A new tool combining multiple-set inflatable straddle packers and a jet pump can control unwanted water production in vertical and horizontal wells.

Andrew V. Padilla is senior production engineer at Chevron USA Production Co. in Midland, Texas.
Jay Miller is in technical sales for Tam International Inc. in Houston, Texas.

Horizontal laterals expose large amounts of productive reservoir rock. This can benefit certain reservoir applications, but it can be a major risk component in waterflooding heterogeneous carbonate reservoirs. Long laterals increase the potential of exposing undesirable geologic conditions. High water-production rates can result in lower oil rates and excessive lifting costs.
By evaluating and diagnosing fluid entry in newly drilled horizontal wells, efforts can be made to control water entry and optimize well performance. A new tool combining resettable, inflatable packers and a hydraulic jet pump has been developed to do this.
Excess water production
In the Permian Basin's McElroy field, efforts to evaluate the potential of horizontal wells to recover unswept oil reserves began in 1995. Initial horizontal well results in this mature waterflood were encouraging, but excessive water production became a major problem. The horizontal producing wells designed to avoid water cycling conditions encountered geologic and reservoir features that resulted in high water cuts. McElroy horizontal wells were designed as re-entries of existing vertical wellbores. Medium-radius laterals were drilled with 43/4in. open-hole completions. Designs included 600- to 1,000ft laterals drilled at 2° to 5° from true horizontal to expose multiple layers of a 30- to 40ft thick target zone (Figure 1). Evaluation was needed to determine a fluid entry profile over the length of the lateral and build sections. Unfortunately, neither practical nor effective logging procedures were available for wells requiring artificial lift methods, so mechanical isolation and short-term swab testing was conducted.
Inflatable packers were an integral component in the evaluation design due to the nature of the open-hole conditions. Long 200- to 600ft sections of the laterals were isolated independently using a bottom inflatable bridge plug and an upper inflatable production packer. Swab testing or short-term full drawdown production testing provided a basic indication of fluid entry rates and oil cut. After moving the inflatable equipment and testing various portions of the open-hole wellbore, a completion design using inflatable isolation assemblies was used to exclude zones of excessive water entry and optimize oil production.
Resettable, inflatable straddle packers combined with a hydraulic jet pump are able to provide full drawdown testing on short intervals. This new method allows for testing multiple zones under full artificial lift drawdown with a single trip in the hole. Initial field trials demonstrated improved efficiency and reduced the cost of evaluating well fluid entry.
Early evaluation methods
Initial production testing of newly drilled horizontal wells with the full length of the curve and lateral exposed frequently showed poor results. Swab testing before the initial completion regularly showed 100% water production. Initial tests using electric submersible pumps (ESPs) showed low oil cuts, high fluid levels and water rates two to five times those of surrounding vertical producers. Wells that showed loss of circulation while drilling were typically the worst performing wells during initial testing. Clearly, there were problems with undesirable water entry along the lateral portion of the well.
A mechanical isolation and swab test procedure was designed to locate the zones of excessive water entry. Inflatable packers were used to provide isolation, and swab testing was used to determine fluid entry rate and oil cut. A rudimentary evaluation of three intervals - the curve and two segments of the lateral - was the most cost-effective design that met the testing objectives. The zones where lost circulation occurred during drilling sometimes dictated how the lateral would be segmented for testing. Mud log data, drill rates, lithology and oil fluorescence data also were used to pick packer seat locations and determine test zones.
Inflatable packers used as bridge plugs were configured with bull plugs and on/off tools to allow for disconnecting and retrieval. Retrieval has been 100% successful. Inflatable production packers were resettable to allow for expanding or reducing the test interval, if needed. Once packers were installed, swab testing was expected to be ideal for obtaining simple and effective results. Although swabbing proved to be a low-cost approach, it also proved inaccurate and misleading. In wells with low reservoir pressure or high inflow conditions, swabbing could not create sufficient drawdown to achieve inflow from the tighter oil-bearing zones. Oil cut results often were pessimistic, with only traces or no oil shows present. At best, swabbing proved a good indicator of the water-entry zones when the fluid level could not be pulled down. The potentially productive intervals were readily recognized because they could be swabbed down to some extent and often showed 5% to 20% oil cut during swabbing. Typical swabbing operations took up to 1 to 2 days per test interval and frequently required 10 to 12 days of rig time to complete a cursory evaluation of the well.
However, the qualitative findings showed oil production potential, which was not apparent during initial well testing.
A modified approach
Because of the inadequacies of swabbing, the testing procedures were modified in two basic ways. First, individual zones were tested for extended periods using ESPs to achieve full drawdown. Secondly, test tool configurations were designed with straddle packers, seating nipples and sliding sleeves so fewer packer sets would be needed to accomplish the testing plan. Submersible pumps with variable-speed drive units provided good flexibility to test zones with different inflow performance at full drawdown conditions. The duration of a typical ESP test was lengthened to ensure valid, stabilized oil cut results. Although test results improved significantly, frequent pulling and rerunning ESP pumping equipment proved time-consuming and expensive.
After testing several zones in Crier McElroy Well 147 for fluid entry, zone 1 toward the toe of the well was plugged off using an inflatable packer. Oil production from zones 2 and 3 was five times that of the original horizontal well, and water production plummeted from 5,600 b/d to 143 b/d. Resettable, inflatable straddle packer systems also were selected to test multiple zones without pulling out of the hole to redress tools and packers. A packer spacing of 200ft was used. Each isolated interval was swab-tested to determine fluid entry and oil cut. Straddle packers worked quite well in improving the efficiency of isolating different zones and allowing for a more complete evaluation of the wellbore. Although the multiple tests were completed efficiently, swab testing again proved inaccurate and generally prolonged testing efforts. A typical test could result in 10 to 25 zone tests in good wellbore conditions.
Overall, the inflatable packers performed well in achieving competent hydraulic seals in the open hole. The Grayburg reservoir is a dolomite formation with good compressive strength. Difficult hole conditions such as oval hole, key seats and washouts were encountered, but most wellbores were found to be competent, in-gauge and suitable for trouble-free testing. Few problems were encountered while setting the packers, and element failures were rare. Use of different setting mechanisms and new element technology improved tool reliability. Because packer seats cannot be pressure-tested in open-hole situations, a practice of picking up and slacking off 10,000lb was adopted to help verify solid packer seats. Fluid movement through the near-wellbore area around the 4ft packer elements has not been recognized as a problem.
Water shut-off options
Once water entry and oil-productive intervals had been delineated, three basic completion designs were used to control water entry. The type of mechanical control device used depended on where the water was found in the lateral. An inflatable bridge plug and cement retainer can control water production from the toe of the well. A scab liner can control water production from the lateral. An inflatable bridge plug and cement retainer at the toe and a scab liner in the heel can control water production from both the toe and heel of the well.
Testing the jet pump
To determine a producing profile and make water shutoff decisions, a reliable, resettable test tool using artificial lift was required. A simple and reliable hydraulic jet pump was joined with inflatable resettable straddle packers to test open-hole horizontal wells. This tool was initially tested in a vertical cased-hole well with multiple perforations.
The first tool string was configured with three packers and a jet pump (Figure 2). The bottom two packers isolated the test interval, and the top packer isolated perforations from the produced and power fluid returns traveling up the casing annulus. The jet pump was placed in the string just above the top packer. A remote setting head above the jet pump and inflatable packers was connected by a 1/4in. stainless-steel inflation line to ensure the three packers functioned simultaneously.
Once the tool string was made up, it was lowered to the desired set depth. The jet pump was pumped from the surface to the carrier and seated. Continued pumping increased the tubing pressure to about 800 psi to inflate the packers. Simply slacking off on the work string traps inflation pressure in the elements. Continued tubing weight was applied during this test. Power fluid rate and pressure to the jet pump were adjusted to establish drawdown and production. Both power fluid and produced fluid rates were measured, with the difference equaling production rate. If no extra fluid was produced, that zone was deemed nonproductive and the tools were moved. If the hole was full, casing volume was calculated to determine when actual bottoms-up fluid could be expected at the surface. For the McElroy tests, produced fluid was calculated to reach the surface in 1.5 hours.
Seven sets of perforations were tested. The top two perforations contributed 100% of the water entry with no oil cut. There was no communication between perforated intervals, and full drawdown was achieved in all intervals tested. The total test effort included eight tool sets in 2.5 days at a cost of US $38,700. An early production test showed water production dropped from 1,400 b/d to 200 b/d, and oil production increased from 6 b/d to 49 b/d. After testing each zone, moving tools to the next test interval required less than 30 minutes. The only difficulty encountered while restarting a test was a loss of blanket gas in the test separator. Care should be taken to assure that all valves on the surface equipment are closed when moving tools. After experiencing effective and reliable tool functionality and seeing the quality of the test results in an open hole, this method of testing was deemed applicable to testing vertical cased-hole wells.
Acidizing can be done during the test procedure without pulling the tools. The jet pump can be removed and replaced with a blanking sleeve, then the test zone can be stimulated, the sleeve pulled and the jet pump replaced. Spent acid can then be produced and the zone retested.
Jet pump details
A jet pump offers several advantages as a means for artificial lift. A range of test rates can be achieved with one pump size. The pump size can be changed to match the well production. The pump can be removed from the well easily by reversing it out of the hole or pulling it with the sand line. Inefficiency in power fluid use is not as much of a problem during short-duration tests as during permanent artificial lift applications. Power fluid can be oil, field water, produced water or any readily available, compatible fluid.
Surface equipment includes a:
portable test separator for fluid handling;
propane tank to supply blanket gas pressure for the instruments and fluid dumping on wells producing low gas volumes;
pump and filter with output of 1.5 bbl/min at 3,000 psi (at McElroy field, the available workover rig pump was used); and
storage tanks for power fluid, produced water and produced oil.
Testing was initiated with 100 bbl of field salt water in a 500-bbl tank. Separate oil and water test tanks made measurement of produced fluid easy and accurate.
Testing in horizontal wells
No modifications to the tool string were needed to test the new tool in horizontal wells (Figure 3). The lower inflatable straddle packers were spaced out with tubing and perforated pups to cover the desired test interval. As before, the jet pump was attached in the string just above the top packer, with the remote head just above it. The remote head and all three packers were connected by 1/4in. stainless-steel tubing to ensure that all or none of the packers is set. The stainless-steel control line was placed inside the tubing in the horizontal application to guard against damage from rubbing when running into the lateral.
One potential problem with this simple tool configuration in a vertical well is that each time the tool is moved, the jet pump is moved higher, reducing drawdown capacity. This results in a greater chance of pumping off that test interval. If the pump were on top of the two packers comprising the straddle interval, this would not happen. With the simple system, the tool string must be pulled and the spacing between the lower straddle and upper packer reconfigured to optimize drawdown capacity.
Positioning the jet pump on top of the straddle packers in the lateral provides maximum drawdown capability for all tests of the lateral. This method will be the primary test method in the future and provide major cost and time savings over previous methods used.
Acknowledgements
This paper is copyrighted by the Southwestern Petroleum Short Course and was presented at the 47th Annual Southwestern Petroleum Short Course Conference, April 12-13.