Phenolic-based thermoset plastic resins have been used for many years in producing oil and gas wells as a water control agent. The most successful application has been to treat excessive water production in gravel packed wells which have experienced bottom water encroachment. Unfortunately, the phenolic plastic resins have many limitations in their range of application. Among these are: typical operating temperature range of 170°F to 220°F (77°C to 104°C), range of material densities, viscosities and control of set/gel time. Also, in liquid form, phenolic plastics are inherently unstable resulting in a short shelf life. To overcome these and other limitations of phenolic plastic resins, a new material with greatly improved properties was developed. TexPlug is a furan-based material that is stable, highly controllable in density, viscosity, set time and applicable to a wide range of temperatures from 75°F to 500°F (24°C to 260°C). These characteristic properties define the product as a superior method for treating bottom water encroachment in gravel packed completions.

Control of water entry
The traditional method that is used for treating water entry is squeeze cementing. The cement squeeze method has been dramatically improved in recent years with the introduction of micro-crystalline cements and is the preferred method in high to medium pressure reservoirs for eliminating voids behind pipe. As long as the fracture pressure is not exceeded, the cement sets properly and dewaters to form a tight plug in the perforations. Unfortunately, the squeeze cementing method cannot be easily applied once a well is gravel packed, especially in the offshore environment where workover operations to remove screen assemblies, squeeze, reperforate and reinstall the gravel pack are cost prohibitive in most cases. The treatment of bottom water entry in gravel packs requires a shut off agent that should ideally migrate as a pure liquid through the screen, gravel pack, sand and perforations, then harden to form an impermeable barrier (Figure 1).

Phenolic thermoset plastic resins
Although some phenolic resins can be chemically catalyzed to a limited extent, they are primarily thermoset agents - they harden as a function of temperature. The phenolics work well in the range of 170°F to 220°F (77°C to 104°C). Consider temperatures outside this range: The "hard set" time for a typical thermoset phenolic is 15 days at 100°F (38°C), clearly not practical in most cases. At temperatures above 220°F (104°C), the "hard set" time is too short to allow sufficient penetration into the formation. If viscosity reducers are added to increase phenolic plastic penetration depth at higher temperatures, the density decreases rapidly to the point of buoyancy. When buoyancy is established, the phenolic plastic will migrate uphole - not out into the formation.

A new solution
TexPlug is a much higher strength polymer material derived from oligimers of furfuryl alcohol to which various additives are mixed to adjust viscosity, density and set time at a specific temperature. The product was originally developed as a high temperature liquid plugging agent to seal steam breakthrough holes in wells subjected to steam stimulation. After setting up, the compound is stable to 700°F (371°C), retains compressive strength of 7,500 psi under cyclic pressure and temperature loading, is totally impermeable, and inert to all common oilfield chemicals, acids, caustics and gases.

Tough stuff
To be effective, a plastic resin must remain in a low viscosity liquid state long enough to allow emplacement and to migrate the desired distance, then once in position, must solidify into an impermeable barrier within a reasonable amount of time and be of sufficient strength to withstand anticipated differential wellbore pressures.
The foremost criterion in designing any treatment is bottomhole temperature. TexPlug is effective over an expanded temperature range of 70°F to 500°F (21°C to 260°C).
The density of plastic resin determines the effectiveness in relation to wellbore fluid density and penetration at high temperatures. Most phenolic resins have a fixed density in the range of 9.8 lb/gal to 10.0 lb/gal at room temperature and decrease in density naturally with increasing temperature. TexPlug has an adjustable density range of 9.8 lb/gal to 15.9 lb/gal which ensures its effectiveness at high temperatures and in heavy wellbore fluids.
Viscosity, measured in centipoise, is a measure of the ability of liquid to flow. Typically, viscosity decreases with increasing temperature. At 210°F (99°C) TexPlug will vary in viscosity from 3.0 cp to 1.5 cp depending on the blend - roughly the consistency of kerosene.
As plastic hardens, the viscosity increases until a phase change from liquid to the solid state occurs. The time required to go from surface temperature to this phase change is known as the gel time. When gel time is reached, the material forms into a soft, malleable, spongy state and will no longer migrate. After gel time is reached, the material will continue to harden until the maximum compressive strength is reached. This is known as the hard set time.
Maximum compressive strength is measured by laboratory load tests. For phenolic plastic, maximum compressive strength ranges from 2,000 psi to 3,500 psi. The new compound has a maximum compressive strength of 7,500 psi; therefore, more effective where large differential wellbore pressures may exist.
Unlike phenolics, TexPlug is resistant to all common oilfield chemicals and has been proven heat resistant to temperatures exceeding 700°F (371°C). The product also has an indefinite shelf life at constant viscosity. Many phenolic resins have an effective shelf life of only 30 days - an important consideration when planning treatment programs in remote locations.

A versatile solution
Although the treatment of bottom water encroachment in gravel packed wells has proven to be the highest value added application, TexPlug has proved successful in many other applications:
• Treatment of non-gravel packed bottom water drive reservoirs with 20 ft (6 m) or more of perforations where the water level has moved up into the perforated interval;
• Sealing leaks in plugs, packers or casing;
• Plugging off a watered-out middle zone within a laminated reservoir;
• Sealing a channel in casing annulus when cementing is not economical or practicable;
• Diverting water flood or steam injection to less permeable intervals; and
• High temperature applications to repair holes and leaks in casing used in steam floods.

Implementation and deployment
Traditionally, TexPlug is placed in the well using electric wireline-conveyed gravity type dump bailers. Real-time depth control utilizing a collar locator or gamma ray tool provides the greatest precision for emplacement.
Slickline-conveyed dump bailers using downhole timing devices have also been successful. This method is sufficiently precise when downhole depth markers are reliably known.
Emplacement with coiled tubing would appear to be technically feasible but has not been attempted to date.

Experience proves the point
During the 1990s, the polymer was successfully used in more than 110 jobs in the US Gulf Coast, California and Venezuela. The product has been off the market since 1997 and has only recently been reintroduced. Below are examples of jobs successfully completed.

Summary
For water shut off treatments in oil and gas wells, TexPlug has expanded the range of applications of plastic resins. It has proven effective in a greater range of wellbore temperatures, has a greater range of density and controllability; has twice the compressive strength, and exhibits superior stability in both liquid and solid state than the phenolic resins. With the growing importance of water reduction treatments, the product is expected to be a valuable addition to the water shut off techniques available to the petroleum engineering profession.