Summary During laboratory drilling tests in a permeable sandstone, the effects of pore pressure and mud filtration on penetration rates were measured. Four water-based muds were penetration rates were measured. Four water-based muds were used to drill four saturated sandstone samples. The drilling tests were conducted at constant borehole pressure while different backpressures were maintained on the filtrate flowing from the bottom of the sandstone samples. Bit weight was varied also. Filtration rates were measured while circulating mud during drilling and with the bit off bottom. Penetration rates were found to be related qualitatively to the difference between the filtration rates measured while drilling and circulating. There was no observed correlation between standard API filtration measurements and penetration rate. Introduction A better understanding of the effects of pore pressure and mud filtration on penetration rates and how these effects relate to standard API filtration properties was sought during full-scale laboratory drilling tests in a permeable sandstone with four different laboratory-prepared muds. Measurements of penetration rate and filtration rate were made as mud type, bit weight, and backpressure on the filtrate produced from the sandstone samples were varied. Other parameters-such as rock type, bit type, rotary speed, flow rate, borehole pressure, confining pressure, overburden stress, mud temperature, and penetration interval were held constant. The filtrate volume vs. time or filtration rate measured while drilling is herein designated "drilling" filtration rate. "Circulating" filtration rate refers to filtration rate measured with the bit off bottom and with mud circulating in the borehole. From the measured filtration rates, pressure drops through the rock samples were calculated pressure drops through the rock samples were calculated using Darcy's law. After the pressure drops were subtracted from the measured total drop across the rock (borehole pressure minus backpressure), the pressure drops across the mud filter cake at the bit/rock interface were determined. The test results are presented in terms of these pressure drops. pressure drops. Changes in penetration rate with bit weight and pressure drop across the filter cake were examined. The relationship between penetration rate and filtration rate was determined. An attempt to correlate the drilling test results with standard API filtration properties was also made. Drill Bit, Rocks, and Muds A 77/8-in. [20.07cm] diameter Smith F-3 bit (IADC 5–3-6) with three 10/32-in. [0.84-cm] diameter nozzles was used for the drilling tests. The bit had a pressure drop across the nozzles of approximately 900 psi [6205.3 kPa] at 240 gal/min [15.14 dm3/s] for the 9.5-lbm/gal [1138.35-kg/m3] mud used during the tests. Four Berea sandstone samples (15.5-in. [39.37-cm] diameter by 36-in. [91.44-cm]) length were drilled. Before drilling, the Berea sandstone samples were evacuated for a minimum of 24 hours, saturated with tap water, weighed, and stored under water. From the saturated weight, bulk volume, porosity, and grain density, water saturation was then calculated. The samples were saturated between 95 and 100%. After the samples were jacketed between steel end caps, the top end cap was filled with water to maintain the saturation. The Berea sandstone samples had a grain density of 2.65 gm/cm3 [2650.0 kg/m3], a porosity of 20%, an unconfined strength of 9,000 psi [62 052.8 kPa], and a permeability of 0.2 darcies. The four water-based drilling muds tested included a low-solids, nondispersed mud (LSND); a low-solids, nondispersed mud with sodium polyacrylate filtration control additive (LSND-SPA); a dispersed (DISP) mud; and an oil emulsion (OIL EMUL) mud. Approximately 120 bbl [19.08 m3] of the LSND mud was prepared first. The LSND mud had a relatively high API filtration rate of 0.92 cu in./30 min [15 cm3/1800 s]. Various materials were added to portions of the LSND mud to obtain the LSND-SPA, DISP, and OIL EMUL muds, which had relatively low API filtration rates of 0.43 cu in./30 min [7 cm3/1800 s]. Composition of the muds is given in Table 1. The methods used to reduce the API filtration rate of the LSND mud reflect typical choices that are available when filtration reduction is necessary during drilling. Ferguson and Klotz performed filtration experiments while drilling with various mud types; Krueger later studied the effects of common filtration control additives on filtration while circulating. Many of these additives require significant amounts of shearing and circulating time to reach full effectiveness. In view of this, the performance of these additives in laboratory-prepared muds performance of these additives in laboratory-prepared muds may differ somewhat from the performance that would be observed in field-conditioned muds. Mud properties were measured before and after each drilling test (Table 2). JPT P. 1671

The effects of size on the performance of three-cone bits were measured during laboratory drilling tests in shale at simulated downhole conditions. Four Reed HP-SM three-cone bits with diameters of 6, 7 7/8, 9, and 11 in. [165, 200, 241, and 279 mm] were used to drill Mancos shale with water-based mud. The tests were conducted at constant borehole pressure, two conditions of hydraulic horsepower per square inch of bit area, three conditions of rotary speed, and four conditions of weight-on-bit (WOB) per inch of bit diameter. The resulting rates of penetration (ROP's) and torques were measured. penetration (ROP's) and torques were measured. Statistical techniques were used to analyze the data. Introduction Drill bit manufacturers generally recommend WOB operating ranges for their bits in terms of pounds-force per inch of bit diameter. The practice of normalizing the per inch of bit diameter. The practice of normalizing the effect of bit size by expressing it in these terms has been widely used and often accepted as a "rule of thumb" in the drilling industry. Many have suspected that this rule of thumb may be an oversimplification because bit design tends to vary widely with size even in the same models, and hydraulic cleaning of the bit and bottom of the hole becomes much more difficult as size increases. A better understanding of the effects of size on bit performance and the validity of the WOB per inch of bit performance and the validity of the WOB per inch of bit diameter rule of thumb was sought by performing drilling tests with various-size bits under controlled laboratory conditions. Drilling tests were performed with four Reed HP-SM three-cone bits with diameters of 6, 7 7/8, 9, and 11 in. [165, 200, 241, and 279 mm]. As many variables as possible were held constant during the drilling tests, including rock type; confining pressure and overburden stress on the rock; mud type, properties, and temperature; and borehole pressure. Nozzle sizes and flow rates were selected so that each bit was tested at approximately the same conditions of hydraulic horsepower per square inch of bit area. Three rotary-speed conditions per square inch of bit area. Three rotary-speed conditions and four WOB per inch of diameter conditions were run. The resulting ROP's and torques were measured at each condition. A detailed statistical analysis was performed on me dam to determine the relationship between the independent variables of bit size, WOB, rotary speed, and hydraulic horsepower per square inch of bit area and the dependent variables of ROP, torque, and mechanical horsepower expended at the bit. Drill Bit, Rock, Mud, and Nozzle Selection Four new Reed HP-SM bits with diameters of 6, 7 7/8, 9, and 11 in. [165, 200, 241, and 279 mm] were provided by Reed Rock Bit Co. for the tests. The provided by Reed Rock Bit Co. for the tests. The HP-SM bit is specified in the IADC code under four classifications (537, 547, 617, and 627). The manufacturer recommends the HP-SM bit for both soft formations containing streaks of harder materials and medium-strength formations. The HP-SM bits have conical inserts except for the chisel-shaped inserts on the gauge row. The manufacturer's recommended operating ranges for WOB per inch of bit diameter and rotary speed are 3,000 to 6,000 lbf [525 to 1051 N/mm] per inch of bit diameter and 45 to 140 rev/min, respectively. Fig. 1 is a photograph of the HP-SM bits. The number of inserts, average insert diameter, and average insert length were measured and are listed in Table 1. The rock formation samples drilled were Mancos shale. Mancos shale is a Cretaceous, gray to black, shale/siltstone formation containing 10% clay composed of illite and chlorite. Samples 15 in. [394 mm] in diameter by 36 in. [914 mm] long were used for the 6–, 7 7/8–, and 9-in.- [165–, 200–, and 241-mm]-diameter bit tests. A sample 17 in. [445 mm] in diameter by 36 in. [914 mm] long was used for the 11-in.–[279-mm]-diameter bit test. All samples were originally cored from a massive surface outcropping located in central Utah and preserved for the drilling tests. Mancos shale has an unconfined compressive strength of 9,000 psi [62 053 kPa] and a permeability less than 1 d. Detailed rock properties permeability less than 1 d. Detailed rock properties and a comparison of laboratory and field shale drilling have been given previously. A "standard" water-based mud with properties listed in Table 2 was selected for the tests. To compare the performance of different-size bits, it was felt that hydraulic horsepower per square inch of bit area (HSI) should be held constant during the tests. It was also felt that similar pressure drops across the bit should be run if possible. Calculations were made to determine the nozzle diameters that would create approximately the same pressure drop at constant HSI conditions. SPEJ p. 473

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