight on bit (WOB) and the choice of nozzles to determine their effects on the rate of penetration in Cattusa Shale [2].       A 6 7/8 inch (175 mm) Diamond Products International (DPI) TD290H PDC bit was run with a flow rate of about 296 gal/min (1,120 l/min) and a WOB ranging from 3,000 to 13,000 pounds (1,361 to 5,897 kg). A standard 12/32 inch (9.5 mm) diameter circular nozzle was tested against a fluted model V12VLAE. Over a wide range of WOB values, the fluted nozzles yielded more than 100% increase in ROP. Over the full range of WOB values the fluted nozzles averaged a 69% ROP increase. Next, a 7 7/8 inch (200 mm) Smith International Incorporated (Sii) F15H rock bit was tested. The rotating speed was about 80 rpm, while the flow rate was about 323 gal/min (1,223 l/min). Three standard 12/32 inch (9.5 mm) circular nozzles were tested against an alternate geometry fluted model V12CLBA. The WOB ranged from about 5,600 to 45,600 lbs (2,540 to 20,684 kg). The ROP enhancement ranged from about 10% on the low end WOB to about 60% near the maximum WOB. Over the full range of WOB values the average ROP increase was about 29%. These tests verify that existing bit designs can yield significant ROP enhancement simply by the addition of different nozzles [2].

Field Test Offset Data

For three years the fluted jets have been extensively field tested in a number of locations. The total drilled hole is more than 250,000 ft. (76,200 m) over 4,000 hours. This includes a combination of vertical, directional, and lateral well-bores in more than a dozen different formations. The longest fluted run is about 138 hours. These field tests, and offset data have shown that the use of the fluted nozzles in standard PDC bits and rock bits can lead to significant increases in ROP. Most of the non-proprietary field test results will be summarized here for both PDC and rock bits. Here ROP will refer to IADC values unless otherwise noted. That is, connection times are included, thus lowering the ROP values.

PDC Field Tests. An initial series of three vertical wells drilled by the Union Pacific Resources Corporation (UPRC) in Brazos County, Texas USA during the period 1993--1996. Security DBS (S/DBS) provided 9 7/8 inch (251 mm) B25-4 PDC bits. More than twenty offset wells were previously drilled using standard nozzle bits so a large historic data base was available for performance comparisons. All wells were drilled in the same area and formations (Midway Shale, Navarro, and Pecan Gap). Offset data for the standard wells and geolograph charts for the test wells were available. The rigs, operator, field supervision, and crews were the same as were the in- and out-depth relative to the formation tops. All wells were drilled using the same mud and hydraulic programs. The resulting data for that group of wells represent over 44,900 feet (13,685 m) of hole and over 900 hours of time on bottom. Most of the fluted runs were about 50 hours long, with the longest being 88 hours. The range of true vertical depth (TVD) was from 9,700 ft (2,957 m) to over 18,000 ft (5,486 m). Most of the runs were in the TVD range of 11,000 ft (3,353 m) to 13,000 ft (3,962 m). The only observable statistical correlation for the increased ROP in the field appears to be the use of the fluted nozzle model V14PLAA. Using IADC drilling data showed an average of 22% increase in ROP, using the earliest fluted design. Dropping data from three questionable outlier wells gives a fluted ROP increase above 40%. The slowest of the fluted runs was for a re-run bit used in the second test. A ROP statistical comparison for those field tests is given in [2].  A more recent series of three vertical wells with comparable offsets were drilled by Enron using the 7 7/8 inch (200 mm) Security DBS model FM2745 bits, in the Carthage Field in Panloa County, Texas USA. All three bit runs were through the same formations with comparable depths and drilling conditions. Two bits equipped with standard circular jets were compared to one equipped with fluted nozzles. The mud weights for the two standard jets, and the fluted run were 10.7, 10.8, and 11.2 lbs/gal (1.28, 1.29, and 1.34 kg/l), respectively, with drilling times of 45.5 hr, 52.1 hr and 49.6 hr, respectively. The corresponding ROP values were 59.1, 62.8, and 85.5 ft/hr (18.0, 19.1, and 26.1 m/hr), respectively. Thus, the fluted model V10CEBA equipped bits increased the ROP average by 40.3%. Figure 1 shows these ROP comparisons, along with the corresponding mud weights and drilling times.

Rock Bit Field Tests. Here we will summarize recent rock bit field results from Amoco, Pan Canadian Petroleum Ltd. (PCPL), and Chevron. All were run with Smith International Inc. rock bits. For the small number of nine comparison wells the average ROP increase was about 48%.       Amoco ran the Sii MF15 8 1/2 inch (216 mm) rock bit, IADC code 517, in horizontal wells in its Sharjah field in Saudia Arabia in the Shuaiba formation, which is a relatively clean limestone. The fluted model V19CEBB and standard runs included logging while drilling (LWD) and are the best instrumented fluted runs to date. The fluted run followed in the same hole after the standard run. The average mud weights for the standard and fluted runs were 9.6 and 9.8 lb/gal (1.15 and 1.17 kg/l), respectively. Both had a flow rate of 508 gal/min (1,923 l/min), and a rotational speed of 212 rpm. The standard horizontal run drilled 883 feet (269 m), in 50 hours, to a measured depth of 15,489 feet (4,721 m), where the fluted run began. It drilled an additional 625 feet (190 m) in 33.5 hours. From the LWD porosity data we know the Vortexx run just happened to begin at a depth where it suddenly dropped from the previous average value of 6.7% to less than half that value, 3.3%. That is, the fluted bit was drilling a much denser formation than the standard bit which proceeded it. One would normally expect a bit to significantly slow down in this situation. Yet, the fluted equipped bit achieved an IADC ROP value increase of about 5.6%, while the LWD on bottom rotating ROP increase was 10.1%. Figure 3 shows the ROP and (scaled) formation porosity as a function of measured depth for the standard run.

      Considering the fluxations in WOB, ROP, formations, and flow rates the first 500 m (1604 ft.) section, before the top of the Viking, was considered a more reasonable segment for performance comparisons. In that top segment, shown in Fig. 6, the fluted equipped F07 bit ROP average of 89.1 m/hr (292 ft/hr) exceeded the standard mini-extended average of 74.6 m/hr (245 ft/hr) yielding a similar average ROP increase of 19.5% for the fluted jet.

      Figure 7 shows the combined ROP, for the two pairs of standard and fluted tests, through the maximum depth. The fluted nozzles (upper curve) had the same area and standoff distance as the standard nozzles. The average ROP increase was 50%. For the more reliable data in the top 500 meters (1640 ft), through three formations, the fluted equipped F07 bit averaged 89.1 meters per hour (292 ft/hr) versus 56.9 m/hr (186 ft/hr) for a standard jet, giving an average ROP increase of about 56% for the fluted jet as shown in Fig. 8.

      Chevron (CPT) tested the Sii MFDSH 8 7 3/4 inch (222 mm) rock bit in a series of vertical wells drilled through chalk in its East Heidelburg field, Laural, Mississippi USA operations. Over the total depth of about 1150 feet, from the top of the chalk, the fluted nozzle run gave an average ROP increase of 41.5%, as shown in Fig. 9. In the bottom 600 feet, the fluted and standard ROP values of 46.8 ft/hr (14.3 m/hr) and 46.2 ft/hr (14.1 m/hr), respectively, were essentially the same. However, through the upper 600 ft (183 m) chalk section the fluted nozzles significantly out performed the standard jets. Figure 10 shows the ROP plots versus depth from the top of the chalk formation. The upper plot of the fluted model V14CEBB bit has an average ROP of 46.3 feet (14.1 m) per hour versus the standard circular nozzle bit average rate of 19.6 ft/hr (6.0 m/hr). That gives an average ROP increase of about 136% due to the use of the fluted nozzles in the chalk.

Fluid Mechanics Concepts

To understand conceptually the fluid mechanics of the asymmetric negative pressure nozzle it is best to start with a review of the classical circular nozzle. The classic axisymmetric nozzle produces only positive, and axisymmetric, pressure when it impinges normally to a standoff surface (the well-bore face) [1-3] There are relatively few parameters that can be varied to control this type of flow. They are primarily the nozzle outlet radius, and angle of the velocity vector around the exit circumference. The latter angle is dependent on the nozzle transition shape from its inlet to outlet. The nozzle exit velocity angle influences the additional entrainment of the surrounding fluid into the fluid jet exiting the nozzle and increases the momentum of the jet before it impinges on the surface. We desire to create a negative pressure on regions of the impingement surface normal to the jet. If we examine a radial plane through the axisymmetric jet centerline, we find that the classic flow does involve one or more vorticity cells with significant negative pressures. This type of flow is easily demonstrated and typical photographs of the vortices are found in books on flow visualization [4], and in computational models [5]. While those vorticity cells do not intersect the impingement surface to produce regions of negative impingement pressure, they can be relatively close to the surface. The modification, or perturbation, of the location of these cells is the key insight to forcing a cell of negative pressure to intersect the surface. Imagine what would happen if we slightly perturbed the nozzle geometry such that the outlet radius and exiting velocity vector slope are both periodic around the outlet perimeter, as sketched for two planes in Fig. 11. Some portions of the negative pressure cell would be pushed down closer to the impingement surface while other portions of the cell would be pulled up toward the nozzle outlet plane. The negative pressure (and vorticity) cells would undulate up and down as one went around the circumference of the jet.

      A solid model of the interior shape (fluid volume) of the two-lobe fluted nozzle used in the ADRL PDC test is shown in Fig. 12. A similar shape with three-lobes was used for the nozzles employed in the Security DBS PDC bit field tests. A third geometry was utilized for the ADRL rock bit test.

Nozzle Pressure Tests

Experimental flow visualization studies and pressure measurements in water were conducted with the classic axisymmetric jet and two-, three-, and four-lobe asymmetric fluted nozzles. The classic positive impingement pressure result was verified and is shown along with the three-lobed asymmetric pressure measurement in Fig. 13. Since those tests were conducted at a low hydrostatic head the Vortexx nozzles actually produced a suction (negative pressure) on a significant portion of the surface, as demonstrated by the deep dips in the pressure carpet (magnitude) plot of Fig. 13. The location of those regions depends upon the flow rate and the nozzle exit geometry. The experimental pressure measurements verify the insight outlined in the above fluid mechanics discussion of the perturbed geometry nozzle. An independent verification of this newly observed flow phenomenon and negative pressure distribution was also obtained from both finite element and finite volume computational fluid dynamics studies.

Computational Fluid Dynamics Studies

The CFD studies were carried out with various flow analysis codes [5-6]. An initial series of axisymmetric studies were run to give insight into the location of the negative pressure cells and the amount of entrainment. Next, a series of turbulent three-dimensional models were constructed for periodic symmetry models of the two-, three-, and four-lobe asymmetric fluted nozzles. These studies utilized the solution of the full non-linear three-dimensional Navier-Stokes equations coupled with the "K-Epsilon" turbulence closure system for the given flow domains [5]. They were initially evaluated with linear finite volume codes. However, since the turbulence and curvilinear geometry were so important most of the CFD work has been conducted with a much more accurate curved hp-adaptive finite element system. This specialized kernel has error estimators that automatically refine the mesh, and/or increase the polynomial orders to improve the accuracy. Thus, most of our turbulence calculations involve locally refined meshes along with 6-th degree polynomial functions to properly resolve the "K-Epsilon" model. Initially, periodic symmetry was employed for simplified boundary conditions so as to reduce the time required for the parametric studies. The computed asymmetric negative impingement pressure contours on the standoff plane were found to agree with the nozzle pressure tests cited above. Standard nozzles were also run as three-dimensional models to verify the programs and for comparison with the new designs. Numerous CFD studies have since shown that the new flow phenomenon can be controlled so as to create an optimum location for negative pressure and/or maximum turbulence. These studies are now being extended to include more of the bit geometry.

Conclusion

A successful new technology for improved ROP in both PDC and rock bits has been demonstrated. It is based on new fluid flow hydraulics for drill bit nozzles. CFD and experimental studies have verified that several new and innovative nozzle designs can produce negative impingement pressures and high recirculation regions. The location and size of these regions can be controlled in part through the geometric design of the nozzle. Field tests, offset data, and drilling research laboratory tests have shown that the use of these Vortexx fluted nozzles in standard PDC and rock bits can lead to significant increases in ROP. They typically range from 20% to more than a 100% increase.

Acknowledgements

The authors from VGI wish to thank the following companies for obtaining and releasing the performance data presented here: Amoco, Arco, Chevron, Chesapeake Operating Co., Diamond Products International, Enron, Exxon, Pan Canadian Petroleum Ltd., Security DBS, Smith International Inc., Sonat, Trans Texas, Union Pacific Resources Corporation, and Walter Oil and Gas.

References