Analysis of Drilling Strengths in Evaporite Rocks

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the Deep Drilling and Production Symposium of the Society of Petroleum Engineers of AIME, to be held in Amarillo, Texas, Sept. 8–10, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract The drilling strength of rock is a finite measurement of crushing resistence similar to compressive strength measured on laboratory specimens. This rock property dictates most of the drillers procedural decisions on a rig. Drilling strength determines which bit is used, the weight and rotary speed applied and mud properties. A system has been developed that computes drilling strength automatically within seconds after each foot is drilled. A small computer is employed in the system with electronic sensors that determine depth, weight and revolutions on bit and time of drilling each foot. The computer calculates drilling strength then interprets it to provide a porosity and pore pressure provide a porosity and pore pressure log at bit depth. Applications of the system, worldwide, have demonstrated a remarkable definition of porosity, permeability and pore pressure in permeability and pore pressure in evaporite rocks. Porosity is generally no more than two porosity percent varience with other porosity tools and pore pressure has an accuracy of + .2 ppg. Experience has shown pore pressure transition over several pressure transition over several hundred feet in massive carborates and significant pressure increase across a hard cap only ten feet thick. This has shown that evaporite rocks have pore pressure distribution detectable pore pressure distribution detectable in drilling response. Introduction The fear of a blowout and disaster has plagued the oil industry since Captain Lucas had the first one in 1901. Up to about 1965 a driller had the choice of drilling painfully slow with mud heavy enough to control any pressure or running light mud and gambling that abnormal pressure would not be encountered.