We all have had experiences, especially in well drilling, where the construction plan looked great on paper but just didn't work out in the field. Horizontally drilled wells, on paper, would appear to produce large amounts of ground water, when in fact the production results usually are disappointing. The length of screened area in a horizontal well may be many times the length of the screen in a vertical well; however, there are a number of factors affecting well yield which are unique to horizontal wells.
These factors include the following:
- The slot size usually is small and often is smeared shut when being pulled through the mudded hole
- Bentonite drilling mud is extremely effective in plugging the formation and well screen, and is difficult to remove from outside of the casing.
- Development is difficult along the screen unless a double packer assembly is used to focus the development at each foot of well screen, i.e., the development water needs to be surged in and out of the well screen in order to remove mud and fine formation particles for the entire length of well screen.
- The head, or pressure, of water outside of the horizontal well usually is on the order of only a few feet, which doesn't provide enough pressure to move much water through the well screen.
- A custom sand pack cannot be set against the screen, therefore the slot size is "best guessed" to match the formation grain size in order to maximize production.
Some of these factors affecting well efficiency can be addressed with the proper use of drilling fluids and development procedures. If bentonite drilling muds are to be utilized, it is recommended to use as "thin of a mud" as possible and to pull a tremie tube with small holes over a few feet along with the screen. This tremie tube can be used to inject fluids in the well annulus to facilitate development. Injecting a polyphosphate solution will aid in breaking down the gel additive of the drilling mud, thus allowing for removal of the clays. The tremie tube, which stays in the annulus, also can be used in the future to inject chlorine or other solutions to minimize plugging of the screen from bioactivity.
Physical development of the well screen - by surging water back and forth through the screen - is by far the most effective method to remove the drilling mud and fines. Favorable development results have been achieved by rapidly pumping water into the well casing and then quickly pumping it out. This can be accomplished in many ways, i.e., using a large centrifugal pump (or fire hydrant) and pumping 50+ gpm at 40-60 psi into a horizontal well, sealed at both ends, for a few minutes. The water should then be pumped out quickly (by suction pump or submersible). This process should be repeated until the water is clear or the volume of water recovered is not significantly increasing. At some sites, this redevelopment is routinely conducted annually to maintain well performance.
