Afterflows and Buildup Interpretation on Pumping Wells

Abstract

Summary Afterflow measurement identifies downhole problems as indicated by the presence of (1) a long fluid column, (2) U-tubing of liquid from the tubing to the annulus, (3) gas coning, and (4) stratification, as evidenced by highpressure gas stringers or high-pressure liquid stringers. Introduction Pumping wells are generally older wells with declining production. They are prime candidates for estimation of skin damage, fracture length, reservoir pressure, effective permeability, and other diagnostic information provided by pressure buildup curves. However, the necessity of removing the rods and pumps to place the conventional pressure bomb and then temporarily replacing the rods and pumps to restabilize the well and get the flowing bottomhole pressure, which is essential to well damage estimates, has meant that buildup curves practically never were run on pumping wells.We have been getting buildup curves on pumping wells by determining the liquid level in the annulus acoustically, measuring the surface annulus pressure, and calculating the sandface pressure. This permits following the liquid level and bottomhole pressure before shut-in to check pump operation or well stabilization and then following the buildup from the moment of shut-in. This also gives direct measures of the well storage factor and the afterflows of gas and liquid during buildup. Calculation of Bottomhole Pressure, Afterflows, and Well Storage A schematic of a typical well configuration is shown in Fig. 1. There is no packer. The gas and liquid entering the well separate in the annulus, the gas rising up through the fluid column to be produced out of the wing valve and the liquid being pumped out through the tubing.Our well sounder has a three-way valve that maintains a pressure above the annulus pressure in an expansion chamber. Upon signal from a preprogrammed self-contained monitor, a pulse of nitrogen from the expansion chamber enters, the annulus, starting a counter. The returning echo from the fluid stops the counter. The speed of the counter is adjusted by a dial calibrated in acoustic velocity. When setting up, we measure the liquid depth with a collar counting device and adjust the counter speed to give feet depth to liquid directly. Under favorable conditions, our reproducibility is about 1 ft. The setting on the acoustic velocity dial gives the acoustic velocity of the gas in the annulus. Knowing the acoustic velocity, we calculate the specific gravity of the gas in the annulus and its Z factor. Knowing the surface pressure, the annulus area, the depth to liquid, and the Z factor at each point along the curve, we can calculate the gas column weight to get the average pressure in the gas and the standard cubic feet of gas in the annulus at each point. The increase in this gas between successive points directly gives the Mcf/D flowing into the gas space during each time intervalthe gas afterflow.The sandface pressure is calculated starting with the measured surface pressure. The weight of the gas column is calculated by integration knowing the gas gravity, temperature, and average Z factor. This gives the pressure at the top of the fluid column.The weight of the fluid column is calculated stepwise. starting at the top, a depth interval is taken such that the pressure at the bottom of the interval will be 1.2 times the pressure at the top of the interval. The average pressure is 1.1 times the pressure at the top of the interval, the annulus area is known, and the rate of gas flow is the previously measured gas afterflow. From these three data, published correlations give the fraction of liquid in the interval. JPT P. 397^

Rate Normalization of Buildup Pressure By Using Afterflow Data

“…The wellbore storage coefficient has been calculated and added following the procedures appearing in Ref. 8. Table 6 lists pertinent reservoir and well data.…”

“…For pumping wells, the ability to calculate reliable buildup pressures and corresponding afterflow rates, along with the wellbore storage coefficient variation with time, has been reported in several papers. [5][6][7][8][9] More direct bottomhole measurements of afterflow and pressure using production logging tools during pressure buildup tests have been reported recently by Meunier et al 10 In the U . S., wellbore storage effects often characterize pressure buildup tests because the majority of domestic wells are produced by rod pumps where afterflow dominates during the buildup.…”

New Well-Testing Methods for Rod-Pumping Oil Wells — Case Studies

“…4 If acoustic travel time is measured, it allows the liquid level in the wellbore to be determined. This liquid level can easily be converted to hydrostatic pressure head by knowing the liquid density.…”

“…4,5 The combined fluid level/surface pressure methods had some difficulty accounting for different fluid gradients in the wellbore. These methods could not account for gas solution into oil as wellbore pressure increases during buildup.…”

Transient Analysis of Acoustically Derived Pressure and Rate Data

“…The first method involves continuous measurement of casinghead pressure together with tracking the movement of the gaslliquid interface during a transient test, using an A WS device. [1][2][3] Translation of these measurements to downhole pressure and rate is made with a wellbore hydrodynamic model/correlation. 1,[4][5][6] The second approach uses the mass-balance principle 7 to accomplish the same task.…”