The Development of an Inhibitive Cationic Drilling Fluid for Slim-Hole Coring Applications

Abstract

ABSTRACT This paper describes a new inhibitive cationic drilling fluid developed for slim-hole coring applications. High rotational speed slim-hole coring requires a drilling fluid that will maintain a gauge hole to preserve the bearing-like tolerances between the drill string and wellbore. A low-solids fluid is also required to prevent drill solids from centrifuging inside the drillstring. This paper describes both the laboratory development and field testing of the drilling fluid. Conventional water-based drilling fluids rely on dispersed clay for viscosity and fluid loss control. Negatively charged (anionic) materials are often added to the fluid to inhibit reactive shale formations. In this study, additives with a positive charge (cationic) were investigated as shale stabilizing agents. The study found that quaternary ammonium salts (cationic surfactants) gave high recoveries in the shale rolling test. The ability of the surfactants to stabilize shale was found to vary with chemical structure. Despite their ability to stabilize shale, the surfactants were unsuitable for use in drilling fluid formulations because of their tendency to foam. Low-molecular weight synthetic cationic polymers that contain structures similar to the surfactants gave shale recoveries as great as the surfactants but without the foaming problems. The combination of potassium chloride with cationic polymer resulted in shale recoveries greater than with either component alone. Drilling fluids were formulated based on a combination of a cationic polymer and KCl. Initial work indicated that many commercially available drilling fluid additives were incompatible with the cationic polymer. Additives were found that were compatible and provided satisfactory drilling fluid properties. Field testing of a cationic/brine drilling fluid suggested that the fluid was highly inhibitive when compared to close offset wells drilled with conventional water-based drilling fluids. The fluid was compatible with the slim-hole high rotational speed coring system. INTRODUCTION Slim-hole drilling and coring is gaining popularity as an exploration tool because of its potential to provide formation information reduce costs and minimize environmental impact. Amoco has developed a slim-hole coring system that uses technology from the mining industry to provide continuous core of sedimentary lithologies. The adaptation of the mining technology for use in the oilfield has required changes and modifications involving rig design, bit design and well control.1,2 A well is designated a slim-hole well if 90% or more of its depth is drilled with bits less than 7 inches in diameter. The Amoco slim-hole coring system features flush joint drillpipe that is slightly smaller in diameter than the drill bit. The small diameter difference between drillpipe and hole size results in an annulus that is much smaller than is found in conventional drilling. The dimensions of a drillstring used by Amoco is compared with a conventional drillstring in Table 1. Another characteristic of the Amoco coring system is the high rotary speeds used to obtain acceptable penetration rates. These high rotary rates (500-1000 rpm) can cause the drilled solids (or barite) to centrifuge on the inside of the drillpipe. Since the core is retrieved through drillpipe using a wireline, a buildup of solids on the inside of the drillpipe can interfere with core retrieval.