Design Considerations for an Inhibitive, Stable Water-Based Mud System

Abstract

Summary. This paper discusses the technique of using high-molecular-weight organic Polymers to stabilize troublesome shales through the adsorption of a protective polymer layer. It specifically covers the performance of a partially hydrolyzed polyacrylamide (PHPA) in respect to shale preservation and in the retardation of cuttings dispersion. Other design factors-e.g., pH control, rheological stability, and lubricity-are also covered. Introduction Researchers and technologists have been working for more than 30 years to develop inhibitive drilling fluids. Inhibition in this sense meant a fluid that would prevent or retard the swelling of expandable clays; therefore, many early researchers equated inhibition with prevention of the hydration and swelling of Wyoming bentonite. Because divalent cations are effective in controlling the swelling of bentonitic clays, it is not surprising that the lime and gypsum treated muds were developments of this earlier period. Unfortunately, there still appears to be a preoccupation with the idea of preventing bentonitic swelling through cation exchange. Though the cation-exchange approach is not without merit, it may be hindering the development of other techniques that could be considerably more effective in stabilizing shale bodies. Today's definition of an inhibitive mud is much broader than simply the inhibition of the swelling of such highly expanding clays as smectite. It means a fluid that exhibits minimal reactivity with the borehole and more specifically with the broad range of shales, claystones, mudstones, and other argillaceous formations. It also means a fluid that will have a minimum dispersive effect on the drill cuttings, thus providing more effective solids removal at the surface. Calcium-treated muds and other fluids that depend on the inhibiting ability of the cation alone do not always meet these requirements for a wide range of shales because the highly expanding clays may constitute only a small part of their composition and thus conditions other than the abnormal swelling of smectite minerals are at work to cause the balling, dispersion, and sloughing characteristics of unstable shales. This paper describes several key factors to consider in the design, preparation. and maintenance of a mud system to meet the expanded requirements of an inhibitive drilling fluid. Much of what is to be presented will be readily apparent to those skilled in the art of drilling-fluids design and control. There appears to be a reluctance or tardiness, however, in practicing many of these known techniques. It is hoped that this paper will instill interest in the further utilization of all drilling-fluid inhibition techniques known to the industry. Inhibition Through Adsorption of High-Molecular-Weight Organic Polymer When a shale is penetrated by the bit, the horizontal earth stresses on the walls of the hole are relieved and the shale adsorbs water from the drilling fluid. This adsorption of water may result in the formation of a sticky plastic mass, while in other cases it may produce only splintering and spalling. Mineral compositions may not be drastically different in these two shales, but moisture content, overall clay content, degree of compaction, and morphology would likely differ considerably. The solution to controlling both shales, however, may lie in the coverage of the exposed surface with a protective film. This protection can come from the adsorption of relatively large organic polymers to form what could be called an imbibition barrier. It appears to be more of a blocking or clogging effect, however, slowing base exchange and hydration. Browning and Perricone used this technique in their investigations and concluded that the hydration and swelling of formation clays could be controlled by the use of chrome-treated lignosulfonates without the concomitant addition of calcium salts. They postulated that multilayer adsorption of the lignosulfonate occurred on the drill chip and borehole at concentrations higher than those normally required for deflocculation. A relatively high concentration of lignosulfonate in solution was used to force increased adsorption of the lignin polymer onto the solid surfaces, thereby creating a viscous layer or film that impeded the diffusion of water into the clay packet. Proof of this barrier's existence can be demonstrated by showing that a chrome-treated sodium lignosulfonatsolution will significantly inhibit the swelling of Wyoming bentonite, as well as retard base exchange. The latter property has been used to prolong the viscosity of prehydrated bentonite on addition to a seawater mud system. Unfortunately, the viscous lignosulfonate layer did not prove adequate to stabilize troublesome shales for a sufficient period of time. This may have been partly because of the large NaOH treatments required to neutralize the lignosulfonate. The free hydroxyls could have led to increased dispersion of the shale and thereby created an unstable borehole despite adsorbed lignin polymer. Clark et al. greatly advanced the adsorbed polymer technique by showing that the use of a PHPA in conjunction with KCl was highly effective in stabilizing shales. KCl was found to be an integral part of the polymer system and was used in concentrations up to 50 lbm/bbl [143 kg/m3]. The use of the potassium ion has proved to be highly effective in stabilizing shales, and its merit for this purpose is not questioned here. As is well known, however, the potassium ion is highly flocculating and its presence in the mud can adversely affect rheological and filtration control, thereby significantly influencing mud cost. Thus KCl concentration becomes a very important aspect of the mud design and should be carefully determined for each troublesome shale. Many of the endurated, brittle shales appear to respond well to the potassium ion, but the softer, highly dispersive shales can be controlled effectively with modifications of the PHPA polymer in the absence of KCl. Nearly 200 wells have been drilled along the Louisiana/Texas gulf coast with a high-molecular-weight (greater than 10,000) anionic PHPA polymer in both fresh-water and seawater systems with excellent success. Many of these wells have been directional with deviations up to 70 degrees [1.22 rad] and located in the most severe gumbo areas along the coast. Thus, more dependence can be placed on the PHPA polymer with less need for KCl than originally believed. It should be pointed out that the intent here is to cover the general performance characteristics of a particular PHPA polymer, not to address specific property requirements for optimum performance. SPEDE P. 331^