Simulated Downhole Conditions Research Articles

Effects of Bit Hydraulics on Full-Scale Laboratory Drilled Shale

Summary The effects of bit hydraulics while drilling shale with a standard three-cone bit are examined in this paper. Tests were conducted by drilling into large-diameter, intact shale samples at simulated downhole conditions in Drilling Research Laboratory's wellbore simulator. The shale samples were recovered from massive surface outcroppings and preserved for laboratory use. The effects of hydraulic horsepower from 20 to 400 hhp and bit weights from 20,000 to 50,000 lbm on rate of penetration are presented. Introduction Most deep shales and some intermediate shales found in the U.S. are typically "slow" drilling formations. Efforts to improve rate of penetration in shale have been performed in both field tests and small-scale laboratory investigations. Until recently, laboratory drilling studies on shale have been confined to microbit1 or single-cutter2 studies because the ability to obtain and preserve large, intact shale samples had not been developed. In addition, laboratory facilities where full-scale drilling tests could be conducted at simulated deep-well conditions did not exist. Massive surface shale formations have been located and techniques have been developed to extract and preserve large-diameter, intact samples. With these samples and the ability to simulate full-scale drilling conditions, a systematic, technical approach was taken where the effects of drill bit hydraulics were examined while drilling shale at simulated downhole conditions. Background and Definitions The in-situ conditions of a typical deep wellbore and surrounding rock formation are illustrated in Fig. 1. The formation is subjected to overburden stress, confining stress, wellbore pressure, and formation pressure. As previously demonstrated,3–5 rate of penetration is influenced strongly by the bottomhole conditions and appears to be most sensitive to the differential pressure between the wellbore and formation. These effects, however, can vary widely depending on rock properties such as rock type, strength, density, permeability, and mud properties such as composition, filtration rate, viscosity, solids content, and particle size.6 Bit hydraulics - i.e., the means of removing cuttings from the hole bottom and cleaning the bit with the drilling fluid - are a key factor in improved bit performance. Bottomhole cleaning theories,7 microbit studies,8 and full-scale laboratory drilling experiments in hard, impermeable rock9 have shown the need for adequate bit hydraulics to maximize rate of penetration and avoid bit balling. Field tests with extended nozzles10 also have demonstrated great potential for increasing rate of penetration in certain formations with improved bit hydraulics. Some nondrilling laboratory studies have shown the effects of nozzle size on pressure distribution at the hole bottom.11

Fluoroelastomer Polymers—Compounding and Testing for Downhole Environments

Abstract A laboratory screening test simulating sour gas downhole conditions has been developed that provides directional information on the factors that control elastomer seal performance in this harsh environment. The results of this test have been correlated to environmental testing of O-rings in simulated downhole conditions using sour crude oil at 203°C and evaluating the pressure failure over the range of 69 MPa to 138 MPa. The results of this testing program have demonstrated the factors that affect overall seal performance for commercially available fluoroelastomer raw gums in downhole environments in the following order of importance: 1. a vulcanized compound modulus at 50% elongation of greater than 6.9 MPa, 2. cure type order of preference of peroxide > bisphenol > diamine, 3. increases in fluorine level from 65 to 69 weight percent have little effect on seal performance in conditions evaluated, and 4. the compound hardness is not as important a factor in predicting compound failure or success as is molecular weight selected that results in elongation of 70–100% at high filler loadings to achieve high modulus. The laboratory screening/environmental testing sequence reported here has proven successful in predicting fluoroelastomer properties necessary to manufacture seals for actual downhole service.

Field Verification of the Effect of Differential Pressure on Drilling Rate

Abstract A field study was conducted on eight South Louisiana wells to determine the effect of differential pressure on the instantaneous rate of penetration in shale. Drilling rate is affected significantly by changes in differential pressure and may be reduced as much as 70 percent as the differential fluid pressure is increased from 0 to 1,000 psi. When the formation pressure becomes greater than the mud-column pressure, drilling rates continue to increase, sometimes at an increasing rate. The sensitivity of drilling rate to differential pressure depends upon the magnitude of the bit load. Overburden pressure and the hydrostatic head of the mud column over the intervals studied had no detectable erect on penetration rate. Increases in formation pressure can be detected by an increase in penetration rate. The careful observation of drilling rate can help avoid kicks and allow the well to be maintained at minimum overbalance hydrostatic conditions. Decreased well-drilling costs and an increased drilling rate will result. Introduction As well depth increases, drilling rates decrease. The number of deep wells being drilled annually is increasing. Associated with these wells are low penetration rates and high costs. The reduction in drilling rate with depth is primarily a function of increased differential pressure. The paper presents a field study of the effect of pressure on the instantaneous rate of penetration in Gulf Coast Louisiana shales. On the basis of laboratory investigations, several authors agree that drilling rate is decreased as the differential between the formation pore pressure and the hydrostatic pressure of the mud column is increased. Maurer conducted crater-volume tests while varying overburden, hydrostatic and formation-pore pressure. He concluded that drilling rate is influenced by a combination of both hydrostatic and differential pressure, but is independent of overburden pressure. Cunningham and Eenink conducted microbit drilling tests at varying hydrostatic, formation, and overburden pressures and found that only differential pressure affects drilling rate. Eckel noted that a pressure differential between hydrostatic and formation pressure is the only pressure that affects drilling rate. Garner et al. conducted crater-volume tests on dry limestone and implied that drilling rate is influenced by both overburden and differential pressure. Robinson found that the yield strength of jacketed rock samples increased as the differential between confining pressure and pore pressure increased. He indicated that this increase in strength may be an important factor in reducing drilling rate. Garnier and van Lingen determined that differential pressure affects both rock strength and chip hold-down, that chip hold-down forces are of a static and dynamic nature, and that these forces are a complex function of almost all known drilling parameters. These studies have provided useful data on crater formation and microbit drilling rates under simulated downhole conditions; however, there has been very little work reported on relatively impermeable rocks such as shale where the actual internal pore pressure is known. In addition, there have been very few field studies reported on the effect of pressure on drilling rate. This study was conducted on eight South Louisiana wells. The depth of the intervals studied ranged from 8,200 to 14,800 ft; hydrostatic pressures varied from 5,000 to 13,900 psi and the formation pressure ranged from 3,900 to 13,200 psi. Differential pressure between formation and mud-column pressures ranged from a positive differential of 1,500 psi (pm greater than pf) to a negative differential of 930 psi (pm less than pf). The average test interval for the eight wells studied was 1,400 ft. A unique feature of this study is the pressure control afforded by the use of shale resistivity methods to predict formation pore pressure. The development and increased accuracy of formation pressure calculations from log-derived shale properties provide a method and a degree of control never before available in field studies of this nature. These methods aid in the field evaluation of the effect of many drilling parameters on drilling rate. Collection and Analysis of Data Detailed drilling data on eight South Louisiana wells were collected by engineers on location. JPT P. 676ˆ

Cutting Action of a Single Diamond Under Simulated Borehole Conditions

Abstract Laboratory experiments were conducted to investigate the cutting of a single diamond on limestone and shale under simulated down-hole conditions. A high-pressure chamber was modified by adding a rock-rotating device so that planing tests could be run. Weight on the diamond, diamond geometry and differential pressure across the face of the rock were varied. It was found thathigh differential pressure reduced the volume of rock cut by the diamond at a given bit weight by strengthening the rock and changing the mode of failure;a finely powdered rock left in the bottom of the grooves reduced the volume cut by as much as 50 percent; andorientation of a single diamond about the axis of a drill point had considerable influence on diamond cutting efficiency. Introduction Diamond bits consist of many diamonds held in a matrix provided with water courses for fluid passage to clean, cool and lubricate the diamonds as they drill formation. Each diamond contributes its share to the overall effectiveness of a bit, but a lack of understanding of the performance of a single diamond has hampered efforts to engineer improvements in diamond bits. The need for those improvements prompted this study. This paper describes the findings of a laboratory investigation of the cutting action of a single diamond where the differential pressure between the wellbore and formation, diamond geometry and vertical force on the diamond were independently varied. The tests were conducted in a pressure chamber that provided relative horizontal motion (planing) between a rock and diamond under simulated down-hole pressure conditions. EQUIPMENT HIGH-PRESSURE CHAMBER WITH ROTATING DEVICE The high-pressure chamber used in previous rock mechanics studies' was modified for this study by addition of a rock-rotating device. The modified chamber (Fig. 1) differs from the old chamber in that the rock can be rotated to provide relative horizontal movement between the rock and the penetrator which may be a small element of a diamond or a drag bit. In this study the penetrator was a single diamond. The rock sample holder inside the chamber is mounted on a thrust bearing and is connected to the external drive mechanism by a stem that passes through two seal elements. The seal elements are needed to allow formation pressure to enter the bottom of the rock sample through a small hole in the stem. The Kapseal seal elements (Teflon boots backed by O-rings) provide low friction (hand free) even at differentials of 5,000 psi. The drive mechanism is a variable-speed transmission with a speed range from 0 to 30 rpm and an output torque of 2,800 in.-lb. A right-angle-drive gear box couples the drive to the stem. The new design has two independent pressure systems (formation and borehole) to simulate down-hole rock stress environment. Formation pressure enters from the bottom of the rock through a 6-in. diameter area, and borehole pressure acts over the remaining surface. A 1/4-in. layer of zero fluid loss oil-base mud poured over the top of the rock and an O-ring seal on bottom allow a differential between the borehole and formation pressures. Bayol 50 was used as the hydraulic fluid in the chamber. The differentials reported here were obtained by elevating the borehole pressure while keeping the formation pressure at zero. JPT P. 937ˆ

A New Approach Toward Elimination of Slug in Shaped Charge Perforating

Abstract Since the advent of the perforation flow laboratory, increased attention has been focused on new designs of shaped charges as applied to oilwell perforating. Data derived from the flow laboratory, in which charges are subjected to simulated down-hole tests, indicate that serious perforation plugging often exists due to the entrance of slugs into the perforated hole. The slug or "carrot" is an intrinsic part of the jet process and its adverse effect is inherently characteristic of many commercial shaped charges which otherwise exhibit excellent performance properties. Numerous approaches have been tried throughout the industry in an effort to prevent this flow-impeding slug from entering the perforated hole. Yet, publicly conducted tests indicate that none of these devices are entirely successful when applied under the clearance and pressure conditions for which the charges are designed. This paper deals with a new, direct approach toward the elimination of the slug or carrot in shaped charge perforating. Herein is described a newly developed bimetallic liner for shaped charges, which when fired under pressure leaves no slug in the perforated hole and yet maintains the high performance of the conventional-type charge. Introduction During the past few years the testing of shaped charge tools, as applied to oilwell perforating, has evolved from simple surface shooting to tests conducted in a flow laboratory in which a study of the charge could be made under simulated down-hole conditions. In the flow laboratory, pressures and temperatures were elevated prior to firing the charge into a target of medium-hard sandstone material. An elaborate system was devised for firing the charge under various fluids and differential pressures. Provisions were made for actually "flowing" the perforated hole under controlled conditions. It was at this point that the term slug or carrot really emerged into oil-well perforating vernacular. While it had been noted that a slug of charge liner material often entered the perforation, these plugged test targets had drawn little attention. Now in the flow laboratory, it was found that this obstructing slug reduced the resultant flow through the perforation to as little as one-half that normally expected! It was then clear that the slug must be either eliminated from the shaped charge entirely or reduced to such proportions that it would be ineffective in plugging the perforated hole.