Full-Scale Well-Model Tests of a New Chemical Plug System for Zone Isolation in Horizontal Wells
- J.A. Gomez (Texas A&M U.) | D.D. Mamora (Texas A&M U.) | L.O. Lilledal (Saga Petroleum)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2002
- Document Type
- Journal Paper
- 83 - 87
- 2002. Society of Petroleum Engineers
- 1.6 Drilling Operations, 2 Well Completion, 4.1.2 Separation and Treating, 6.3.6 Chemical Storage and Use, 4.3.4 Scale, 4.1.6 Compressors, Engines and Turbines, 3.2.5 Produced Sand / Solids Management and Control, 2.7.1 Completion Fluids, 6.5.2 Water use, produced water discharge and disposal, 1.8 Formation Damage, 2.4.5 Gravel pack design & evaluation, 4.1.5 Processing Equipment, 3 Production and Well Operations, 5.5.2 Core Analysis, 2.4.3 Sand/Solids Control
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A new method for zone isolation in horizontal wells with chemical packers has been developed and tested with a 60-ft-long, full-scale horizontal-well apparatus. The method involves setting two chemical wellbore plugs in the horizontal section to straddle the problem zone, squeezing gel into the problem zone, and cleaning out the borehole before production. A crosslinked hydroxyethyl cellulose was used as the wellbore plug, while a monomer gel was used to shut off the problem zone. Tests confirmed the viability of the new zone-isolation method.
The two main types of horizontal well completions are openhole and slotted (or preperforated) liner completions. An openhole completion is the simplest and most basic completion method. Openhole completions may be used in a strong and consolidated formation where borehole collapse or sloughing is not expected to occur. In a weak or poorly consolidated formation, in which borehole- collapse and sand-production problems are expected, a horizontal well would be completed with a slotted liner. For some formations, borehole-collapse or sand-production problems may occur at a later time as a result of production or reservoir pressure depletion. Slotted-liner completions may be gravel-packed to alleviate sand-production problems.
Openhole and slotted-liner completions suffer from one main disadvantage in that the producing intervals along the horizontal section are in direct communication with one another. One or more of the producing intervals may begin to produce extraneous gas (for an oil well) and/or water. For a horizontal oil well, the extraneous fluid production may be caused by one or a combination of the following.
The horizontal section has been inadvertently drilled too near the oil/water contact or gas/oil contact.
Water/gas production from a high-permeability interval (e.g., a fracture).
Several methods exist that may be used to shut off production from a problem zone in a horizontal well. Some of the most widely used methods are as follows. For openhole completions, an inflatable, external casing packer (ECP) may be run and set across the problem zone, and the well can be completed with a slotted liner. An inflatable straddle-packer is run on coiled tubing and set across the problem zone; a gel is injected into the problem zone, and the packer is removed when the gel has set. These remedial methods for openhole completions may not work if the inflatable packer can not seal against the borehole wall because of either a badly washed out or an irregularly shaped hole. For slotted/preperforated liner completions, there is no mechanical way of sealing off the liner-borehole annulus.
Therefore, the motivation for our research was to develop a method to isolate problem zones in horizontal wells, particularly with slotted/preperforated-liner completions. As it turned out, such a method could also be used in horizontal open holes as well as in vertical and deviated wells.
Chemical Plug System
The injection of polymer gels to shut off water-/gas-producing zones has been widely tested in the field.1 However, the use of chemical wellbore plugs for zone isolation in horizontal wells has been limited to laboratory investigations.2 In 1995, Mamora et al.3 proposed the use of a chemical wellbore plug designed specifically for zone isolation in horizontal wells with slotted-liner completions. The technique would involve the following sequential stages (Fig. 1).
Run coiled tubing and spot a chemical wellbore plug just upstream of the zone to be isolated, and then pull back the coiled tubing.
When the chemical plug has hardened, spot the formation gel across the problem zone, close the surface-tubing rams, and inject the formation gel with a controlled injection of brine in the casing annulus.
When the formation gel has set, wash out (or make a round trip to drill out) the chemical plug and the excess formation gel before bringing the well on production.
CEA 88 (Phase I).
Research on this new zone-isolation technique was conducted as a joint industry project, CEA 88 (Phase I), sponsored by Chevron, Halliburton, Shell, and Texaco. Three commercial gels were selected and tested for potential use as chemical wellbore plugs-a polyacrylamide, a monomer, and a plastic polymer. Laboratory experiments were conducted to investigate the viability of the new zone-isolation technique, specifically the placement and holding pressure of the chemical plug.4-7Gel-Placement Experiments.
In these experiments, a 1/4-in. tubing (the coiled tubing) was placed inside a horizontal 4-ft-long by 1.5-in. inside diameter (ID) clear plexiglass tube (the horizontal wellbore). Brine was injected by means of the tubing to fill the plexiglass tube. The liquid gel was then injected through the tubing, spotted in the plexiglass tube, and allowed to cure to form a chemical plug. During the curing period, the chemical plug was observed from outside the tube to determine whether the plug slumped. Experimental results showed that slumping of the chemical plug is a function of the density and viscosity of the completion brine, gel injection rate, and gel type (Table 1).
In these experiments, PVC pipes, lined internally with sand, were used to simulate the roughness of the horizontal wellbore. The PVC pipe was secured in a vertical position, and one end of it was capped off. Liquid gel was introduced into the PVC pipe and allowed to cure by placing the pipe and its contents in an oven set at a constant temperature of 150°F. After the gel set, the bottom end of the PVC pipe was sawn off, and water was slowly injected at the opposite end of the pipe. The water-injection pressure was measured to determine the maximum pressure (holding pressure) when the chemical plug yielded.
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