No abstract

Oil production from a fractured reservoir, composed of a gas cap and an oil zone, is usually accomplished using surface or submersible pumps. The production method is called controlled gravity drainage (CGD). In the CGD mode of production, the pumps usually operate under a constant withdrawal rate until gas breakthrough, at which time the pumping rate would be influenced by the presence of gas at the production well. In this paper, we describe immiscible displacements in fractured porous media to have a better understanding of the process. Oil−gas CGD displacements were conducted using a laboratory flow apparatus in the fractured glass bead systems. A detailed dimensional analysis was conducted to scale up the experimental results based on the physics of the CGD process and experimental findings. Dimensional analysis of immiscible two-phase flow in porous media allows for quantification of the influences of petrophysical properties of the fractured media and physical properties of test fluids on some important aspects, such as critical pumping rate and recovery factor at gas breakthrough. In this work, an empirical model based on the dimensional groups of the system obtained from the Buckingham π theorem was employed to investigate the gravity drainage process in a fractured porous medium. A model was developed to predict the critical pumping rate, maximum withdrawal rate, distance between the gas−liquid (G−L) interface positions within the matrix and fractures, and recovery factor just before gas breakthrough for fractured porous media undergoing the CGD processes. The model was tested against experimental and field data of oil production under CGD. The results demonstrate that the model gives satisfactory prediction for the oil−gas drainage systems. The procedure outlined in this paper also has potential applications in modeling immiscible displacements.

Turbulent drag reduction behaviour of a mixed nonionic polymer/cationic surfactant system was studied in a pipeline flow loop to explore the synergistic effects of polymeric and surfactant drag reducing additives. The nonionic polymer used was polyethylene oxide (PEO) at three different concentrations (500, 1000, and 2000 ppm). The surfactant used was cationic octadecyltrimethylammonium chloride (OTAC) at concentration levels of 1000 and 2500 ppm. Sodium salicylate (NaSal) was used as a counter-ion for the surfactant at a molar ratio of 2 (MR = Salt/OTAC = 2). Relative viscosity and surface tension were measured for different combinations of PEO and OTAC. While the relative viscosities demonstrated a week interaction between the polymer and the surfactant, the surface tension measurements exhibited negligible interaction. The pipeline results show a considerable synergistic effect, that is, the mixed polymer-surfactant system gives a significantly higher drag reduction (lower friction factors) as compared with pure polymer or pure surfactant. The addition of surfactant to the polymer always enhances drag reduction. However, the synergistic effect in mixed system is stronger at low polymer concentrations and high surfactant concentrations.

Although extensive research work has been carried out on the drag reduction (DR) behaviour of polymers and surfactants alone, little progress has been made on the synergistic effects of combined polymers and surfactants. In this work, the interactions between drag-reducing anionic polymer (copolymer of acrylamide and sodium acrylate, referred to as PAM) and drag-reducing cationic surfactant (octadecyltrimethylammonium chloride, OTAC) are studied. Solutions are prepared using both deionised (DI) water and tap water. The measurement of the physical properties such as electrical conductivity and viscosity are used to determine the surfactant-polymer interactions. The addition of surfactant to the polymer solution has a significant effect on the properties of the system. The critical micelle concentration (CMC) of the mixed surfactant-polymer system is found to be different from that of the surfactant alone. With the addition of surfactant to a polymer solution, a substantial decrease in the viscosity occurs. The observed changes in the viscosity of mixed polymer-surfactant system are explained in terms of the changes in the extension of polymeric chains, resulting from polymer-surfactant interactions. The anionic PAM chains tend to collapse upon the addition of cationic OTAC. The pipeline flow behaviour of PAM/OTAC mixtures is found to be consistent with the bench scale results. The DR ability of PAM is reduced upon the addition of OTAC. At low concentrations of PAM, the effect of OTAC on the DR behaviour is more pronounced. The DR behaviour of polymer solutions is strongly influenced by the nature of water (DI or tap).

Turbulent drag reduction behavior of mixed polymer–surfactant systems (anionic polymer/cationic surfactant; nonionic polymer/cationic surfactant; nonionic polymer/anionic surfactant) was studied in a pipeline flow loop to explore the role of surfactants in mechanical degradation of polymers. The polymers investigated were nonionic polyethylene oxide (PEO) and anionic polyacrylamide (PAM). The surfactants studied were cationic octadecyltrimethylammonium chloride (OTAC) and anionic sodium dodecyl sulfate (SDS). The pipeline flow results obtained for PAM/OTAC mixtures support the idea that the coiling of polymer molecules does not protect the polymer molecules against shear degradation. The addition of oppositely charged cationic surfactant (OTAC) to anionic polymer (PAM) results in coiling of polymer molecules. The coiled polymer molecules undergo faster mechanical degradation than stretched polymer molecules. The addition of surfactant (cationic OTAC or anionic SDS) to nonionic polymer PEO increases the resistance of polymer molecules against shear degradation. This is reflected in the pipeline flow results. The effect is more significant for anionic surfactant (SDS) than for cationic surfactant (OTAC) especially at high concentrations of surfactant; the smaller size of the headgroup of anionic surfactant monomers allows them to have a greater influence on the polymer molecules. These results support the idea that extended polymer chains are more resistant to mechanical degradation as compared with coiled polymer molecules. The polymer chains undergo extension due to repulsion between the neighboring surfactant micelles attached to the backbone of the polymer chains.