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

Abstract

Summary A new well-testing method that provides high-resolution bottomhole pressure (BHP) measurement has been developed for use with rod-pumping oilwell completions. This new well-testing technique is conducted by running a high-resolution downhole pressure gauge in a specially designed carrier assembly with the downhole pump. The gauge assembly is run on the rod string without pulling the completion packers or tubulars. With the new method, a downhole pressure system is in place while the well is flowing during the entire well test. High-resolution downhole pressure data can then be recorded during all flow and shut-in periods of the well test, providing more accurate pressure data and enhanced capability for reservoir analysis. Traditional techniques use wireline-conveyed pressure gauges, tubing-conveyed pressure-recording devices, and surface pressure-monitoring systems with an advanced fluid-level measurement to record data from pumping well completions. These methods are limited in their ability to provide accurate test results when low-pressure conditions prevail. The new method allows operators of low-pressure oilwell completions to use advanced well-test-analysis software to analyze actual pressure data to obtain reservoir properties and quantify skin damage. Data can be acquired from many types of rod-pump completions, including wells with downhole gas separators. Excellent pressure resolution has been obtained from this new testing technique with either of two downhole assembly designs depending upon the completion configuration. The results and the ensuing well-test analyses in several pilot pumping wells will illustrate that the new process permits more flexibility in well testing and allows for greater accuracy and better interpretation of data in low-pressure rod-pumping oil wells. Introduction Traditionally, low-pressure rod-pumping oil wells have been difficult to test. This is true because previously used methods have not allowed downhole pressure data to be captured from both flow and shut-in periods. To obtain a test of a completed rod-pumping well, most operators would flow the well and then pull the rods before running downhole pressure gauges on wireline or tubing. With traditional methods, the rods had to be pulled out of the well before the gauges could be run. Unfortunately, unseating the pump would dump a tubing volume of fluid on the formation. Also, by the time the pressure gauges were on bottom, the flowing pressure had changed, and the possibility of an accurate measurement of flowing downhole pressure was lost. These occurrences would compromise the performance of the pressure transient analysis. Soliman presented the use of superposition in pumping well-test analysis.1 Kabir introduced the use of deconvolution 2 and a segmental analysis approach with automatic convolved type-curve methods3 to improve the analytical capabilities of existing welltest analysis techniques. Many of the new rod-pump testing methods focus on analytical changes and do not address the method's capability to capture high-resolution flowing and shut-in BHP data during a pumping well test. It can be shown that the radius of investigation for a pressure transient test can be increased with higher-gauge resolution. This increase provides still another reason for using high-resolution downhole sensors in testing procedures. Methods developed in the late 1980s tried to combine fluid level measurement with surface pressure data to estimate the BHP.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. The presence of foams and the typically high storage volume associated with pumping wells makes distinguishing the exact shut-in time from the reconstructed pressure data more difficult with this technique. Also, the combined fluid-level/surface-pressure methods do not have the high-resolution capabilities necessary for rigorous derivative-type curve matching with modern well-test-analysis software. Many authors have addressed various analytical methods of dealing with slug-test or closed-chamber data.6–9 Although most of these analytical procedures were developed for drillstem testing, several papers have been presented showing early applications of this method in rod-pumping wells. Once a well is completed for production, it will be difficult and costly to perform a slug-type test on rod-pumping wells with established slug-testing procedures. Most slug-testing methods require a downhole valve and a pressure gauge to obtain shut-in data because fluid does not reach the surface during the test. Also, matching anomalous pressure-derivative changes in early- and late-time regions is difficult if actual downhole data are not available with a high-resolution gauge during both flowing and shut-in periods. This paper addresses the mechanical issues required to obtain high-resolution downhole pressure data. This new method allows the operator to run pressure gauges with the pump assembly by simply pulling and running the pump with the rod string. Two alternative methods were developed. The operator may run the gauge assembly below the pump seat if there is no downhole gas separator. Otherwise, the gauge assembly can be set in between the rod pump and the pump seat. Both methods allow a pressure gauge assembly to be run in place during flow and shut-in periods without requiring that the tubing string be pulled out or that the recording of the flowing downhole pressure be sacrificed. This allows accurate BHP measurement during the entire testing sequence without the need to estimate the flowing downhole pressure or fluid type, amount, or density. Two rod-pumping horizontal-oilwell cases are presented here to show the advantages of this new testing technique. Previous Downhole-Gauge Testing Methods Previous attempts to use downhole gauges on rod-pumping wells have been costly and difficult. Methods used included drillstem tests run during the drilling or initial completion, slug and closed-chamber tests incorporating various analysis techniques, and wireline- and tubing-conveyed tests. Earlier attempts to run gauges on the rod string below the pump have not been effective because a gauge had not been developed that was sufficiently rugged to withstand pump vibration. Also, previous instrument dimensions have required large carrier assemblies. In tubing-conveyed methods, the operator must pull rods and then pull and run tubing. This can be costly because the operator must acquire a workover rig to perform the testing operation.

Case Study of the Application of Pressure Transient Testing to Characterize the Pumping Wells in the Calcutta Field in Suriname

“…4 shows the gauge configuration. Guoynes, Azari, and Fairbanks, et al (2002) showed two different gauge configurations for well testing of pumping wells with reduced rodpumping noise.…”