Initially Gas Wells usually flow at sufficiently high rates to remove all liquids from the wellbore. When the gas flow rate decreases below critical rate 1 some intervention such as compression, surfactants, and/or velocity strings is required. At a later time when either reservoir pressure and/or gas flow rate is sufficiently reduced, then other means of artificial lift (usually such as plunger lift, gas lift, or sucker rod lift) must be added to maintain production. Acoustic fluid level testing can be used to investigate the producing condition of a gas wells. Developments in digital acoustic fluid level technology have resulted in the operator being able to undertake fluid level measurements and use this technology to investigate the status of his gas wells. The act of acquiring a fluid level on a gas well is an inexpensive and non-intrusive process. Examples of acoustic tests are presented, where the test are performed on shut-in or flowing above or below critical flow rate gas wells with or without surfactant treatments acquired down the tubing or the tubing/casing annulus.
Techniques for acoustic liquid level analysis are discussed for gas wells where unusual conditions exist such as very shallow liquid levels, very deep liquid levels, noisy wells, high bottomhole temperature, and low or high surface pressures. Some gas wells have gas lift mandrels, liners, multiple zones of perforations, tubing holes, flush pipe and other conditions which result in the acquisition of difficult to interpret acoustic traces. This paper describes analysis techniques used to determine the distance to the liquid level in gas wells with these unusual conditions. The analysis is based on data obtained at the surface without entering the wellbore and yields accurate representation of the conditions existing on the surface, within the wellbore and within the reservoir.
The principal objective of the acoustic measurements in a flowing gas well is the determination of the quantity of liquid that is resident in the tubing (or annulus when the tubing is used for deliquifying the wellbore by means of a pump) and whether the liquid is uniformly distributed over the length of the well as a mist or annular flow pattern or has fallen back, accumulating towards the bottom of the well.
In the first case when the gas flow rate is above critical rate, when the liquid is uniformly distributed, the gas velocity is sufficient to continuously carry liquid as a fine mist or small droplets to the surface, establishing a relatively low and fairly uniform flowing pressure gradient. In the second case when the gas flow rate is below the critical rate the gas velocity is not able to carry liquid to the surface, accumulating liquid in the lower part of the well. The flowing pressure traverse in the well bore will show two different gradients, a light gas gradient above the gas/liquid interface and a heavier gradient in the lower section of the well below the gas/liquid interface. The gradient of this fluid below the gas/liquid interface is controlled by the gas flow rate through the liquid with zero net liquid flow as the gas bubbles or slugs of gas percolate through the liquid and only the gas flows to the surface.
Knowledge of the flowing gradient and fluid distribution in the well is importance in determining if there is back pressure acting on the formation due to liquid loading in the tubing. When gas velocity drops below critical rate, production rates are reduced by liquid accumulation in the tubing and this liquid requires application of a deliquifiyng technique such as installation of plungers, pumps, redesign of the flow string to increase gas velocity or addition of surfactants reduce the gradient.
The acoustic test is designed to determine which flowing gradient conditions exist in a well by performing a series of fluid level and surface pressure measurements while the flow at the surface is stopped for a length of time sufficient to identify the behavior and distribution of the fluids in the tubing or tubing/casing annulus. The advantages of acoustic test over wireline flowing pressure surveys include lower costs, equipment is very portable, and lower risks since measurement tools are not introduced into a flowing well.