Mature fields and reservoirs provide operators an opportunity to access untapped production or reserves with the potential for the same or better returns than the original stimulation treatment of existing wells in the region. This can be done by drilling infill wells, using water injector wells to increase reservoir pressure and sweep hydrocarbons to nearby producer wells, or restimulating existing wells.
Restimulation requires pumping additional fluid and proppant or acid into new and/or old zones of an existing well. Existing wells are commonly restimulated for the following reasons:
- Initial stimulation was inadequate due to low proppant or fluid volumes, overflushing or damaging fluids;
- To restore lost fracture conductivity or poor connection to the wellbore; or
- Access (previously bypassed) reservoir rock (e.g., closer fracture spacing).
Slimhole completion systems provide a robust restimulation option by installing an entirely new liner inside the existing well using permanent mechanical isolation packers to isolate each zone.
There are several treatment methods used today to restimulate wells. This includes ball sealers, chemical diverters, isolation tools conveyed by coiled tubing (CT), cemented liner inserts, expandable liner patches and slimhole systems using packers for isolation.
Ball sealers and chemical diverters are low cost and operationally simple restimulation methods often used by operators. Both of these methods are used as plugs to divert fracture treatment fluids to other entry points along the wellbore. Challenges for chemical diverters include determination of diverter pill size and concentrations, fluid leak-off, damage caused due to inadequate cleanup and overflushing of proppant stages. With ball sealers, if the fracturing operation is temporarily halted, much of the effect of the ball sealer diversion can be lost. The effectiveness of this method also depends on the type of completion method being used, whether it is perforations of a plug-and-perf (PNP) system, a slotted liner or a system using sliding sleeves. Ball sealers also might be difficult to retrieve if nondegradable balls are used.
CT-deployed tools provide flexibility for operators with tools that are rapidly deployed and where a rig is not required for installation. Major challenges of this system include limited pump rate capacity due to the smaller diameter of the CT, the inability to successfully deploy in long laterals and high costs.
Other methods, such as expandable and cemented liners, can be inserted inside an existing system and require PNP setting similar to new well completions. Scales and fines can, however, negatively impact the expansion operation resulting in sticking or lost tools.
Slimhole systems offer a means of permanent isolation inside an existing system using mechanical packers and ball-activated sliding sleeves. Packers provide an effective means of stage isolation by permanently setting inside the existing casing. The arrangement of sliding sleeves in the wellbore enables exact placement of stimulation fluids into the wellbore. An additional benefit of slimhole systems is that a new liner is installed inside the existing well, mitigating the need to compensate for any damage to the original liner.
Slimhole systems can be used to restimulate existing wells and also to re-enter existing vertical wells into sidetrack drilled horizontal wells to increase reservoir coverage.
Restimulation of a PNP well
A well more than 10 years old in the Glauconite Formation in Canada was completed with 4.5-in. casing to surface. There were two existing sets of perforations in the casing from the original stimulation treatment approximately 30 m (100 ft) apart and one-third of the way into the lateral. To maximize the effectiveness of the well and cover the entire lateral, the operator shot three new sets of perforation clusters approximately 150 m to 300 m (492 ft to 984 ft) apart. Using the Packers Plus Stack- FRAC Slimhole system, a solution was designed for the operator consisting of four stages using a 2.875-in. liner.
The ball-activated FracPORT sleeves of the slimhole system were strategically placed to target the fracture treatment through the new perforations as well as to allow production from the existing perforations. Based on five-month monthly production averages for both prestimulation and post-stimulation, a factor increase in the production rate of 59 times was observed. In one year, restimulation with the StackFRAC Slimhole system resulted in more than 12 MMcm (435 MMcf) of additional gas.
The total end completion cost for the original well was $514,000 and $805,000 for the restimulation. After more than 11 years of production, and based on an estimated current oil price of $70/bbl, the difference between the barrels of oil equivalent produced with restimulation and the estimated production without restimulation is 197,667 boe. This amounts to $13.8 million of additional revenue with the restimulation treatment, more than 10 times the total end completion cost of the original liner and the new slimhole liner combined.
Re-entry into vertical wells
The Cleveland Sand tight gas reservoir in Texas was discovered in the 1950s and initially developed with hydraulically fractured vertical wells. With many wells having flat decline rates, operators looked to re-entry as a cost-effective solution to boost production and reduce the cost of drilling a new well. Six vertical wells that initially began producing in the 1970s through the 1990s in Lipscomb County were targeted by an operator between 2005 and 2008. The wells were re-entered using whipstock connections to drill sidetracked horizontal laterals from the existing vertical section. Each well consisted of three to five completion stages, stimulating lateral lengths up to 610 m (2,000 ft). Up to 320,000 lb of proppant was pumped for each well. Based on the monthly production averages for both prestimulation and post-stimulation, a factor increase in the production ranging from 10 to 468 times was observed.
Restimulation of existing wells is a useful, field-proven technique that can be used to increase production from mature or depleted reservoirs. Slimhole systems enable operators to enter existing wells that need restimulation using an entirely new liner and permanent packers for reliable zonal isolation to target new, old or previously bypassed zones. Carefully selected candidate wells for restimulation have the potential for massive returns.