Adsorption of polymers used in drilling fluids on the inner surfaces of carbon steel pipes

Polymer Engineering and Science, Oct, 2008 by Luciana S. Spinelli, Aline S. Aquino, Elizabete Lucas, Arnaldo R. d'Almeida, Rafael Leal, Andre L. Martins

INTRODUCTION

Drilling wells to find economically recoverable hydrocarbon reserves starts with the opening of vertical holes in exploratory, or pioneer, fields. When a deposit of oil and/or gas is discovered, it is then delimited by drilling other holes, starting the development phase of the field. In this phase, the operations generally involve drilling inclined holes, often with long horizontal stretches within the reservoir. During all these drilling operations, a fluid must circulate within the drill hole. This fluid returns to the surface passing through the annular space (column-well walls). The fluid can be of various compositions, but is basically constituted of a continuous phase (water), clay and chemical and polymer additives [1-3]. Aqueous biopolymer solutions, such as xanthan gum, have rheological properties suitable to the needs of the drilling process, besides being benign to the environment. The polymers most frequently used in drilling fluids are xanthan gum, starch, and partially hydrolyzed polyacrylamide (PHPA). PHPA, besides providing good rheological properties to the fluid, also is less expensive than xanthan gum.

The drilling fluid performs several functions, among them carrying the rock fragments to the surface, sustaining these fragments at moments when the fluid circulation is stopped, cooling the drill bit, and especially lubricating the drilling column in the segments of contact with the cased hole (metal X metal) and open hole (metal X rock) [4, 5]. Despite the practical importance of the adsorption process of these polymers on the surfaces of carbon steel, used in making casing for oil wells, no information on this was found in the literature.

The adsorption of polymers and the interfacial load density of adsorbed polymer layers on solid-liquid interfaces play a significant role in controlling adhesion, wettability, and colloidal stability or biocompatibility [6, 7]. Various aspects of the adsorption process remain elusive and are the subject of continued attention. Among these aspects are the effects of the heterogeneity of the surface and the role of the polymer chain's dynamic under nonequilibrium conditions (8).

The adsorption of polymers on metal surfaces can be calculated from the measure of the contact angle between a liquid drop of the polymer solution and the metallic solid, known as the interfacial tension between these phases (8).

Schultz et al. (9) developed a method based on contact angle measurements of a drop of liquid (L) on a solid (S) in a nonaqueous phase liquid. In this configuration it is possible to obtain the adsorption of a substance dissolved in the drop. This adsorption is selective, because only some molecules present in the solution adhere to the solid. Figure 1 shows the scheme of this configuration (8).

[FIGURE 1 OMITTED]

The aim of this work is evaluate the adsorption of polymers from their aqueous solutions on surfaces of carbon steel pipes used in making casing for oil wells by means of visual observation and contact angle measurements to discover the polymer that best adsorbs to the metal pipe.

MATERIALS AND METHODS

Materials

Xanthan gum, obtained from Kelco, neutral hydroxypropylated starch, from HP Amido-MI Drilling Fluids, and PHPA, donated by CENPES/Petrobras, were solubilized in distilled and deionized water. Carbon steel pipes, used to case oil wells during the drilling step, were also provided by CENPES/Petrobras. The models of chemical structures of the polymers used are shown in Fig. 2.

[FIGURE 2 OMITTED]

Experimental Part

Two methods were used to evaluate the adsorption of polymers on metal surfaces. One of them was visual observation of the fluid-metal contact, captured by photographs, using a Sony DSC-P52 Cyber Shot 3.2 digital camera and the other was based on obtaining the adhesion and adsorption energy values from measuring the contact angles between the metal surface and drops of polymer solution in a hydrocarbon solvent medium or not.

Preparation of the Polymer Solutions and Test Bodies. A mother solution of 10 g/L was prepared by dissolving the polymer, at 50[degrees]C, in distilled and deionized water, under vigorous magnetic or mechanical stirring, at 330 rpm, for 24 h. Then successive dilutions were performed to obtain the following concentrations: 1, 3, 4, 5, 7, and 8 g/L.

To prepare the test samples, a pipe of 3 inches diameter used in completion operations of oil wells was obtained, which had a certain curvature. Therefore, pieces in 5 X 2.3 X 0.5 [cm.sup.3] dimensions were cut and placed them under a high-pressure press to eliminate their curvature as much as possible. Then these were sanded with emery paper and washed them in water and acetone.

Visual Observation Test of the Fluid-Metal Contact.

The metal plates were bathed, in a vertical position, in a recently prepared polymer solution. The system was maintained at room temperature and under magnetic stirring to reproduce to a certain extent the flow of the drilling fluid when in contact with the pipe. A pair of plates was removed at pre-established times (10, 20, 30, 40, 50, and 60 min) to observe the influence of contact time on the polymer-metal adsorption. Then the surfaces were dried in an air-circulation oven from Nova Etica at 50[degrees]C for 3 h, again in the vertical position, after which digital photographs were taken. Images were obtained of the two faces of each metal plate, which correspond to the internal and external faces of the pipe. In this study, the result of interest involves the internal face, which is in contact with the liquid during its circulation in drilling operations..