Effects of Pore Pressure and Mud Filtration on Drilling Rates in a Permeable Sandstone
Authors: Black A.D. , Dearing H.L. , DiBona B.G. 
Published: December 2007

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