Are Field Results Consistent with Calculations Using the Darcy Equation?
(Yes.)
As shown above, basic calculations using the Darcy equation indicate that flow profiles are not expected to be improved by gel treatments in unfractured wells (i.e., radial flow) if zones are not protected during gelant placement. During the past ten years, we have actively sought field examples that contradict this prediction.10-12To date, we have found no definitive contradictions. Those that claim contradictions fall into four categories.10-13In the first category, wells with fractures were incorrectly assumed not to have fractures. Often, the actual injectivities or productivities for the wells (i.e., the left side of Eq. 3, q/Δp, in BPD/psi) were five or more times greater than the injectivities or productivities calculated using the Darcy equation for radial flow (i.e., the right side of Eq. 3).
The high injectivity or productivity values suggest that the wells experienced a problem with fractures or formation parting. Since vertical fractures allow extensive crossflow in the formation, flow profiles taken at the wellbore were usually meaningless.
In the second category, the wells experienced a serious problem with flow behind pipe. This type of problem can usually be diagnosed using temperature surveys (especially for liquids) or noise surveys (especially for gases).14As was the case when vertical fractures were present, flow profiles taken at the wellbore were usually meaningless.
In the third category, too much faith was placed in the resolution of the tools and methods used to measure flow profiles.11In the wells selected for these field tests, at least two distinct zones should be present that are separated by an impermeable barrier. These zones should be separated by a sufficient distance (10 feet or more) to ensure that injection profiles can clearly distinguish flow into the different zones. Also, depending on the circumstances of their use,14one should recognize that the profiling tools (e.g., spinners or radio-tracer profiling tools) have limits on their accuracy (e.g., ±10-20%).
In the fourth category, unsupported conclusions were drawn about the effects of gel treatments based exclusively on overly optimistic interpretations of oil production decline curves.11-13
Certainly, it would be valuable to find well-documented field cases that contradict the predictions from the Darcy equation. These cases could reveal important new principles that could be exploited to optimize gel placement. However, in the absence of these cases, field engineers would be well-advised not to apply gel treatments in wells with radial flow unless hydrocarbon productive zones are protected during gelant placement. For those who remain skeptical of the Darcy predictions, you are challenged to find a convincing counter example.