Pre-frac Injection Test Saves Reservoir from being Damaged!
Tue, 31 Jul 2007
Any engineer would agree that accurate data is a prerequisite to informed decision making.This concept also applies in the oil and gas industry where pressure build-up and drawdown tests are perhaps the most common methods to obtain reservoir information. However, in wells with multiple, low perm pay zones, the time required to test each sand is prohibitive.Pre-frac injection testing was developed as a cost-effective method to optimize multi-layer, low perm sand completions and to estimate pore pressure and permeability in each sand. SPE Paper 39932 describes pre-fac testing as a technique in which,
"A large amount of information is available from a simple pre-frac injection/falloff test to aid in fracture treatment design.The key requirements for the pre-frac injection test are fairly simple: High resolution pressure recording is mandatory.A normal service companyís gauge resolution of 5 to 10 psi is unacceptable.Ideally, a quartz surface or bottomhole gauge, with a resolution of 0.01psi should be used."
Also known as a Minifrac, FET (Fluid Efficiency Test), MFO (Mini Fall-Off Test), or DFIT (Diagnostic Fracture-Injection Test), a pre-frac injection test is often performed prior to the stimulation treatment of a formation.It usually consists of pumping a small amount (500-2,000 gal) of water, often containing a clay control additive, into a sand at fracture rates ( bbl/min).As in any well test, pressure and rate are measured throughout a fracture-injection test and recorded for subsequent analysis. Many operators choose the SPIDR surface gauge for their injection testing needs because of its dual-quartz surface transducer with a resolution of 0.01psi and a sampling frequency capability of 1 second and its ability to simultaneously record injection rates and volume along with injection pressure.
A major operator had questions about a well they had scheduled to stimulate with a traditional cross-linked gel.They believed their well might be under-pressured and, if true, it would necessitate a change in the fracture program.Performing a gel-frac on anunder-pressured well could damage the formation since there would not be enough pressure to recover the frac fluids.In order to confirm the design of scheduled fracture, which was only 2 weeks away, they installed a SPIDR surface gauge for a pre-frac injection test.The injection test was run on the lower zone and monitored for 24 hours.
After the test was completed and the data analyzed, their suspicions were confirmed.The well went on vacuum proving that the zone was extremely under pressured.The scheduled 2-stage cross-link gel fracture stimulation was redesigned as a much larger 3-stage CO2 energized stimulation.If the fracture had gone on as scheduled, the formation would have been seriously damaged.
The use of the SPIDR for the pre-frac injection test allowed the operator to avoid serious damage to the reservoir and the redesigned stimulation treatment has resulted in a productive well.Use of the SPIDR system enabled the engineers to make an informed decision while avoiding the costs and risks associate with running downhole gauges.
