Convolution Analysis of Surge Pressure Data
Authors: Simmons Jeff F. 
Published: December 2007

Pressure data from underbalanced perforating or backsurge perforation washing are analyzed by convolution analysis. The method perforation washing are analyzed by convolution analysis. The method presented allows closed-chamber test evaluation of reservoir transmissivity presented allows closed-chamber test evaluation of reservoir transmissivity and wellbore skin by a straight-line graph analogous to a Horner plot. As a well-test analysis method, the closed-chamber test avoids surface pressure buildup, which often prohibits conventional drillstem testing. pressure buildup, which often prohibits conventional drillstem testing. "The underbalanced well surge, or closed-chamber test, provides a method to evaluate well-test parameters safely and discreetly in areas where data security and/or environmental concerns prohibit production testing or conventional drillstem testing."

Introduction

General Description. Underbalanced well surging has proved to be an effective perforation-washing technique. The surge perforation-washing technique. The surge procedure, whereby a small volume of procedure, whereby a small volume of reservoir fluid is produced at extremely high rates, often improves well productivity. Two methods of controlled well surging are common. Before installation of sand control, a temporary well completion similar to a drillstem test (DST) is often used to cause a "backsurge." Alternatively, a well surge may be performed by perforating the well "underbalanced "while the tubing string is occupied by low-pressure gas. In either method, the underbalanced surge results from production into a closed chamber that is initially occupied by low-pressure gas.

The backsurge operation uses a work string composed of upper and lower remotecontrolled valves, a temporary packer, and pressure recorders (Fig. 1). The assembly is pressure recorders (Fig. 1). The assembly is run into the wellbore with an enclosed air chamber formed between the two surge valves. The chamber is initially occupied by air or nitrogen at essentially atmospheric pressure. Surge-chamber volume is usually pressure. Surge-chamber volume is usually sized to a specific volume per perforation and typically ranges from 5 to 20 bbl [0.8 to 3.2 m3].

The backsurge is performed by either mechanical or hydraulic (by means of annulus pressure) opening of the lower surge valve. pressure) opening of the lower surge valve. At the instant the lower valve is opened, the formation sandface is exposed to a minimum pressure, which results in a high rate of pressure, which results in a high rate of formation-fluid influx. The surge of fluid cleans residual perforating debris from the sandface and causes limited sand failure near the well. U.S. gulf coast wells were observed to produce more than 20,000 B/D [3180 m3/d] during the 1- to 2-minute floperiod of a typical surge.

Upon completion of the surge, the packer is released, allowing produced fluids and sand to be circulated out of the wellbore. Because all production is contained downhole, well control is easily maintained. Fig. 2 illustrates the pressure response of a typical backsurge.

When the surge is effected by underbalanced perforating, the mechanical configuration is analogous to the backsurge. An underbalance is created by displacing the tubing string with low-pressure gas. The perforated casing functions as the lower perforated casing functions as the lower surge valve and the wellhead replaces the upper surge valve.

Monitoring the pressure response of either procedure by attaching a pressure recorder procedure by attaching a pressure recorder to the tail pipe suspended below the temporary packer has been a common practice. Pressure data historically have been used to Pressure data historically have been used to measure initial reservoir pressure and to determine whether a drawdown occurred, which indicated that a successful underbalanced surge was obtained. Pressure data have also been used qualitatively as a measure of expected well productivity, by noting the relative duration of the drawdown.