The paper presents new experimental results from laboratory studies on the use of gas-blocking foams for controlling gas influx into production wells. Foam performance is measured using the well-qualified gas-blockage test procedure at the conditions of four reservoirs where pilot testing of foams is currently planned or on-going. New foam formulations are described where polymer is incorporated into the foaming-agent formulation. For one reservoir, cross-linking chemicals have also been included, allowing comparison of conventional foams stabilised only by surfactant polymer-fortified foams, and gelled foams.
The results show that strongly improved gas-blockage performance may be obtained by addition of polymer to the foaming-agent solution. In the most favorable cases, foams of essentially no gas-blocking ability can be transformed to very efficient gas-blocking agents by adding polymer. However, in some (repeated) experiments, polymer addition did not result in a better gas-blocking foam, and concentration effects are observed so that adding too much polymer can give sharply reduced foam efficiency. Gel foams also are not necessarily superior to polymer foams as gas-blocking agents.
It is concluded that polymer-enhanced foams are of great potential value for gas influx control, but that more knowledge of surfactant/polymer interaction in the thin film network that constitutes the foam is required in order to predict the effects of adding polymer to gas-blocking foam formulations.
Introduction
Production at high gas/oil ratio (GOR) is a problem in many reservoirs that contain both oil and gas and occurs because of the higher mobility of gas in the formation. The negative consequences include reduced oil rates, environmental pollution resulting from excessive amounts of flared gas, loss of drive energy and recoverable oil, downstream processing problems and ultimately shut-down if production becomes uneconomic. At the near-well level, high GOR can be traced to gas coning, cusping or channelling as illustrated in Figure 1d-f.
In recent years, foam has been developed into an attractive GOR-control agent. Foam is composed of gas dispersed in a continuous liquid phase, with the gas occupying typically 50 to 99% of the total volume. A foam confined inside the pore network of a reservoir rock consists of thin liquid films that span the pores, thus making the gas phase discontinuous. This drastically reduces gas mobility without having a comparable influence on the liquid relative permeability curve. A snapshot of foam in typical sandstone pores is given by Figure 2. Such confined foam can be generated by displacement of a suitable foaming agent solution by gas at appropriate conditions. Foam whose purpose is to block gas influx to an oil-production well should ideally be created in situ wherever breakthrough of gas may occur and then remain stagnant, maintaining the strongest possible gas mobility reduction for extended periods of time.
The macroscopic flow characteristics of sands containing static foams were first described more than 30 years ago and later termed the gas-blocking state. Readily obtained by gas displacing a foamer solution from a porous medium of sufficient length at fixed differential pressure (P), this state is characterized by low production of the foamer-containing liquid (i.e. near-constant saturation of that liquid) and low gas flow rate through the network of liquid films. Effective foams can be maintained in the gas-blocking state for months at or below the pressure gradient used for the original displacement, and early field pilot-test results are encouraging.
In some field cases, foam GOR control treatment may not be sufficiently effective with a straightforward foamer formulation consisting only of a foaming agent at a suitable concentration.P. 71^
