Advances in Estimating Gas Well Deliverability

Abstract

Abstract In the past, drawdown tests to estimate well deliverability and diagnose wellbore damage, although simple in themselves, have been of limited value, often because it is inconvenient or costly to obtain the constant flow rate required to analyze them reliably. This paper presents a new drawdown analysis technique developed with the aid of a digital gas-well simulator. The new method does not require a constant rate, is as simple to analyze as the conventional method and is considerably more accurate. It can thus heused in gas wells to analyze the early time data from a back-pressure test when the rate has not yet "stabilized" to a constant value. The new technique takes into account turbulent flow in gas-well systems - a prerequisite to calculatinga gas well's current and potential deliverability. These calculations areeasier if at least two drawdown tests, and preferably, more, can be conducted on the gas well. Fortunately, the practice of running a multiplicity of drawdown tests is already embodied in the four-point isochronal AOF test, where each flow test is separated from the others by a sufficiently long period of shut-in. This type of test lends itself to reliable drawdown analysis. However, in the four-point flow-after flow test, where the flows at each rate are consecutive, the deliverability estimates often are not reliable. The new drawdown technique is not limited to use in gas wells. It can also be used toestimate well productivity and diagnose wellbore damage in oil or water systems. Introduction Gas flow in a porous medium is different from oil or water flow in that itdoes not obey the Darcy Law. In the gas-reservoir-engineering literature (1), this difference is now commonly called "turbulence." Turbulence occurs becausethe low viscosity of gas (compared to that of oil or water) results incorrespondingly higher gas velocities under a given pressure gradient. At these higher velocities, the gas suffers inertial energy losses as well as the usualviscous energy (Darcy) loss.