FAQs
Many of the questions we get asked are offered here along with some brief answers. If you are unable to find what you are looking for here, please do not hesitate to contact us with your question.

 
  • What types of reservoirs can be tested using the Single Well Chemical Tracer method?
    SWCT tests can be performed on sandstone or carbonate reservoirs. Open hole or cased hole completions are commonly used.
     
  • What reservoir conditions are acceptable for testing?
    We have successfully performed SWCT tests for the following ranges of conditions:
     
    Interval Height 8 - 104 ft
    Porosity 7 - 34%
    Reservoir Temperature 80 - 249 deg F
    Brine Salinity 1,200 - 270,000 ppm TDS
    Permeability 4.5 - 6,000 md
    Oil Gravity 12 - 49 API
    Sor (no EOR applied) 7 - 45%
     
    We are always interested in working to accommodate parameters outside of these ranges, if at all possible. The interval size should be small enough to allow us to focus on a given reservoir for the Sor measurement. If large intervals are tested, the SWCT test will work fine but a wide range of Sor numbers will sometimes be the result. For most purposes, the operator is interested in the Sor from a particular reservoir zone. This focus is accommodated by keeping the test perforated interval small.
     
  • What oil / water cuts are acceptable for a candidate SWCT test well?
    For most of our tests - including residual oil saturation measurements and EOR evaluations - we will work with anything from a 100% oil producer to a 100% water producer. We simply flood the region surrounding the candidate well with water until we have achieved (or very closely approached) residual oil saturation conditions, and then perform our suite of tests. The well will return to normal production after the water flood is back produced. For connate water saturation measurements, we require 100% oil producing wells.
     
  • Can an injection well be used for a SWCT evaluation?
    No. We do not recommend using an injector as a test well because the oil composition near the injector, after large volumes of water injection have occurred, is not representative of the oil in the balance of the reservoir. After extensive water injection, the oil in the pore space surrounding an injection well is stripped of gas and some aromatic compounds and does not represent oil that will be encountered in the reservoir by the EOR process. Producers are the best choice for candidate wells.
     
  • What special equipment is required to run a SWCT test?
    Ordinarily SWCT tests can be accomplished with very little specialized equipment and no additional rigging. CTI will mobilize any specialized equipment that cannot be sourced at the test site. Generally, we need a well capable of producing, electrical power and one or two portable water storage tanks to carry out the test.
     
  • What type of well completion is needed for a successful SWCT test?
    The selection of a good candidate well is critical to the success of any SWCT field study. The test well should be completed to a single homogeneous zone to achieve ideal test conditions. The candidate well should penetrate the test zone at a point above the original oil water contact, have a means of producing fluids (rod pump, sub. pump, gas-lift equipment, etc.) and the test zone not artificially fractured and propped. The SWCT technique should be carried out from a producer to best represent reservoir conditions. Generally, newer producing wells give better test results than older wells. This trend is primarily due to the newer wells having better quality cement and having fewer conduit problems such as holes in the casing, leaking squeeze cement repairs, channels behind casing, etc. Normally, wells that have adequate equipment to serve as producers (flow line in place, electrical power, artificial lift equipment, etc) and are producing from the test zone without serious mechanical problems are suitable candidates.
     
  • Can we conduct an SWCT test on a zone that has been artificially fractured and propped?
    No. We do not recommend testing a hydraulically fractured zone. The fluid flow streams are dominated by propped fractures and the oil saturation in the proppant is not representative of the reservoir rock.
     
  • Are there any geographic limitations for CTI services?
    No. We have carried out tests in every oil producing region of the world, offshore and onshore, from the desert to the arctic.
     
  • Will the chemical tracers affect the operation of the test well?
    No. A key consideration in the design of our tracer test methodology is to ensure that the test is non-destructive. Following the conclusion of our testing, the well simply needs to be put back on production and it will return it to it's original operational state.
     
  • What chemicals are used as tracers in SWCT tests?
    We select the suitable chemical tracers based on reservoir and production parameters specific to the test well. All of our standard tracers are non-radioactive hydrocarbons that are readily sourced from commodity chemical manufacturers worldwide. They generally require no special handling beyond what is required for flammable solvents.
     
  • What injection / production rates are required for the well to be a good SWCT candidate?
    Candidates should be capable of producing a minimum of 20 bpd. We need to be able to recover the
    SWCT test fluids in one or two days. There is no upper limit to the production rate.
     
  • How does a SWCT test work?
    A fluid (usually water) containing a reactive chemical tracer is injected into the reservoir through a production well. The tracer bearing fluid is then pushed out into the reservoir by an additional volume of fluid. The well is then 'shut in' while some of the reactive tracer is allowed to undergo a hydrolysis reaction within the reservoir. Following 'shut in', the well is produced to bring back the fluid, which is analyzed for tracer content. The reaction product, alcohol tracer and unreacted ester tracer undergoes a chromatographic separation during their return journey due to their dramatically different oil/water partition coefficients. The magnitude of this separation depends on the oil/water ratio in the reservoir and the ester partition coefficient, thereby allowing us to calculate Sor.
                                                                                                                                         
  • Are all artificial lift mechanisms suitable for SWCT testing?
    Gas lift, rod pump, ESP, free flow and PCP support SWCT testing without serious complications. Jet pumps incorporate substantial dilution of the reservoir produced fluids with the drive fluid and should be avoided. 
                                                                                                                     
  • How long does one SWCT test take to complete?
    Generally, one SWCT test is completed in about one week. The shut-in time is dictated by reservoir temperature. Typical shut-in periods are from 2 to 8 days. Injection is carried out in one day and production is usually completed in one to two days.
     
  • How many SWCT tests have been carried out so far?
    The first SWCT was conducted in 1968 in the East Texas field. Since then, about 650 tests have been conducted in most oil producing provinces of the world. CTI has conducted about 475 tests and Charlie Carlisle, president of CTI, has taken part in about 600 SWCT tests since 1979. 
     
  • What do I need to provide to carry out a SWCT test or One-Spot-Pilot?
    CTI will provide the specialized equipment needed to carry out the injection of filtered water and tracers and chemical analysis of all of the injectants. The operator supplies one or two vertical tanks, the producing well and electrical power, about 25KW. If the test is carried out using a pumping unit, the power to the pumping unit is more than adequate to supply the CTI field unit. A one liter sample of stock tank from the test reservoir is also needed to accommodate partition coefficient measurement.
     
  • What information do you need to design a SWCT test and prepare a proposal?
    The following base reservoir information is needed:
     
    • Reservoir temperature
    • Test well production rate, or typical production rate for the possible candidates
    • Perforated interval size and average porosity
    • Reservoir depth, artificial lift method
    • Type of EOR method to be tested (in the case of a One-Spot-Pilot). This can be CO2, miscible hydrocarbon, WAG, ASP, SP, or AP