We report here a laboratory study that considers the potential of jetting technology for removing formation damage produced in the drilling process.
The effective placement of chemical treatments over long open hole sections is an unresolved problem. The treating chemicals are often lost to the formation in relatively small areas resulting in poor clean-up in large sections of the wellbore. The jetting technique, which has been investigated as part of an on-going research program, could be used with coiled tubing or conventional drill pipe and would provide effective coverage along the whole length of the wellbore. The use of barite as a weighting agent in the drilling fluid rather than calcium carbonate is possible because the material is removed physically and not chemically. This is beneficial as it means that the total solids volume in the drilling fluid is lowered.
We have considered the effects of abrasive laden jetting fluids on two sandstones of differing permeabilities using two water based drilling fluids and two types of weighting agent. Further tests using water only as the jetting fluid are reported.
We find that in all cases the jetting removed any external filtercake that was present, and is also capable of removing rock material. On the higher permeability sandstone we see no improvement to the level of formation damage from jetting even with the filtercake removal. SEM studies reveal pore space blocked by particles generated during the jetting process and or by the abrasive material. For the lower permeability sandstone we note an improvement with jetting and no evidence of invaded solids material.
We have further considered the pressure required to initiate flowback after jetting compared with production straight back through the filtercake. We find that the pressure to initiate production is reduced by over 40% for all the fluids tested on both rock types.
The use of jetting technology has many applications under surface conditions, water jets have been used for such diverse activities as surface cleaning, metal cutting and mineral extraction. Water jets with abrasive are widely used in the constuction and demolition industry for the cutting of reinforced concrete.
At surface pressure water jetting removes material by cavitation; the process whereby small bubbles form in the fluid jet stream. These bubbles are formed by the large pressure release as fluid passes through the nozzle, the bubbles collapse on impact with a surface causing a strong erosive effect. Previous research at Schlumberger Cambridge Research1 has shown this effect is suppressed downhole under hydrostatic borehole pressure, the cutting rate can be reduced by a factor of 4 or more. To remedy this the addition of a small amount of solids, 1-5%, by weight can improve significantly its ability to erode material.