The success or failure of drilling and completing Geothermal Wells depends greatly on available cementing technology. This requires not only having the knowledge of temperature stable competent materials but also a complete understanding of hole preparation, casing running procedures, mud displacement, lost circulation and the mixing-placing of the cement slurry.
Many steam fields ore located where the formation contains poorly consolidated sedimentary deposits, rubblized shales and fractured volcanic rock. These formations are fragile in that some upheaval may have occurred disrupting the structural stability, and with low reservoir pressures, make them prime candidates for lost circulation problems. Many of the geothermal wells have to be drilled through and/or into these fractured formations. Loss of drilling mud or any other fluid put into the hole usually occurs creating problems which are expensive-to eliminate. This paper offers methods which have proven successful in combating lost paper offers methods which have proven successful in combating lost circulation during the drilling and casing cementing operations on geothermal wells located in fields all over the world.
COMBATING LOST CIRCULATION DURING DRILLING (1)
When drilling a geothermal well, lost circulation is to be expected. It apparently occurs repeatedly and is difficult and expensive to combat. If the hole is being drilled with mud and the mud loss becomes too great, the normal procedure is to add lost circulation materials to the mud, and hope that full returns can be regained. Too often this is not the case and another method must be tried.
Plugback cementing usually becomes the next method because it has been used Plugback cementing usually becomes the next method because it has been used to eliminate the problem. If cementing equipment is not already on location, a call has to be made requesting equipment and the proper materials for high temperatures and lost circulation conditions from a cementing service company. From the time they were first called until they arrive on location, several hours will have elapsed. The drill string will have to be round tripped to remove the bit and the cementing equipment made ready to mix and place the cement slurry in the hole.
The cementing composition selected for this type of job is usually API Class "G" Cement containing 40% Silica Flour and Retarder designed to be drilled out in 12 – 16 hours. The cement slurry is mixed and displaced. Because it has not been possible to fill the hole, the displacement may be cut short to allow the cement slurry to "drift" into the zone of loss. The drillpipe is then raised to be sure it is above the cement to wait a few hours before trying to fill the hole. This should allow the cement slurry time to thicken and become resistant to flow when subjected to an increased hydrostatic head.
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