Determination of the "Gas-Free" Liquid Level And the Annular Gas Flow Rate For a Pumping Well

Abstract

Abstract This paper describes two methods of determining the "gas-free" fluid level and the annular gas flow rate in a pumping well without significantly disturbing the balance of tire system. The methods consist of direct or indirect measurements of the annular flow rate and observations of the pressure buildup when the annulus is closed in at the surface. These data allow the calculation of the volume of gas in the annulus and the normal annulus gas venting rate. The "gas-free" fluid level is then determined directly from the volume of the annular gas. This fluid level, in turn, allows one to derive tire bottom-hole pressure using a pressure gradient that is consistent with the temperature and pressure of the oil column. Introduction Knowledge of the dynamic and static bottom-hole pressures in a pumping well enables the operator to determine the optimal surface and lifting facilities for present and future operations. The bottom-hole pressure under flowing conditions is usually obtained by first determining the fluid level with an acoustic device. The height of the liquid column is calculated and an assumed pressure gradient for the fluid is then applied to the liquid column to arrive at the dynamic bottom-hole pressure. Whenever there is doubt about the fluid level or the pressure gradient, the operator often closes the annulus in order to lower the level of the liquid to the pump seating nipple. The pressure can then be determined accurately, because a known gas pressure gradient exists from the surface to the pump seating nipple. However, the pressure data so obtained may not agree with those at the flow rate prior to the closure of the annulus valve. Therefore, a new stabilized flow rate, which corresponds to the altered pressure, must be allowed to establish itself. This procedure is time consuming and may lead to a loss of production, because flow of free gas through the pump reduces the pumping efficiency. This paper describes a single-step and a two-step method of determining the "gas-free" liquid level find the annular flow rate of gas without a significant change in the bottom-hole pressure. Knowledge of the liquid level (with the gas phase absent) allows the calculation of the bottom-hole pressure under flowing conditions using a pressure gradient of the liquid which is consistent with its pressures and temperature. Theory The volume of gas in the annulus can be considered as being contained in a vertical cylinder, the volume of which can vary, dependent upon the "gas-free" liquid level in the annulus which forms the base of the cylinder. Under normal operating conditions, gas is vented at the same rate as it enters and no change of mass occurs. If, however, the flow rate of gas being vented differs from the input flow rate, then the muss of the system will change according to the following mass balance equation for single-phase gas flow.